44 By Michael Hackleman (Rob Harlan is a general and solar contractor with 25 years of experience with solar water heating systems in Mendocino County, California. Rob primarily designs and installs photovoltaic systems today.) Mh: Rob, will you give a brief his- tory of the last 30 years of solar- water heating system design and implementation? Rob: Solar-water heating systems got a real boost in the 1970s when tax credits were offered by state and fed- eral programs to help folks make the investment. These systems were intended primarily for domestic hot water, i.e., showers, dishwashing, cooking, and clotheswashing. They were also popular for heating the water in pools and hot tubs. This movement slowed to a snail’s pace when the tax credits ran out. Mh: As I recall, a lot of manufac- turers also disappeared when the tax credits went away. Of course, some of these systems were poorly designed, used cheap components, or lacked adequate protection against freezing, overheating, or corrosion. I know that you’ve upgraded solar water heating systems over the years, or older systems from homes and businesses in favor of newer designs. What’s your experience of the design and hardware from 30 years ago? Rob: Some designs were indeed flawed—poorly implemented, overly complex, or incorporating untested ideas. Still, even good designs require some maintenance. The lack of knowledgeable service personnel and parts crippled some systems. The solar collectors from these systems are actually pretty rugged and often find their way back into new installa- tions sold “as is” or used. Today’s manufacturers of solar water heating systems and components have bene- fited from the lessons learned long ago. Things are back to a steady pace, 7 Solar Water Heating System Designs for domestic hot water, radiant floor heating, hot tubs, and pools — in any climate An interview with Rob Harlan of Mendocino Solar Services NERGY ORKS E W Drawing: Norm Ehrlich, Six Rivers Solar Fig. 1: Solar water-heating systems will handle multiple heating sources and various applications. with a variety of manufactured sys- tem types. Most offer good reliability, are warranted, and generally follow time-tested designs. Mh: There are a few parts that are basic to most solar water heating sys- tems (Fig. 2): collector(s), storage tank, heat transfer medium, and inter- connecting plumbing. The collector intercepts the sun’s rays and converts it into heat which is transferred to the storage tank using a fluid such as water or antifreeze. An expansion tank is used in closed systems to accommodate the slight changes in volume that result when water or antifreeze is heated and expands. If glycol (a non-toxic antifreeze liquid) is used, a heat exchanger is needed to transfer the heat from the collector to the water that will exit the faucet. A T&P (temperature and pressure) relief valve is a common safety device found at the top of water heaters. If the water gets hotter than it should or the system builds up too much pres- sure, this valve will open, releasing water until the temperature or pres- sure drops to safer levels. The sim- plest control system disables the backup heating system (gas or elec- tricity) during daylight hours, giving the sun a chance to heat all of the water in the storage tank. Rob: And—on active systems, a controller turns a pump on and off as solar heat is available. Let’s define a few terms used to describe these sys- tems—active vs passive, open vs closed. An active system is one that uses pumps to move the heat about. A passive system is one that contains no pumps, relying instead on natural convection, conduction, or radiation to move heat. An open system means the water circulating through the col- lector is the same water you’ll use in a shower (Fig. 3). A closed system circulates the separate heated fluid from the collector through a small loop that includes a heat exchanger, usually located in the storage tank (Fig. 4). Mh: I understand why some people choose passive over active designs. Pumps, controls, relays, and motor- ized valves all require electricity. Electricity is a very specialized and sophisticated form of energy. Folks who live in the country beyond the grid know what a luxury electricity is. We know it’s a luxury because it’s expensive to make. And very expen- sive to make a lot of it. It’s a shock for folks who have lived most of their life with utility power to move beyond the grid. A pas- sive solar heating design for making domestic hot water or warming a home requires little or no electricity to operate. Fewer parts, less to go wrong, less to take bites out of your pocketbook. With passive, it’s all in the design. Considered experimental in the 1970s, passive solar heating has proven itself worldwide in a wide range of climates. Speaking of cli- mates, why would someone choose a closed system over an open one? Rob: Freezing protection. If the water in the collector freezes, it will burst a tube or header. It’s messy, it dumps your hot water, and it must be repaired. You don’t have to live in a place with hard freezes. Water in a collector open to a clear sky can actu- ally freeze when the ambient air tem- perature is as high as 40 degrees F. September/October 2000 Backwoods Home Magazine 45 Fig. 2: (above) Block diagram of a solar water- heating system Fig. 3: (right, above) An open system Fig. 4: (right) A closed system 1. Integral collector/storage 2. Thermosiphon 3. Three-season 4. Drain-back 5. Drain-down 6. Re-circulation 7. Active closed-loop Seven types of solar water-heating systems Drawings by Michael Hackleman unless otherwise noted. STORAGE TANK WATER HEATED BY COLLECTOR IS THE SAME AS THE WATER USED IN HOUSEHOLD WATER OR GLYCOL COLD WATER IN This condition is called night sky radiation. Mh: Incidently, there are two rea- sons why water that freezes will burst its plastic, metal, glass, or stone con- tainer. Actually, they are simply properties of water. One, water is virtually incompress- ible. Two, water expands slightly as it changes from a liquid to a solid. Water immo- bile inside a small tube or pipe and exposed to a freeze, then, will begin to expand as it becomes ice. Unable to compress itself, it makes a bigger volume by breaking whatever contains it. Rob: True. It’s actually the differ- ent strategies used to combat the potential of freezing that define the major types of systems and their rela- tive complexity. I’ve categorized existing systems into seven types: integral collector/storage, ther- mosiphon, three-season, drain-back, drain-down, re-circulation, and active closed-loop. Mh: Will you describe them all, first generally and then assess their merits and liabilities from your own experience? Rob: I would be glad to. I must say first that my experience with solar hot water is limited to my service area (coastal northern California) which is a fairly benign climate with occasion- al light freezes. I ask your readers to keep this in mind as I speak of vari- ous systems. 1. The integral collector/storage is the simplest and historically oldest type of solar water heating system. Paint a tank black, put it in a big crate, insulate it on all sides except the one covered by glass or plastic, and point it at the sun. Water in the tank is heated directly by the sun and stored in the same unit. In the trade, this is also know as a breadbox-type system. An example of a manufac- tured unit of this type is the Servamatic™. Produced in the 1970s, many are still operational today. The same principle can be seen in today’s ProgressiveTube™ unit (Fig. 5). These are also in-line units, posi- tioned between the well and the shower. You get as much hot water as they collect and store. Mh: This is a popular design in homebuilt units, too. Simple, cheap, and often made with recycled materi- als. I once took a shower at a ranch I was visiting from water heated in a long thin 20-gallon tank inside an old, big refrigerator with a transpar- ent cover pointed south. I had a long, hot shower in the cold night air. Good experience. Rob: I have very rarely had to serv- ice an integral collector/storage type system, which is a testament of their durability. 2. The thermosiphon system is another solar water heating method (Fig. 6). Sunlight strikes tubes and fins inside a collector box through which water or glycol is circulating. The inlet and outlet of the collector are plumbed, respectively, to the inlet and outlet of the storage tank. If we were talking about electricity and polarity, we’d say the collector is in parallel with the storage tank. Still, it forms a loop. The heated fluid moves from the collector to the storage tank and back to the collector through a process called thermosiphon. This is a natural convective action. If you plumbed this as an open system, the storage tank could be your own water heater. Mh: I’d like to elaborate on a few things you’ve said. Thermosiphon results when water heated in the col- lector expands and rises, pushing cooler water in the rest of the loop September/October 2000 Backwoods Home Magazine 46 Fig. 5: (above) An integral collector/storage unit Fig. 6: (above) Position the stor- age tank above the collector to prevent nighttime reverse flow. Fig. 7: (left) A check valve pre- vents reverse flow when tank is even with collector. Drawing: Thermal Conversion Technology GLAZING GASKETS CASE ABSORBER/ STORAGE TANK FLUID CONNECTION INSULATION GLAZING September/October 2000 Backwoods Home Magazine 47 into flowing. Cooler water is pushed out of the bottom of the tank and into the bottom of the collector. Once cir- culation starts, the process continues unabated all day. Just as the sun heats the water in the collector, the night sky can cool a collector, causing reverse flow. Think about it. Water in the collector is cooled by nighttime stag- nation. Cold water is heavier and sinks, push- ing the entire loop into reverse flow, moving warmer water from the tank to the collec- tor which is, in turn, cooled. This will quickly give away some of that hard- earned hot water. The easiest way to avoid this is by positioning the bottom of the tank above the top of the collector (Fig. 6). This is a physics trick that will pre- vent reverse flow. Sometimes it’s not possible to elevate the tank above your collector. Thermosiphon will work even if the tank is positioned level with or even somewhat below the collector. In this case, the addition of a check valve will prevent reverse flow (Fig. 7). Avoid the standard pressure-type check valve. It’s too resistive to thermosiphon flow. Instead, use a gravity-type check valve. Angle it in with the plumbing for minimal pressure to open, mini- mum backflow to close. The solar collector itself is some- thing of a mystery to many folks and I get many questions about it. A com- mon configuration uses a box, a grid of water tubes, insulation, and glass or plastic glazing (Fig. 8). The box is a large shallow pan, with designs varying smaller and larger in width and length than a standard sheet of 4x8-foot plywood and 4-6 inches in depth. Manufactured designs use stainless steel or aluminum for the boxes but most homebuilt units use plywood. If properly glued and screwed and sealed against weather, they are tough. Homebuilt designs start with a 4x8- foot sheet of plywood ½ or ¾-inch thick. From it (or another sheet of plywood) cut two 4-6 inch strips from each dimension, supplying the mate- rial for the box’s four sides. Large diameter (1½-inch to 2-inch) copper header tubes at the top and bottom of the collector are oriented horizontally and plumbed together with smaller vertical tubes (i.e., ½-inch tubing) spaced 3-6 inches apart. Tin or cop- per fins or sheet is mechanically and thermally connected in a variety of methods to the tubes. Tubes and fins are blackened with paint or through electrochemical processes. Fittings are added for connection to external plumbing or other collectors. Sheet foam insulation is added behind and to all sides of this assembly when it is mounted in the box. Glass, greenhouse fiberglass, or some other translucent plastic glazing is added to complete the unit. Glass is available in a range of sizes, particu- larly if it’s recycled. UV (ultraviolet)- resistant fiberglass is available at local hardware stores in several widths. Don’t burden yourself with plastics that will crystallize in one or two seasons from exposure to the ultraviolet rays of the sun. Select your glazing first. The best economy results when the box is sized to the glass you already have or can get. Rob: I am reluctant to endorse building one’s own collectors, given the availability of used collectors. If you do build your own, don’t use aluminum absorber plates. They will react adverse- ly with copper tubes. Also, it is best to silver solder any joints within the collector. The col- lector goes through large temperature swings. This is hard on standard solder joints. Mh: Indeed, the experience of building one’s own collector usually brings about an appreciation for how inexpensive used collectors really are. So, my recommendation to the enthusiastic do-it-yourselfer is: don’t commit to building a whole bunch of collectors without first building one. Rob: A few more comments on thermosiphoning. If you ther- mosiphon with water and live in a cli- mate with freezing temperatures, your collector will freeze and burst. Sometimes passive freeze protection valves are installed in such systems. Often called Dole valves, these are designed to open at a preset tempera- ture, 34°F or 45°F. They drip water to create a flow through the collector and, in this way, prevent freezing. In my experience, these valves are not reliable, so I cannot recommend them. Mh: I haven’t used Dole valves personally but I know that some peo- ple in the area, including Stephen Heckeroth, do trust and use them. However, it is also my understanding that Dole valves must be periodically inspected and cleaned. If you’re the type of person who isn’t good at reg- ular maintenance, you’d be better off selecting a different system. Rob: If you live in a climate zone without freezing temperatures, an open thermosiphon system will work NERGY ORKS E W Fig. 8: (above) A traditional solar flat-plate collector Drawing: Florida Solar Energy Center well. If not, I still recommend using glycol and a heat exchanger for the thermosiphon loop. 3. The three-season system is another tactic for handling freezing. The idea is to use the solar water heating system for three seasons and drain it for the fourth. It can be a thermosiphon or pumped system and assumes the owner will use another source of energy for heating the water. 4. Drain-back is another type of solar water heating system (Fig. 1). This drains the water in the panels into a tank when there’s no heat avail- able from the sun. The panels are empty of water, then, and cannot freeze. A non-pressurized tank is used to capture this water, and a pump refills the panels when the sun’s warmth is detected. 5. Drain-down is a variation of the drain-back solar water heating sys- tem. Here the water is dumped onto the ground. This is a fairly common design, particularly in older systems. It uses a Sunspool™ valve to fill the panels for operation. The same valve, when it reaches a lower temperature, opens to dump the water that’s in the panels onto the ground. 6. Another type of solar water heat- ing system is re-circulation. This method of freeze protec- tion activates a pump to circulate a little bit of hot water from the storage tank back into panels when low ambient temper- atures are experienced. 7. Active closed-loop is the final type of solar water heating system on my list (Fig. 9). This design uses any fluid in the collector-to-storage loop that won’t freeze at the low temperatures the system is likely to experi- ence. The heat gathered in the collector is transferred to the water in the storage tank via a heat exchanger. What fluids won’t freeze? I’ve seen systems use glycol, silicon oil, and methanol. Automotive anti-freeze might seem a good candidate, but it’s poisonous. The most popular heat transfer medium is polypropolene glycol, a food-grade dough extender used in the baking industry. It costs about $20 a gallon and is mixed with water. A 10% mixture will protect the collectors down to 20- 25°F. The ratio of glycol to water is increased for lower temperatures. I use a 50/50 mixture in my service area. There’s a lot to be said for using pure water in a solar water heating sys- tem. Water is non-toxic, widely available, and cheap. Also, it is the most efficient heat transfer fluid and does not degrade in use. Glycol is also non- toxic but it does break down over time. Exposed to high temperatures, it becomes acidic and will eventually begin to eat your plumbing. So, glycol needs to be checked periodically. I use litmus paper to check its pH. It’s a fairly simple matter to refresh the system with a new glycol-water mix. Incidentally, there are some types of systems that don’t really fit into any of these seven categories. The popular Copper- Cricket™ is one example. This system used a 20% methanol mixture under a vacuum to actually “pump” heated fluid down to a stor- age tank without a pump. It operates on the same princi- ple demonstrated in a coffee percolator to transfer heat. Another is the Sun™-family of solar thermal collectors. These use columns of evacu- ated tubes to collect and transfer heat. There’s more basic stuff, too. Some folks just spiral plastic pipe on the ground to pre-heat the water that goes into their standard water September/October 2000 Backwoods Home Magazine 48 Fig. 9: Components of an active closed-loop system. Drawing: Florida Solar Energy Center There’s a lot to be said for using pure water in a solar water heating system. Water is non-toxic, widely available, and cheap. Also, it is the most efficient heat transfer fluid and does not degrade in use. September/October 2000 Backwoods Home Magazine 49 heater. It works but if a sudden freeze doesn’t ruin it, long term exposure of the plastic pipe to sunlight will. Mh: The softer, more flexi- ble black plastic tubing you’re referring to is identified as PE, or polyethylene tubing. Ultraviolet radiation from the sun breaks down any kind of plastic, disintegrating the bonds of the polymers and turning the plastic brittle. The black tubing sold in rolls is neither designed to work in direct sunlight nor withstand elevated temperatures. Hot water, particularly with soft water, will leach stabilizers and joint cement from the tubing, too. This is great for showers but you don’t want to drink this water or cook with it. Rob: If there’s one thing I’ve observed, it’s that most folks who build their own system try to re- invent the wheel, and their designs sometimes reflect a lack of under- standing of the basic principles. With good plans, most people could build a good system. Still, many folks don’t want to do it themselves. Mh: I prefer doing my own system yet I have to admit that I have often overrated my ability to be there when the system really needed me. Rob, will you go back through the list of systems and give us your thoughts on the advantages and disadvantages of each type? Rob: The integral collector/stor- age system has the advantages of low cost, simplicity, and the lack of pumps or controls. Even homebuilt versions are long-lasting. The tank has enough thermal mass to avoid freezing except in hard-freeze areas. The disadvantages? This design is relatively inefficient and the water often doesn’t reach a very high tem- perature because the glass-to-mass ratio is small in a breadbox-type sys- tem. Heat losses from the collector are high at night, so there is definitely a time of optimal use of the hot water produced, usually afternoons and evenings. The collector/tank combi- nation is heavy, too. Filled, it may reach 650 pounds and tax an unrein- forced roof. The newer ProgressiveTube™ col- lectors of this type (Fig. 5) are simple and use 4-inch copper tubes and fins with special “selective” surfaces. They extract more of the sun’s energy than blackened surfaces and resist re- radiation of this energy at night. I recommend ProgressiveTube™ sys- tems for my climate zone. The thermosiphon system has the advantages of simplicity and good efficiency. It doesn’t require electrici- ty and is therefore unaffected by a utility blackout. One disadvantage of thermosiphon flow is that the plumb- ing must follow strict guidelines— bigger tubing, gentle turns, no low spots, and no restrictive valves—to ensure a smooth, unrestricted flow. An air pocket at a high spot or a large bubble somewhere in the system will stop thermosiphon flow. Mh: I’d like to add to your com- ments on thermosiphon. I’ve found this to be a neat, natural way to move heat from a collector to storage or use. Water pumping in rural locations can eat a big portion of anybody’s energy pie. Any process that will pump water and the heat it contains through a pipe without external power is a blessing. But—ther- mosiphon will not tolerate poor plan- ning or a sloppy installation. It wants free, unrestricted motion. Even the check valve must be a gravity-type rather than a pressure-type to avoid becoming restrictive. Tests have shown that thermosiphon doesn’t start until the collector reach- es a critical temperature (Fig.10). Flow commences rapidly, slowing to a more constant rate. A bubble big enough to block a tube will stop flow immediately. The collectors can get hot enough to blow a T&P valve and still no flow. It’s exciting to see water and steam shooting up into the air but, alas, not very productive. Steeply-pitched pipes will ensure a good flow. I know that in-line, centrifugal-type pumps are used in radiant floor sys- tems to periodically purge the ther- mosiphon loops of air bubbles. Theoretically, thermosiphon can push water through the pump when it’s off. The pump has another use. It enables the owner to pump more heat into the floor from storage at night. I added a small purge-pump to one thermosiphon system in the 1970s. I wanted to use primarily ther- mosiphon but the system included existing plumbing—naturally inac- cessible—and the thermosiphon flow kept getting blocked with bubbles. I added a small 12-volt pump in paral- lel with the check valve (Fig. 11) to occasionally purge the system with a NERGY ORKS E W Fig. 10: (Left) The temperature needed to start thermosiphon flow is greater than the one that will sustain it. Fig. 11: (below) A small DC pump can be used to purge the system’s lines of air bubbles. Temperature Time 10am noon 2pm thermosiphon starts thermosiphon sustained max temperature reached faster flow rate. I used a positive-dis- placement type to avoid any flow of fluid through the pump when it was off. Rob: I’ll go on. The three-season system has the advantages of using the existing water heater as a backup, being inexpensive, and requiring only a small pump. The disadvantages are that it is susceptible to freezing and depends on the owner being there to drain it when the weather is cold. There is an overall limit to the size of this system when it’s plumbed to a water heater of a specific capacity. The drain-back system (Fig. 1) is relatively simple, versatile, and freeze-proof. The tank used in this type of system is long-lasting and there is lit- tle maintenance required. During a blackout (or other loss of electricity to the system), the panels are empty and will not over- heat. It’s even possible to set up the system so that thermosiphon will get the heat to your water heater. The disadvantages are most evident in off-grid systems, where the energy used in pumping is relatively high. This is because the pump must be sized to fill the collec- tors daily rather than just circulate water through them. As well, the tank must be located below the panels so that the water that is drained back will have a place to go. This is my favorite choice of a system for freez- ing climates. The drain-down system has the merits of high efficiency and is a freeze-proof system. It uses a small pump with small energy use. The dis- advantages? Lots of expensive parts, including a complex controller, and the need for periodic inspection and maintenance. However, in any appli- cation with a limited supply of water, the daily dumping of water from the collectors onto the ground will be an issue. The re-circulation system has the advantage of using a standard hot water heater to double as the storage tank. And it’s freeze-proof if the sys- tem is small. It has the disadvantage of wasting a lot of energy. If it’s real- ly cold, the backup heating system, say an electric element, has to heat water that is simply being radiated away from the collector at a signifi- cant rate. The active closed-loop system (Fig. 9) is freeze-proof and contains quality components. One disadvan- tage is that it is complex, meaning it has pumps, valves, and various con- trols. The tank with heat exchanger is expensive but adds a lot of useful, well-insulated ther- mal mass to the system. If utility-powered, the pump won’t work during a black- out. Mh: There’s merit to the idea that if the system depends on electricity, the electricity should be gener- ated from the sun, too. If there’s sun for the collec- tors, there’s sunlight to make electricity to power the pump and move the heat. In all of these systems, if the collec- tors overheat, a T&P relief valve will provide protection. There’s a down side with the T&P valve blowing. First, it gives away a lot of hot water since the valve won’t close until both the temperature and pressure fall. And, second, dumping the heat trans- fer medium can be expensive—if it’s a glycol/water mixture. I want to thank you, Rob, for turn- ing me onto the fact that a P-type (pressure-only) relief valve is manu- factured. I want to use one of these in my next installation. I suspect it will keep the system from dumping all the hot water since it should close as quickly as the pressure is relieved. The pipes in the collector can take heat, but have a tougher time surviv- ing pressure. Rob: I guess my critique of the advantages and disadvantages of these systems reveals my bias. Generally, I have found with solar hot water, the simpler the better. The sim- ple systems seem to last longer, as a rule. Mh: Bias? I appreciate your review and advice. I’ve learned a lot. Will you describe how you size a system to the application and match compo- nents with each other? Rob: Almost every hot water sys- tem has a backup. I design for 70% solar usage. A four-person family is a good standard. Two 4x8-foot collec- tors will supply the hot water needs of four people. The tank should be sized to the array. In my climate, I’ve found that 1.8 gallons of fluid per square foot of collector is a good ratio. So, two collectors of 32 square feet each will require a storage tank of 115-gallon capacity. For radiant floors, I’ve found that the collector area should be about 10% of the floor area. The same two 4x8-foot collec- tors, then, will handle about 650 square feet of radiant floor. Mh: What’s the average cost of water heating with electricity, propane, and natural gas for a 4-per- son family? Rob: Yes. Using electricity at 12¢ per kWh, the cost of water heating is about $46 month or $551 annually. Propane at $1.41 per gallon costs about $26 a month or $307 per year. Natural gas and fuel oil are less, as is electricity in other parts of the coun- try. Of course, when a solar water heating system is installed and has returned the investment, the energy from it thereafter is free. Mh: Will you give me an idea of how long it will take to pay off the cost of several of these systems based on these rates? Rob: I have that information, too. First, let me say that these figures do not include the cost of maintenance, the rise in the cost of utility electrici- ty, the lost interest on the investment, September/October 2000 Backwoods Home Magazine 50 Two 4-ft by 8-ft collectors will supply the hot water needs of four people ... (or) will handle about 650 sq.ft. of radiant floor. September/October 2000 Backwoods Home Magazine 51 and no tax on the savings. In my experience, these balance each other out. A new integral collector/storage system using the ProgressiveTube™ design will cost about $2,500 parts and labor to install. After 7.3 years, the system cost will equal the cost of electricity to heat the same water dur- ing that time. With propane, it’s about 13 years. If the owner installs the sys- tem, the cost is about $1,600. The payback is 4.8 years for the avoided cost of using electricity and 8.7 years if using propane. A new drain-back system costs $3,500 parts and labor. This is equal to 8.5 years of electricity and 15.2 years for propane for domestic hot water. A system that will heat a hot tub will cost about $4,800. When heated electrically, the payback com- putes to 7.5 years. Mh: In my experience, folks who install their own solar water heating systems usually begin by putting one collector in a loop to the existing water heater. If you shower in the morning, what’s the conventional method for preventing the water heater from using electricity or propane to reheat this water before the sun gets a chance at the task? Rob: In an electric heater, it’s easy. A 24-hour timer can be set to lock out the backup heating during day- light hours. The owner can manually override the timer with the flip of a switch during bad weather or unusu- ally high demand. For a propane or natural gas heater, turn the gas valve to the pilot position. Mh: There is a proper way to plumb the solar collector to the stan- dard water heater, too. Today’s water heaters position the cold-water inlet and hot-water outlet at the top of the tank. Cold incoming water to the tank actually drops through a tube inside the water heater which ends just above the bottom of the tank. For thermosiphon flow, this is not a good arrangement; you want the cold water return to the collector to exit directly from the bottom of the tank (Fig. 12). Fortunately, water heaters have a drain valve. There is a way to re- arrange this plumbing (Fig. 13) so that the collector will use this orifice for its thermosiphon loop while you retain the ability to drain the tank. If someone wanted to assemble their own solar water heating system, what’s a good source of information and parts, beyond the library and internet? Rob: A wonderfully detailed overview of solar hot water systems, complete with schematics and techni- cal information, is found in the Solar Water and Pool Heating Design and Installation Manual from the Florida Solar Energy Center at (407) 783- 6300. Triple A Solar in Albuquerque, NM (800-245-0311) sells used solar- thermal collectors at good rates. Check out local sources of used pan- els to avoid shipping costs. Six Rivers Solar (816 Broadway, Eureka, CA 95501) at (707) 443-5652 sells a high-quality, rectangular thermal storage tank that integrates the inputs and outputs of collectors, auxiliary heating sources, DHW, radiant floors, and hot tubs (Fig. 1). ∆ Rob Harlan, Mendocino Solar Services, 42451 Road 409, Mendocino, CA 95460 Michael Hackleman, PO Box 327, Willits, CA 95490. E-mail: