Build Your own Passive Solar Water Heater
(Page 4 of 7)
October/November 2007
By David A. Bainbridge
If you prefer new tanks, you can order glass-lined electrical water heater cores — minus the heating element, outer insulation and sheet metal cover. Stainless steel tanks also may be available in some areas. (See “Solar Sources” below.)
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3. Ensure that your system will retain heat. There are a number of options for glazing the top and the south-facing wall of your unit, including single- and double-paned glass or fiberglass and plastics designed for extended solar exposure. In most cases, you should use two layers of your chosen glazing material with an air space between to provide maximum heat retention. Glass is generally a suitable glazing for owner-built heaters, unless you get severe hail. Twin-wall polycarbonate is a tough alternative. Be sure to flash the glazing carefully to avoid leaks and to caulk and seal the panes to avoid condensation, which can limit energy capture.
4. Size your heater appropriately. To determine the size you need, allow 30 gallons of hot water per person in your household (a conservation-minded family might get by with only 10 gallons per person). Depending on your environment and glazing, you’ll want to plan for 1 to 2.5 gallons of water per square foot of glazing as a general ratio for good heating. A smaller water-to-glazing ratio speeds up heat gain considerably but can increase the freezing risk. If you can’t meet the ideal, don’t lose heart. A smaller system will still provide economical solar water pre-warming and conserve nonrenewable energy and cash. Batch systems with relatively small tanks and simple enclosures are common, and many are still doing well after 30 years, long after most of the more complex systems have disappeared.
5. Make an efficient, freeze-resistant connection to the backup system. Minimize pipe runs and insulate the pipe carefully using foam or fiberglass insulation with aluminum jacketing. Build it to last — if you just use foam it will break down within a few years. It can take up to 72 hours at 12 degrees to freeze an exposed water heater tank, but pipes are much more vulnerable. In a brief freeze, you can leave the hot water on slightly to keep the pipes from freezing. In very cold winter climates, drain the collector tank and pipes in the fall.
Set the system up so you can turn off the backup heater and run solar hot water directly to users, and so you can bypass the solar water system if you want to drain it and shut it down in winter. Make sure the connections are building code approved. After finishing the plumbing system, bleed the air out of the tank through the screw plug at the top of the tank until the system is full of water.
6. Build your system to last. Use the best materials you can afford or scrounge, and take proper care in the construction of your unit. It should work for 20 to 30 years, so it’s worth doing it right. Be safe: Make sure the tank supports are strong enough to bear the load they’ll carry, and get some help with moving tanks safely. If you build a high performance heater, consider adding a tempering valve near the backup heater so no one gets scalded in a shower.
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