Solar hot water + tankless
Does anybody have such a system? I am considering piecing something together soon. The idea is to keep a tank as hot as possible (up to 170* or so) at all times using solar energy, and feed the (electric) tankless from it. The idea being that during the summer I get free hot water almost all the time (because the tankless would not kick on), and cheap at other times (because the tankless would not have to provide as mush temp. rise).
Your idea works!!
Essentially, this project involves using the tank from a hot water heater to construct a "batch solar water heater", an insulated box in which the hot water tank is used as a reservoir and heat sink. The box is constructed of Plywood and there is a double-glazed window that lets in the sun, thereby inviting the solar gain. The article mentions plumbing in a crossover pipe and two valves (essentially an isolation loop) to bypass the batch heater when the weather is too cold--this allows you to drain the tank during the winter, too :yes: .
The piping instructions/diagram for the isolation loop, as well as tanks purpose-built for batch solar hot water heaters (just in case you don't have an old hot water heater tank lying around), can be found here:
Cost estimate given in the article is $200, but I suspect that is using the puprose-built tanks. It would cost considerably less if you could find an intact water heater tank to salvage--look for one from an electric water heater, the gas-fired types have a flue pipe that runs down the center of the tank and that results in the gas-fired type having a physically larger tank for the same capacity.
This sounds like what you want, the plumbing from the solar batch water heater's tank is inserted into the cold water supply line to the water heater (using that isolation circuit I mentioned). When somebody cracks open a faucet somewhere, the water drawn into the heater will come from the batch solar water heater's tank, and you're correct, I quote from page 29 of their article: "On a hot, sunny summer afternoon, the solar heater can warm water enough that the water heater stays off all day"! Keep in mind that they were plumbing it to supply preheated water to a standard electric tank-type hot water heater--if the solar heater keeps that much water hot enough to preheat the tank enough to keep it from turning on all day, I'm sure it will be hot enough that your HWOD unit will not need to turn on, either!
Good luck, you might be able to get a copy of the April, 2008 "Kansas Country Living" magazine directly from the Pioneer Electric Cooperative at:
If that fails, send me a PM with your name and address and I'll send an XC of the two page article via snail mail (that offer is open to anyone who might be interested). This looks like a very simple DIY project, even the cabinet for the water tank is simple!
Here's the link:
Looks quite easy to construct.....might be a bit expensive.
Lots of good info herehttp://www.homepower.com/home/
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