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Solar Water Heating Fundamentals

Solar thermal collectors


Craig and Bobby setting up a panel at the Jennings Street installation. Contracted by Bright Power for energy cost reduction.

• Glazed flat plate

This is a durable aluminum framed box with a tempered glass aperture area. It has the appearance of a typical solar panel. Inside the box is either harp or serpentine style copper tubing welded to a copper and chrome absorber plate. The dimensions vary from 3"-6" deep, 3'-4' wide and 6'-10' high. They are durable, have high pressure ratings and no components that can fail.

• Unglazed (pools)

These collectors are unglazed because the absorber surface is not encased by glass. Made from black polyethylene plastic or EPDM rubber, they heat water very efficiently to high temperatures when there is enough insolation (sunlight). Large volumes of water can be heated but they cannot be pressurized. They are very light weight and come in a wide variety of dimensions. They are perfect for heating an outdoor swimming pool and save the home owner literally thousands of dollars that would be spent to heat the pool with propane. A lot of collector area is needed though the panels are very affordable.

• evacuated tube

A 50 tube Sunmaxx, Diversified heat transfer productssystem in Harlem

Tubes differ in many ways from flat collectors though the overall amount of heat produced per sq ft of roof area is nearly identical to flat plates, each performs better under certain conditions. Evacuated refers to the method of insulation in which the absorber area is sealed by vacuum filled glass tubes. There are two distinctly different types of evacuated tubes, water tube and heat pipe. In a water tube collector the heat transfer fluid, such as propylene glycol, circulates through the evacuated are. Water tubes cannot be used in a drain back systems and contain very small passages for the fluid resulting in losses and possible points of failure. Heat pipes carry heat out of the vacuum using a medium such as a refrigerant to a point outside of the tube which is then immersed in the heat transfer fluid. The top of the heat pipe will exceed 400 degrees quickly when left dry in the sun.

• Integrated collector storage

ICS units have a tank that is attached to the collector and goes on the roof. Hot water moves into the tank from the panel by a natural convective process. DHW is usually drawn directly through the tank/ panel assembly. If there is any risk of freezing the system must drain down completely.

• other technology

Solar air heaters

These are wall mounted panels that gather heat and circulate it through the building with a small DC powered fan. No piping is needed, only two small penetrations in a south facing wall. This is a great solution for areas where utility service or heat system piping is not possible. A 4'x8' panel will produce 6000 btus/ hour in direct sunlight.

Parabolic trough and parabolic dish

Solartron parabolic dish installed by E2g Solar with help from SPD, for Paul Adler. Featured in Feb 2012 Home Power magazine.

Parabolic collectors focus the suns heat on a small area where no absorber plate is needed. The focused sunlight creates intensely high temperatures, tens of thousands of Btu/hr. from a 1 sq ft collector surface area. The dish shown in the picture is 12' across and the aperture area is only 12"x 12". Trough collectors operate on the same principal but there design makes them suited for very large arrays, the collector surface is a tube. Parabolics' need direct sunlight to function so they are always equipped with sun tracking technology which also enables them to collect when the sun is low in the winter.


link to learn

Concentrating Solar Power is a step up from the parabolic dish and constitutes a technology of its own because the intense temperatures created are usually use to power electricity producing steam turbines. Temperatures of over 700 degrees are gathered along the collector surface, a tube. Molten salt is a very good heat transfer medium and is manageable in extreme temperatures but requires high tech metals to withstand corrosion. The heated molten salt can then flash water to steam and powers plants to produce mass amounts of electricity. It is living proof that thermal energy IS the creator of electricity.

BIS in Germany by Wagner Solar

Building integrated, various

Building integrated solar is a loose term. It includes using solar panels as a deck cover, using panels as a building material in the construction of the façade or even embedding collector tubing into a roof or wall surface. Perhaps more artistic than experimental, there are known to be many feasible and productive methods the question is which one best suits the design conditions. Since each project is unique and site built, high engineering costs and complicated installation may challenge the investment capability.

Choosing equipment

Artisan Solar roofing shingles by Don Follett

Your installer will either have a preferred brand or recommend one based on design conditions. Often tank and collectors are not available from the same manufacturer. Chose brands that have a base in the US, are experienced and secure. Make sure they can be contacted when advice, packing mistakes or warranty action is needed. Warranties on solar tanks should be 6+ years and 10+ years on the collectors. Look at the SRCC ratings that will tell you the expected production and the collector efficiency and pressure ratings.

Choosing an installer

The loudest is not necessarily the best. Look for integrity and experience, check references. Your installer needs to have a strong background in plumbing and construction, should work for a licensed plumber and be aware of local laws. A good warranty is critical it also proves that the installer is confident with his/ her work. Consider what their insurance covers and understand your roof warranty. A local installer who is on hand and who is also involved with a solar network is the least risky.

Becoming an installer

Miriam and Shamika, students from Non traditional Employment for Women and the Community Environmental Center, field training at Sleepy Hollow

Get as much experience in plumbing and construction as possible and then learn how to design with solar. There are many great solar learning opportunities available locally. Being accredited will help you get a job with a company installing or enable you to recommend and participate in an installation. NABCEP offers the best certification in the field and is esteemed among professionals. Certification is an excellent goal and pursuing it will guide you through the process of gaining the knowledge needed.

Roof mounting

• Orientation

High noon, winter solstice (Dec. 21st), St Johns between Underhill and Grand Army Plaza

It is optimal to mount collectors facing due south. The tilt angle of the collectors in relation to the angle of the sun is also very important. To stay within your projected performance range, arrange panels to face less than 30% from solar South. On East/ West pitched roofs, East and Westward oriented arrays can use separate controls or multi input controllers to operate each set of panels alternately or independently based on the period of the day they are receiving direct radiation. Consider the angle of the sun throughout the seasons. In NY 42 degrees Latitude means the average angle of the sun is 42 degrees and a collector mounted at a 60 degree will collect more when the sun is closer to the horizon in winter.

• Shading

Place the panels where they are not shaded, if possible. Performance will diminish in proportion to the shaded area. Also consider foliage growth and future building plans. It is sometimes hard to visualize how shading will change during the seasons when the sun is in a different position. Tools such as the Solar Pathfinder reflect the site surroundings on a lens which simulates the curve of the earth and shows where the shadows will fall at each season.

• Attachment methods

This plank is securely adhered and bolted to the roof and then covered with a roof membrane

Attaching panels to the roof affects the building structure and roof surface, so it is a serious consideration. Though flat panels filled with fluid weigh less than 4 lbs/ sq. ft. and tubes are even lighter, the upward forces exerted by wind can be as much as 160 lbs/ sq. ft. The depth of the snow on the roof makes it necessary to give the bottom edge of the panel a standoff so the entire panel is above the snow line and so snow can slide off.

Remember it will be harder to resurface the roof after panels are attached, so it is better if the roof is new.


If the joists are in good condition and the roof surface is new, the panel racks can be bolted through the center of the roof joists making sure to pre drill for a burly bolt and locating the exact position of the joist. Some owners want the penetrations to be covered by a warranty and if this is not done by the original roofer the warranty may become void for the rest of the roof. The installer is usually capable of sealing the roof so if the owner agrees, bolting to the roof is easiest. If the joists are exposed in the attic but are weak or staggered, the rack can be bolted, not to the joist, but by using a rod and bar to hug the joists from the underside.

64 Sunmaxx panels with attached ballasting system


Weighing down the rack works if the roof can handle the continuous load which must equal the maximum expected wind load. It will also present an added difficulty if roof resurfacing is needed.


The benefits of embedding a platform for the panel footing into the roofing material are that the roofer has complete control, the roof can be resurfaced, the roof structure does not need to be interfaced and the panels can be at any orientation in relation to a flat roof. This can be done on flat or shingled roofs by an experienced roofer.

Structural steel

One of Bright Powers' many efficient, high quality designs. Mounted on steel for longevity and integrity

Sometimes the value of the roof outweighs the cost of attaching steel beams to the supporting structure of the building. Where steel spanners are located for solar all of the above benefits apply and in addition, if structural work on the roof became necessary it could be done without interference to the array. Though expensive, this is certainly the method which is best for the longevity of both the roof and the solar components.

• Racking and panel connections

Avoid using metals which are galvanicly dissimilar, most notably; steel should not be in contact with copper or aluminum since accelerated corrosion will occur. Where aluminum racking is mounted on steel put an EPDM or butyl rubber layer between the two. Triple check your racking hardware order because if you come up short on nuts or bolts you won't be able to substitute them with galvanized or zinc plated steel bolts but you may replace them with stainless steel which is compatible with most other metals, stainless steel is OK.

Closeup of a Sun Earth racking attachment

Panel manufacturers sell racking kits to mount their collectors on. This usually consists of some sort of foot that may or may not attach to a rail. The side rails are aluminum struts that have channels for the bolt heads of the bolts provided for assembly. The top varies but is usually a type of aluminum strut. There will be clips to attach the panels to the bars. Be sure to follow the instructions exactly since precise engineering led to the design of the clip.

The top and bottom copper manifolds of the panel must be joined with a level of precision unseen in regular plumbing so it is super critical that the panels are absolutely level with each other with no dips or curves where racks are coupled together, particularly in panels connected by "unions". Some brands allow the headers to be joined by soldering which is more forgiving during installation and is more durable but more difficult to repair later on. Union type connectors usually have gaskets which are very sensitive to damage during installation and are not designed to "squish" or allow any deviation in the position of the flanges. Some with brass compression fittings may need tightening after start up. The space between the connector and the panel is very slim and the copper may be delicate so it is necessary to be prepared with 2 or 3 low profile wrenches.

The best way to avoid problems with union type panel connectors is to adjust the panels so the header is very straight and tighten the unions BEFORE committing the panel clips.

• Penetrations

PVC trap with flashing collar

A 4"-8" PVC pipe is inserted through the roof with a hook on the top so water will not get inside. A prefabricated flashing collar is used to seal the PVC to the roof membrane. There is plenty of room inside the PVC pipe for the solar pipe insulation and sprayfoam can be used to seal the remaining spaces. Try to use sweeping bends or flexible tubing where the solar pipe passes through as it creates a small up trap in the pipe which could trap air.

Coolie cap

Essentially a copper cap two pipe sizes than the solar pipe can be soldered directly to it thus covering the penetration and allowing insulation to butt right up to the cap. Some thermal losses will occur where the soldered cap, touching the pipe, is exposed to the outside air but the profile is close to the roof so an essential pitch can be maintained.

Pitch pocket

A square of tar is cut out around the penetrations. Sometimes the pipes might pass directly through the new bed of tar. But the pipes are hot so a shield should be around the pipe with a rain cover, such as a coolie cap. The area is then filled on with new tar.

At the foundation wall

2 holes core drilled through a granite wall leaving a 1" annular space for insulation

This implies you are running the solar pipe down, along the outside of the building. Support the pipe with struts or straps at least every four feet so it won't bend, slap the building or blow around in the wind. You must put a weather protective shield on the exposed insulation. Always be mindful of pipe expansion and the thermal shock that occurs at start up.

Usually the base of the building will be poured concrete, concrete block, stone or brick. Using a hammer drill or core drill make an opening large enough for both pipes and insulation to pass through. Make sure you are not harming the structural integrity. The pipe must be encased in a strong shield that will bond with the cement and reinforce the structure, such as a piece of cast iron pipe. Cement corrodes copper so don't make contact with these materials. Fill the area around the opening with cement and use a rubber cap if further weatherproofing is needed around the solar pipes.


Sometimes the array is sighted away from the building it is supplying. This is often the case with pool heating. Trenches can be dug by hand or machine and the trench must be at least 2' deep to with stand impacts from above. Since the pipe is a low spot it is a wise addition to put an access box with drains somewhere along the channel. The pipe must still be insulated and the minerals in soil will corrode copper pipe. Fiberglass insulation will deteriorate quickly and hold moisture so use elastomeric insulation. Additionally, foam board can be placed around the insulated pipe to help protect it from the dirt.

Freeze protection and overheating control

• Glycol

Propylene glycol is non-toxic anti- freeze. Brands designated for solar have re-soluble inhibitors. The inhibitors prevent the glycol from becoming too acidic for the pipe. The glycol should be rated for at least 325 degrees. It will keep its' integrity for 5 years under normal conditions and less per occurrence of overheating conditions. When glycol degrades it becomes acidic enough to eat through copper especially where there is extra friction in the pipe such as at elbows. It also loses its freeze protection gradually, as the inherent chemical compounds eventually separate.

• Fluid expansion

For the most striking description of fluid expansion consider that 1 part water occupies 17,000 x its' original volume when it is steam. Steam formation temperature is retarded by pressure and most strategies make the goal to avoid steam either by limiting the temperature or increasing the pressure.

The fluid in its liquid state also expands dramatically according to temperature. Where steam is to be avoided, the expansion tank should have the capacity to hold 1/3 the liquid volume of the system. The expansion tank should be compatible with glycol. The tank has a Schrader valve on it and should be pre charged to within 5 lbs of the average system pressure. If the expansion tank cannot take up the expanded liquid, the system pressure will increase until the pressure relief valve opens.

• Pipe expansion

Coil copper expansion loop with covering.

The pipe material also expands dramatically when temperatures rise. Each material has its own coefficient of expansion but here we will refer to copper. Often overlooked, the expansion and contraction of the pipe itself is the leading cause of leaks on roofs. Leaks in a solar loop come at a high price. Because there is usually no make-up fluid, damage to the pump and loss of expensive glycol will occur. When the pipe expands it will stretch lineally so damage is usually found at bends. Mechanical expansion joints can be purchased or a U can be made from soft copper or an assembly of copper sweep elbows, being mindful not to inhibit flow. The formula used to calculate how much a pipe will expand is: E= (coefficient of linear expansion in/in F degree)x Length of the pipe x delta T (at start up the delta T of incoming fluid might be 100 degrees or more. "E" represents the deformation that needs to be absorbed by a bend. The coefficient of expansion for copper pipe is 9.4 x 10(-6). So a 1200"(100') pipe with a delta T of 100 degrees would have an "E" of 1.128. To find the length of pipe in a U bend in feet which will absorb this deformation (K): 6.2 (constant) x (square root of OD of pipe x E). The Outside Diameter of 1/2" copper is 5/8", so, K=(.625 x 1.128=.705) square root is .839, x 6.2: K= 5.20 ft or a U 1.73 ft on three sides. You would need a 5' 2 1/2" U bend of extra pipe to compensate for the expansion in this example, a significant amount.

• Heat dumps

When the storage tank reaches its maximum safe temperature the controller will signal the circulation pump to shut off. See below for more detail. Now temperatures in the collector will continue to climb. If a heat dump strategy is employed; rather than the pump turning off, the relay will activate a diverter valve that will instead direct the fluid through a radiator located outside of the building. Thus the fluid is kept from boiling until hot water demand brings the tank temperature to a point where it can accept more heat input. Swimming pools make excellent heat dumps and also energy on pool heating. Simply divert to a pool heat exchanger. The extra heat can also be stored in a seasonal storage pit and used later in winter as a contribution or with a heat pump.

• Drain back systems

Drain back system installed with designer Peter Skinner of e2g Solar.

Drain backs serve as both freeze protection and overheating protection because when the pump is not running the fluid from the collectors drains down into a holding tank. Among the many benefits of a drain back system is that, when properly sized, it also accounts for fluid expansion. It is a mechanically sound strategy for addressing 3 major concerns in one design feature. Each time the pump starts back up it needs to lift the water back to the collectors so requires a larger pump for the startup process and a controller that can signal this initial pump. Therefor it is better to locate the drain back tank at the top of a building. An audible trickling sound results when the water drains down but can be dealt with by installing some flow restriction on the return piping. Most importantly, the collectors and piping must be installed at a pitch so all fluid will drain out of the collectors and also all piping on the roof and to the drain back tank.

• High pressure and steam back systems

High pressure system

In this heat protection method, high pressures are used to control steam formation. All components must have at least a 150psi pressure rating and withstand temperatures up to 350 degrees. A well sized expansion tank is still needed to take up extra fluid volume. Pressure relief valves on this type of system do not open until temperatures reach 350 degrees.

Steam back system

These would be set at up to 50 psi for a boiling point of 300 degrees, usually the setting is lower. Here boiling is anticipated. Steam forms in the collectors and pushes the liquid down. The water hides safely in the cooler zone while the collectors continue to heat the steam portions of which then also condense. The pressure spikes when the liquid is about to turn to steam and then levels out as the liquid is forced to occupy the expansion tank which is the specialty object in a steam back system. Expansion tanks up to 50% of the volume are used and the pipe to the expansion tank is as short as possible with no check valves to block the flow of water to the expansion tank. Tank and glycol must have exceptional temperature tolerance.

• Storage

The basis of tank sizing is that the tank should able to hold and utilize all of the extreme heat that may be collected and is there for the useful way to control overheating. Tanks sized for space heating are designed at a ratio that will allow the tank to be heated entirely by the solar array in winter and in the summer will need to have a heat dump plan. In multi-family building with high consumption the water may never reach its' point of use temperature but will still contribute significantly to the water heating load. See more about storage and use below.

• Plumbing components

This header wasn't installed straight so the gasket failed. When a plumber (not me!) tried to fix it with a standard plumbing material, it failed even worse. What a mess!

Plumbing components used to install solar must be able to handle 250 degrees under normal operating conditions but the conditions are not always normal. All contingencies should be considered when selecting components. What if two out the three families are all on vacation for two weeks in the summer??? Under normal operating conditions fluid and pipe expansion occur to extreme degrees. Use schedule L copper or steel. Be sure the diaphragm in the expansion tank is pressure charged correctly with a 350 temperature rating and is compatible with glycol. If you are using a double pumped drain back system the second pump must be bronze to with stand corrosion from the air. If you use a mechanical flow meter be sure it can handle the heat and pressure. There are many fine points that need to be considered to avoid disaster when plumbing the solar loop all of which are addressed by using good components and a thought out piping design.

Storage tanks and heat exchangers


• Tanks

The daily consumption needs to be determined when sizing the tank or any other hot water source. Tables can be found in plumbing code books and many other reference sources that show the rate of use at each type of fixture. A suggested rule of thumb is to assume the first two residents of a family will use 20 gallons/ day each and additional member will use 15 gallons. If it is intended that this hypothetical family will heat their use entirely with solar March- September, we now know that the storage tank will need to be at least 70 gallons. Now consider occupancy rates and weather conditions such as: what if they vacation for a week in the summer? What if it is rainy for 3 days and I'm only getting 1/3 of the seasonal irradiation? The possibility of a lapse in occupancy results in the necessity of an overheating protection method. This concern is not normally applicable in a multi- family building. Industrial facilities would want to be able to store the heat during weekends and holidays but residential systems do not usually have room for that much storage. If possible you might size the system for sub- prime production with a high collector to storage ratio, such as in a space heating system, and divert the storage to an additional tank when the input is great or there is not enough use.


Colder water is heavier than warmer water and will sink or stay at the tank bottom. Conversely, the water will be hotter at the top of the tank. The effect of stratification could easily create a temperature difference of 40+ degrees between top and bottom in a 100 gallon tank. Cold water needs to put in at the bottom of the tank so it does not cool the warmer water and hot water should be drawn from the hottest top of the tank. Usually the effects of stratification are utilized and turbulence in the tank is counter productive. When putting heat into a tank be sure to give solar the priority by injecting it lower that an auxiliary source. The heat will fill the tank from the top down naturally and so the auxiliary source will not be signaled. When there is no sun and the auxiliary source is relied upon, the top portion of the tank must hold the load of the demand and this is another reason why solar storage tanks are often larger than fuel fired tanks.

Collector ratio

Each 1 Sq. ft. of collector area should have a minimum of one gallon of storage. The collector area is not always apparent as with evacuated tubes and the rules change with other collector types. It is preferable to base storage needs on the collector output rating, referenced from the SRCC (Solar Ratings and Certification Committee). The rating will show the daily performance of a collector under various weather conditions. New York latitude puts us in category C of the ratings tables for the summer. If, for example, your collector has a rating of 40.000 Btu on a clear day in the summer and the family you are serving uses 55 gallons each day and needs to raise that water 100 degrees to make it hot enough: 40,000/ 8.34 (lb.s/gall)=4,796 (gall/degree), 4,796/100= 47.96 gallons heated 100 degrees each clear day March- September. In large commercial systems where the use is much more than can be produced with the usable roof area the tank size determines how much usable heat can be transferred and not necessarily stored. Imagine if the tank temperature was the same at top and bottom on a sunny day because the heat was being consumed at the same rate it was produced. Even were that only 30% of the total load it would be very efficient for heat transfer and the system would always be productive.

• Heat exchangers

Two flat plate heat exchangers installed for Earthkind Solar, rated for 350,000 BTU/HR.

Heat exchangers transfer heat from the source, solar, to the medium, water. Those are the materials used in this example. There are heat exchangers where fluids pass each other in unitized package of fluid passageways. There are brazed plate heat exchangers and shell in tube heat exchangers among others. There are also heat exchangers where the source fluid is piped through a submerged coil or other submerged passage ways. Submerged coils offer the least frictional resistance to the fluid flow.

Heat exchangers are rated in Btu/ hour of heat transfer potential. The Btu production/ day can be divided by 6 prime solar hours each day but when finalizing the calculation add an additional 25% to the production. To begin the calculation: determine the collector output and the flow rate recommended for that panel (say 1 GPM per panel/40,000 Btu day). Convert the GPM into pounds per hour because hour answer will be in hours, not minutes. 60GPHx 8.34 lbs=500.4 lbs/hr. 500 pounds of liquid are moving through the system each hour. Convert 40,000 Btu per day into hours 40K/6 hr=6,600 Btu/hr. 6,600 Btus need to transfer into 500 lbs.= 13.2 degrees continually transferred from a 6.600 Btu rated heat exchanger. Add 25% for seasonal variation and to keep the returning water temperature optimized. You do not want to transfer all the heat because you can't raise it all the way back up from 0 when it makes the next pass through the panels. Having a slow flow rate in an attempt to raise the fluid more than 20 degrees will cause scale to form on the heat exchanger and reduce the operating hours. In this case the temperature in the panel will need to be at least 13.2 degrees hotter than the bottom of the tank to be effective. This is the primary Delta T that will signal the solar pump to turn on, when it knows there is enough heat in the collectors to transfer to the storage water.


• 60-1000+ gallon steel tanks with 1-12 exchange coils in the tank

Sub merged coil heat exchangers are not specified by Btu. They are sold as proportionate to the storage volume so are pre- engineered. Tanks up to 130 gallons are available for sale with one or two submerged heat exchangers built in. Larger tanks for domestic water can be custom built on or off site with heat exchangers welded in to specification. The tank must be insulated with 2"-4" of foam and should have a sacrificial anode.

• 40-2000+ gallon steel tanks requiring external heat exchanger

Piping in two giant storage tanks at Westchester Community College for Earthkind Solar.

This includes everything from a small existing hot water heater, with the elements disabled, to industrial sized tanks, fabricated on site. Tanks up to 750 gallons can be purchased pre-insulated from manufacturers. When you are using a heat exchanger the tank must have at least 4 points for connection. Small domestic retrofits can make use of the drain and pressure relief ports but be sure any tank used has a properly sized pressure relief valve. The tank must be insulated and should have an anode.

Plate HX

Brazed plate heat exchangers are compact external units piped to and from the heat source and also to and from the storage vessel. They can be purchased from a variety of sources and selected based on their Btu rating.

Tube in shell

For large volumes and applications where flow is critical but an external heat exchange unit is required. The choice for pool heating or applications with very large pipe diameters. Tube in shell or tube in tube is more like a submerged coil but is external to the tank.

• Collapsible EPDM tanks

Nice EPDM lined tanks, like these by STSS, are collapsible for access and are availble up to 1500 gallons.

Also referred to as unpressurized or atmospheric tanks, the water in the tank is not potable but used as a heat storage medium. Available up to a 2000 gallon volume, these tanks can be fitted with submerged coil copper heat exchangers to specification. The domestic water is heated as it passes through a heat exchanger, drawing heat from the volume of the tank. Heat from heat sources is similarly injected into the thermal mass. The best thing about these tanks is that they are compactly packaged to fit through small access ways.

• Other storage methods

Thermal mass

Excess heat produced during summer can be stored in sand, earth, salt or water for use during winter. Since the chamber is not pressurized the temperature can be allowed to climb as high as the piping and insulation will allow. It is feasible to heat a building all winter and yet reduce the necessary collector area.

District pools

District heating serves separate residences or multi-family units by piping stored hot water from a large thermal mass such as an underground pool located away from the building. Large, ground mounted arrays can supply hot water and heating remotely. This is commonly practiced in European countries.

Molten salt

The salts being used to store thermal energy are compounds like Sodium and Potassium Nitrate that melt at over 750 degrees and don't vaporize until they reach thousands of degrees allowing them to hold mass amounts of latent heat. The salts then flash water to steam which powers electric turbines. The ability to store heat at such high temperatures means it is usable overnight and through unfavorable weather conditions making it a stable and plausible source for grid tied electricity production.

Back up heat source

• Integrated into solar tank

Designed by Caleffi, makers of the world's best hydronic components.

If the solar input is not providing all the needed heat, the same tank activates an electric element. If designed well, a special gas burner can be used in the solar tank alternatively. The requirement is that the auxiliary heat source only heats the top portion of the tank so that the solar loop will be activated to heat the bottom of the tank. Many common brands offer solar tanks with electric back-up and a few brands offer gas back-up tanks with a pass through burner midway up the tank. Regular electric tanks can be retrofitted by removing the bottom heating element and supplying the tank bottom with water from a solar heat exchanger outside of the tank. The cold port at the top of a water heater has a dip tube that causes the cold water to be injected to the bottom of the tank, hot solar water can be similarly injected into the tank bottom 1/3 ^rd, and the drain port can be used to draw the water for circulation.

• Supply hot water heater

The classic prefeed system supplies the cold inlet of a hot water heater with preheated solar water usually from a solar storage tank with a heat exchanger to boost storage capacity and regulate the solar control strategy. Here the HWH will respond to the present temperature all or some of which will have been boosted by the solar input.

• Supply tankless

Caleffi is THE best, thank you Caleffi!!

As long as the selected tankless hot water heater has a burner activated by incoming water temperature and not activated by flow, a tankless works fine for auxiliary heat. Prefeed the cold inlet with solar heated water and verify that the tankless construction can handle the hot inlet temperatures. It might be necessary to mix down the inlet water temperature to protect the tankless. A tankless capable of use for space heating would be sufficient. A properly sized solar storage tank is required to store and maximize the solar production.

• Supply boiler

The same strategy and burner activation requirements apply to the boiler as to the tankless HWH. Modulating boilers are designed to fire according to hot mixed inlet temperatures which is a great efficiency advantage. If there is a recirculation line supplying the boiler the solar should be diverted back to its' own storage tank until its' temperature is hotter and otherwise must be designed to not interfere with the recirc loop. Solar storage is required and a pre heated domestic tank may be necessary especially if there is recirculation.

• Boiler, tankless, or hot water heater supplying solar tank

Caleffi rocks.

Any of the above appliances can circulate auxiliary heat through the top coil of a dual coil solar tank or do so by use of an external heat exchanger. Merely set the controller to activate that zone valve or pump(s) when the top of the tank falls below the required temperature. The effective difference in use of the heating appliance here is that the auxiliary heat circuit is a closed loop requiring closed loop components. One is not interfacing with the domestic production of the auxiliary appliance. The appliance will not be used to produce domestic hot water in this configuration as it will be manufactured within the solar tank.

• Heat pump

A heat pump could be used be used to extract usable heat from the solar tank even if the tank was maintaining a lower temperature. This might require a larger than usual heat exchanger from or in the top of the tank because heat pumps need a high flow rate. If there is a geothermal system, the winter solar temperatures, with an average heat exchanger and a large enough heat pump, could contribute enough to the production to meet 100% of the space heating and domestic loads. If the geo system was capable of contributing a heating loop to solar storage it could be used at the top of the tank for a seasonal temperature supplement.

• Direct injection

Certain thermostatic mixing valves will accept pre-heated water to be injected on the cold inlet port.

The valve will not introduce a hot water supply to the hot port except to boost the temperature of "cold" water. Of course, a domestic mixing valve will be installed down stream to temper the supply to the fixtures. In this way none of the solar heat is unusable or lost to the operating parameters of the auxiliary heat source.

Integrating with domestic hot water piping

• Mixing valves

Setting the standard for high efficiency mechanical rooms. By Bright Power.

It is a safety and code requirement that the domestic water must be distributed at 120 degrees or less and so a thermostatic mixing valve must be installed between the heat source and distribution system. A check valve on the cold side of the mixing valve is needed to prevent the backflow of hot water into the cold water supply.

• Check valves

Thermal migration

A check valve on the solar loop should be installed to prevent convective flow in the pipes from temperature, even when the pump is off. Swing checks offer low head losses while a spring check offers a positive seal though creates flow losses. Even when flow is arrested at the turn off setting, temperatures can still migrate within a pipe run. If temperatures persist in escaping from the tank, additional check valves might need to be installed.

Flow control

Check valves are located to direct flow in only one direction and disallow unwanted mixing of sections of connected pipe.

Back flow prevention

Hot, warm or circulated water cannot be introduced into municipal water supplies. Pressure differentials will result where backflow is prevented because of draw and temperature dynamics so a vacuum breaker is needed to stop backflow if the drawing side still has a pressure higher than the main.

• Plumbing permits

A plumbing permit is required for the alteration of domestic water pipes where they are rerouted and connected to the new tank or appliance.

• Label the pipes

Label the pipes for safety and maintenance.'

Pipes of systems containing other than water should state this and the type of fluid contained in the pipe. This is both for emergency work and to prevent future accidents. Solar plumbing systems are not commonly understood so the direction of flow should be clearly labeled and where the fluid is going. This will help the owner/ operator understand the piping network and prevent mistakes that would damage the system. Pretty pipe labels can be ordered from your plumbing supplier or on line.

Space heating

• The importance of low temperature heat emitters

Radiant tubing: easy to install in or under the flooring.

Heating with low temperature water consumes less fuel. Of relevance too is how to make the best use of heat already created from solar and put into storage. The slower that water is moving through a radiator and the more surface area the radiator has, more Btu will have a chance to radiate out to the environment. The temperatures required are lower than those expected to be used for domestic hot water so a draw dedicated to space heat can be maintained by a solar tank.

• Sizing the solar array

First the load of the heating system is calculated based on the room volume and heat loss in the heated area. Usually the heat load will be 3 times higher than the domestic hot water load. The panels will be collecting less energy for fewer hours and will collect best during winter at a 60 degree tilt. Refer to the collectors SRCC rating to see the variation in seasonal output. You will notice evacuated tubes have less of an efficiency drop when collecting for winter, but they don't shed the snow so chose the location carefully. The output will be around 1/2 of the summer output. An array designed to meet all space heating loads will be up to 9 times larger and a strategy to store or disperse extra heat during the summer is necessary.

• Combi tanks

High Powered combi tanks from Wagner Solar.

Tanks designed for space heat maximization are like dual coil solar tanks. One of the coils is for the domestic hot water and has a lot of surface area and capacity. Hot water for heating is the water in the tank. The tank water is circulated in series with a boiler or tankless as a closed loop.

• Forced air


A popular means of heat distribution, forced air can be easily supplemented by inserting a fluid filled heat emitter inside the ductwork.

Solar heaters

For space heat only on south facing walls or roofs products are sold or can be fabricated that are designed to capture and hold heat from the sun. A small fan powered by a solar PV panel will move the warm air into the room. Only a small hole is needed to penetrate while the unit is mounted and sealed to the outside if the structure. These units are sized up to 6000 btus.

• Passive Solar

This refers to a building designed to incorporate solar heat gains and heat retention by placement of windows and walls and good insulating practices.

Solar air conditioning

Solar air conditioning, like this absorption unit, solar heating and PV equipment from Solar Panels Plus.

Heat from a solar source can be used to circulate the refrigerant in an absorption chiller which otherwise needs gas or electricity to operate.

Piping, insulation and controls

Piping considerations

• Balanced flow piping

Balanced flow piping

When using multiple tanks or groups of collectors it is important that each unit receives the same amount of passing fluid, heating up at the same rate. The effects of this would be most noticeable if one tank was cooler than others. Balancing can be achieved with valves or valves with flow meters. Reverse return piping is the best way to achieve a balanced flow. In order for it to be effective, pipe lengths, turns and obstructions must be identical on each supply run and on each return run.

• Routing the pipe

Solar Plumbing Design running a line set for Green Power Solutions.

Supply and return piping are all that stand between the tank and heat source and is the only part of the system where losses from installation can occur and is why some systems work better than others. Though options of where and how to run the solar pipe can seem limiting because of construction considerations and improper construction planning, an all out effort should be made to adhere to the following:

• Pipe elevation

Carefully setting the pitch of the pipe so the system can be drained completely and won't trap air. (

Avoid having air entrained anywhere in the system including the pipe. One air bubble will block the flow of liquid and if it reaches the pump will stop the pump from running. If there is a hill in the pipe route air may remain hidden there for days before it presents a problem. Sometimes a lot of air will stay stuck to the top side of a long horizontal pipe run. For this reason, even though it may not be a drain back system, it is preferable to pitch the pipe slightly and use smooth pipe. If the pipe is very smooth and any joints in the pipe are reamed the air will find its way to the top where it will be manually removed. Flow rates are between .5 and 1..5 GPM per panel and so is very slow in parts of the system, making it difficult for air to be carried away by just the small circulating pump.

It is also recommended to avoid low spots where debris can settle. This debris could be bits of solder, copper shavings, pipe dope or burned glycol. Debris is inevitable and should be allowed to flush from the system. Low places can cause difficulty when flushing the system or repairing the system and can never be completely dried. If you must design with a low spot, please, put a durable drain on it.

• Types of pipe

Copper pipe comes in 10', 20' or coils up to 100' lengths. There are four grades of copper pipe with different wall thicknesses, medium to high to high pressure applications where the PH might be low should use "L" copper. "M" copper could degrade quickly under stagnation conditions and, since the glycol is slightly acidic, will have a shorter life span. Copper pipe can be soldered or brazed and is easy to cut and repair. PEX, cross linked polyethylene, is used in heating applications and can handle temps up to 225, is impervious to acid, comes in long rolls and is flexible. However the temperatures of a solar thermal loop may exceed the tolerance if the system is not designed with a positive heat dump, no other pressure stagnation protection method will work. Flexible, Stainless Steel line sets are a popular choice and often sold by the collector manufacturer. The temperature tolerance and chemical resistance is excellent. Drawbacks include moderate pressure ratings, ridges that disrupt the flow, difficulty repairing and the attachment method requiring specialty gaskets. Iron pipe can be used in closed loop heating and closed loop solar applications. It is cheap and has very high tolerances but requires more skills, time and equipment to install.

• Sizing pipe

Formulas for sizing pipe are based on the flow rate required and the frictional resistance of the pipe. These are the limiting factors and your pump will be selected based on the flow rate and friction once the pipe size is selected. The amount of fluid traveling is usually rated in gallons/ minute (GPM) and the speed at which it is travelling is usually rated in ft/second (FPS). Greater GPMS means greater FPS and when the pipe size increases more GPMs can travel at less FPS. More than 8 feet/ second will greatly accelerate wear on the pipe and the pipe can only sustain a certain amount of fluid flow before the frictional resistance will cancel out flow entirely. Take as an example 100' of 1/2" cooper; at 5 gpms the head loss due to friction is 65' of head (Hazen- Williams formula), each elbow adds another .5' of head, and the collector will add another .5-1..5' of head. In other words, even a giant pump will not make it possible to supply 5 collectors on a 4 story building. For this hypothetical installation we want to minimize friction losses so we can reduce the size of the pump and not put excessive wear on components. We have 5 collectors in a string so we need to run the flow fast enough to gain about 15 degrees as the fluid moves through the array, a bigger heat gain is less efficient since hot water collects less heat when passing through. Product specifications usually recommend the best per panel flow rate and you want to increase with each added panel so let us aim for a flow rate of 5 gpms. We have a 50' pipe run (100' of pipe), 20, 90 degree elbows, 3' of head loss at the collectors. The formula tells that 5 gpms can barely make it through 3/4" pipe at under 8 fps so we will need to use 1" pipe and calculate the losses to determine the size pump to use. The 1" pipe loses 2.2' of head in friction, .05' for each elbow = 1' of head and we need to overcome 5' at the panels. We need 1" pipe to deliver enough flow and we need a pump with a 8.2 head at 5 gpms. That is how to size the pipe, pump and set the flow rate.

• Minimizing friction loss

As can be seen in the example above, friction loss sets our operating parameters. By comparing 1/2" to 1" pipe we discover the benefit of using larger diameter pipe but the elbows and fittings are an equally substantial part of the equation and though larger pipe offers less resistance, larger fittings offer more resistance. Minimizing elbows is key. Long radius elbows have almost 1/2 the resistance of regular ones and 45 elbows are about 1/2 as much as 90s and tees create a lot of friction and should never be arranged to bullhead flow causing flow imbalances. Avoid bumps and sharp edges within the piping as this could again double the friction. It is important to ream the pipes. Use full port valves and fitting with smooth, amply sized ports.

Types of insulation

Elastomeric and fiberglass insulation suitable for solar. Shown with some jacketing options.

• UV protection

Good exterior insulation is expensive and important. Most elastomeric brands say "UV resistant" on them but everyone knows this is not the case. It will turn to brittle powder and shrink long before your warranty is up unless you put pipe coverings on it. It also should not be wet because you will lose precious heat. PVC or aluminum jacketing is cheap compared to the insulation and worth the effort in effort and energy saved

• Fiberglass

Inside where there is not excessive moisture, 1" wall fiberglass pipe insulation can be used. It has an adhesive seal that commonly becomes unstuck so it is good to put some extra insulation tape on the seams. A roll of special tape to go with the insulation will be available from your retailer. You can miter the elbows or get PVC elbow covers for cheap. A bread knife works great for cutting the insulation. It is often difficult to get 1" wall insulation and accessories from plumbing suppliers so it saves time and money to get it from an insulation distributor. Pipe insulation sizing is based on the OD of steel pipe so it doesn't correspond to copper pipe sizes. Be sure to specify you are applying it to copper pipe, when you order.

• Elastomeric

"Elastomeric", "rubber" or "closed cell foam" insulation all refer to the dense, flexible type often seen on refrigeration piping. Unlike fiberglass it will not degenerate if it becomes wet and it has a higher R value, therefor it is best for use outside. It is easy to cut with scissors. It can be bought with or without seams and can be mitered and taped or glued at corners. The tape is a roll of similar material with an adhesive backing. Use 1" wall insulation. Remember, you may need to go directly to the distributor and specify the size is for copper pipe.

• Avoiding heat loss from conduction

Have you ever had to handle metal when it is cold outside? It is not as bad if you are wearing gloves. The gloves with the fingers cut off are almost like not wearing any gloves if you are touching metal. For this very reason all pipe supports need to be on the outside of the insulation or losses will be substantial, not 100's but 1000's of Btus. Just get hangers that are as big as the outside of the insulation. Where the weight of the pipe is being supported a saddle made from a PVC pipe or piece of flu pipe can be used to keep the insulation from getting crushed in the hanger. Insulate everything.

• Aesthetics

The insulation clad pipes are a cause of aesthetic concern for most owners. Color choices are very limited so lending a smooth look to the available materials will be very important. Shop around at you insulation supplier to see what your options are and be prepared to spend time on applying the insulation. Building owners need to understand that pipe insulation makes up a sizable portion of the job.

Control strategy and monitoring

• Pumps

Size the pump according to pipe and collectors it supplies. The pump is the primary flow controller in the system. Preassembled pump stations need to be checked for compatibility with the size of the array and the piping. To match a pump to the system, first find the systems friction head as covered in the section on sizing pipe, then look up the pump curves for the brand you are interested in. Pumps are now available that have adjustable speed control making them more versatile for changes in pipe design and seasonal needs. Some smart pumps now have integral temperature sensors that will change the flow based on heat gain at the collectors and they can interface with the solar controller. Drain back systems need a second pump that is activated on start up to assist in lifting the water up into the collectors until gravity assisted circulation begins. Pool systems use the pool pump to circulate the water through the pool panels en route to the pool, the pump size should be verified to be sufficient and usually is.

• Valves

A Caleffi balancing valve, a ball valve and a spring check valve.

In addition to functioning as on/off and fill/drain devices, several different kinds of valves can be used to control the system. Zone valves are automatically controlled valves that can close or open flow to heating loops or heat dissipaters or other devices on the storage tank. Three way or four way diverters, like zone valves, can also redirect the rout of flow depending on heating needs. Balancing valves are often needed to serve the important function of providing equal flow to groups of collectors or tanks in larger arrays. Balancing valves can be set according to the desired gpm or also contain a means of measuring the flow. Check valves are a simple mechanical valve necessary to stop backflow and to aid in the prevention of thermal migration.

• The function of the controller

A basic solar controller would need only two temperature sensor inputs, one for the bottom of the tank and one for the collector temperature. It would also send 120 v power to the solar pump when the collectors were hotter than the tank bottom, to activate the solar loop. Then there is a need to limit the temperature of the tank, for safety, so we also need a tank top temp sensor. The tank top sensor can stop the power to the solar pump. If the tank top is not hot enough for use then we need call in the auxiliary heat source to supply the demand so a 24 v signal can also be activated when the tank top is too cold or a 24 v signal could be activated also if the tank top is too hot that activates a diverter to a heat dump. The solar controller usually has a transformer to provide up to 10 different 24 v signals for various control strategies, each one independently programmable to activate a valve or device within certain parameters. Most controllers have at least two 120 v power outputs that are programmed to activate pumps within the set parameters. More sophisticated controllers can vary the speed of the pumps or take flow and temperature data to calculate btu production or have an output to connect to a internet based monitoring system. Some controllers even contain wireless capability. The controller can be used to activate almost any function in a heating system. Drain back and pool systems have unique operating requirements and need a controller specific to them.

• Flow meters, pressure and temperature gauges

Mechanical pressure and temperature gauges are a common and important item that can be purchased from a plumbing supply store. Some thread directly into the pipe or specialized ones have immersion wells. Temperature sensors that connect to a controller operate on resistance, OHMS, which changes according to heat. Pressure sensors that connect to a monitor or controller come with specialized wells for the sensor and can alert the user to a leak. Flow meters are usually included in a pre assembled pump station and are important especially if there is no digital flow monitoring. Mechanical flow meters can be purchased best as a solar specialty item from meter makers. Flow meters that take digital readings are available and needed to monitor btu production. It is desirable to have temperature gauges on the supply and return of the solar piping as this is a visual indication of production and proper flow rates.

• Sensors

Temperature sensors are located wherever the controller needs the temperature data. A sensor goes at the collector outlet, where the hottest fluid is exiting on its' way to storage. If there is not a well for the sensor it can be clamped to the pipe with SS. Don't forget to run the temperature wire along with your pipe, on the outside of the insulation. 18 gauge wire can be used to run the signal. Most solar controllers accept 10K thermistor type that takes a reading of resistance in OHMs, the OHMs go down as the temperature goes up. Another type of sensor is an analog sensor or RTD and here the OHMs go up as the temperature goes up. These types of sensors are not compatible with the same controllers. Operation can be verified with an electrical multi meter.

• Relays

Relays open or close a 24 v signal after it has been stepped down from 120 v by a transformer. Each automatic valve or burner switch and some mechanical sensors need 24 volts and controllers will have relay out puts for optional functions.

• Monitoring production

Though electricity is commonly metered, metering heat production is only now coming to demand due to an increase in awareness about the cost of heat and need to conserve it. Monitoring your solar thermal functions can include programs to monitor BTU production if a digital flow meter is installed in the piping. If a pressure sensor is also installed it will alert to failure and could pay for itself in one call, by preventing permanent damage to the system. Claims based on peak performance often sell a system and it may help to verify the promised performance to maintain integrity. There is a great need for real production data to be made available so the amazing results can be seen and unnecessary disappointment can be avoided. The utilities need the data to calculate their load reduction based on solar and those savings, in turn, fund incentive programs. Among the many ways that the cost of monitoring will pay for itself is by ensuring a performance based relationship with the installer.

Motivation and financial benefits

How much energy does it really make?


When purchasing the system you will access the SRCC data on production for your chosen panel. The rating will be based on your climate and you can use a category C for peak performance and average the rest of the year in a category D. Doing this, for most types, every 4' of width will be making around 30,000 BTUs per day in category C. Three of these panels makes nearly 100,000 BTUs each day for the warmest 6 months and about half of that for the winter months, a low estimate of 274,500,000 BTU each year which is 7965 KWH, equivalent to the peak production of 24-20 sq. ft, 300 Watt PV panels. Therms cost at least $1.20 each. 8,235 KWH is the equivalent to the production of 48, 8 sq. ft., 75 watt PV panels at 1/4 the required roof area.

Environmental reasons

• Emissions

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In only 100 short years we have purged almost all the CO2 buried underground for millions of years into the atmosphere and created a critical climate change situation that is nearing the point of being irreversible. State and City plans are working hard to avert the impending disaster and your contribution is required. Nationally, 29% of emissions from fuel are from heating buildings and NY accounts for 4% of the countries' energy consumption. About 35% of emissions are created by generating electricity with fuel and the rest is from transportation. Since 2009 emissions have been decreasing but are still far from safe levels and we need to continue and increase the trend by investing in the sustainable energy and energy use reduction. If carbon emissions do not reach low levels by 2030 the change will be irreversible and by the end of the century we will see the climactic effects on our food supply and weather patterns.


There are more solar thermal systems installed, world wide, than solar electric systems. Solar thermal panels collect 4 times as much of the suns' energy/ sq. ft. than do electric panels. In the example above see how 3 thermal collectors convert to the equivalent production of electricity. 274 therms = 8,235 KWH. Three 32 sq. ft. thermal panels = 48, 8 sq. ft., 75 watt PV panels at ¼ the required roof area. In the US, low public awareness and the low cost of subsidized fuels have, so far, kept the power of solar thermal a secret. Even if created sustainably, it does not make sense to waste electricity on heating when the resource is available. The only cost effective way to make heat sustainably is with solar thermal panels. The only reason we can waste fuel on making heat is because of the artificially low cost of heating fuel in the US and this is bound not endure.

• Making energy sustainably

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Because of the population density and proximity of daily needs, living in the city is very energy efficient. It will take a combination of all the technologies and lifestyle adjustments to reach a sustainable level of consumption. Solar thermal offers the most promising sq. ft. per energy production use of roof space in Urban environments since usually, on up to a 5 story building, 20% of a roofs area will provide 75% of the hot water needs. As building envelope and boiler technology continue to drastically improve, medium sized, newly constructed buildings can meet 75% of the total heating load with 30% of the roof space.


Since the mayor implemented plan 2030, the Greater Greener Building Plan, emissions have been dropping steadily and are now 13% lower than in 2005. The elimination of #6 heating oil had a huge impact on emissions and public health. Several new local laws are making it easier to use more roof space for solar. Limitations on energy consumption are now being written into the local codes and these reductions will allow a better portion of energy to be generated on site. All hands are on deck to fund and ease the installation process.

Financial reasons

• Reducing bills

Some quick calculations regarding fuel heat: Nat gas appx. $1.20/100,000 btu, Propane appx. $3.00/92,000 btu, Oil appx. $3.50/140,000 btu. A 3 panel system makes about 27,375,000 btu/year and costs in the range of $10,000.

If you have a heating system older than 10 years, an expertly done system overhaul could reduce the bill by as much as 10% and cost in the range of $4000.

If your hot water heating is a gas unit older than 10 years, you should change it anyway for about a 5% energy savings at a cost of around $1,600. If you got a solar upgrade this would be included.

If you have an oil/steam system that makes hot water also you must take action. Don't use the steam boiler to make DHW in the summer the options are many. Ideally you would convert to a high efficiency natural gas heater and convert your radiators run on hot water. This alone could cost $12,000 and save up to 50%. Add solar to any upgrade option and save an additional 65%.

Tax benefits

Installing solar water heat entitles the residential home owner to an automatic 30% federal personal income tax reduction which carries over successively until the full amount is credited. State income tax incentives can be obtained as well for water heat or space heat, up to $5000.

Other incentives

Commercial customers can find incentives through utility companies and federal, state and municipal programs. Utility programs and NYSERDA programs as well as local weatherization programs have cash rebates or tax credits for equipment upgrades, pipe insulation and oil to gas conversions for residential customers. All incentives can be researched starting with

Property value

Buildings with new heating systems and a low utility consumption report are worth more than the cost of installing the system. Soon, as more people continue to install solar, tax assessors will automatically increase the value of the building and there is already a law in place that prohibits the tax bracket from increasing because of solar.

Calculate your payback

• Pools

If fuel operating costs are $1500/year, an $8000 installed solar pool heater would cost $0 to operate under the same parameters.

• Air heaters

An 8'x4' solar air panel produces 6,000 btus /hour when sunny and costs $3500 installed. If offsetting natural gas for 100 days/year saves more than $70/year and gets a 25% tax credit.

• Domestic hot water

When considering the cost of a solar heating installation, subtract from the equation the cost of installing a conventional appliance instead. Solar thermal systems are very durable and will operate for 30+ years at an extremely low operating cost compared with only slightly lower up-front costs to install other conventional appliances.

If exactly 100% of the hot water demand is made during the summer, the savings can be calculated according to the fuel bill. Production in the winter can be expected to be 50% of this. Each resident uses between 15-20 gallons of water each day, 10,000 btu. Each person, $0.12 day x 365=$43.50. Minimum possible system size is 30,000 btus/ day(20,000 annual average) or $100/ year. Costs $6000 installed- 55% incentives=$2700/$100. The bigger the system and demand the faster the payback, for example, a 3 panel system costing $12,000 installed saves at least $300/year if consumed.-55%= $5400/$300. 6 panels, $15,000-55%=$6750/$600, etc…It is important to consider a fraction of the cost that offsets the purchase of non-renewable equipment, for example, a hot water heater sized equivalent to a 3 panel system would probably cost $3000 installed, where as it is included with the solar water heating system.

• Space heating and water

In order to produce 100% of winter, water space heating needs, the array will need to be about 3 times the size of one for DHW only. Space heating not only requires higher temperatures but operates during non-prime months. The savings are, however, proportionate to the added production and the price per panel of installation decreases a lot with each additional panel. A back-up heat source is necessary for cold days and overheating must be avoided in the summer. Even single family homes commonly spend $600 or more in space heating needs alone with natural gas plus another $200 (4 people), at least, for hot water, so if the space heat system installed costs $22,000-55%=$9900/$800.

• Whole building efficiency

Energy retrofit project

The point of it all is to stop wasting heat so that it can be made renewably. A well-insulated building can be heated with as little as 2000 btu/ 1000 sq. ft. At this rate heating systems could be run entirely on low temperature water from solar at installation costs equivalent to those of conventional appliances. Heat loss and cooling loads account for 75% of building energy consumption. Paint the roof white and/or use water cooling. Minimize electric consumption and someday soon all of the electric load can be generated renewably, on site.





June 14, 2012