I am looking to invest some money into a solar hot water heating system. We are currently heating our hot water with in-line hot water from an oil furnace. It is nice not having to worry about a leaking tank, but there is a price: temperature fluctuations in the shower. Hot, then cool, furnace kicks in, cold, and then the water finally warms by the end of your shower. A water tank in front would certainly buffer this, but an oil-heated tank runs about $1700. I can
ugrade to solar for another $500-800.
Now, to link the solar heater to the furnace, I need a differential thermostat to monitor the temperatures of both and then provide power to the regular control circuit when it is really needed. If the solar water is hot, the furnace never needs to run. Are there any U.S. vendors that provide programmable differential thermostats, and can these be hooked up to the thermocouple that the furnace already uses for the hot water?