solar hot water questions

[mikeyny]

I have a home brew hot water solar panel that works real well, but, I don't use it in the winter. I heat our house with a wood boiler and that takes care of the domestic hot water most of the time. When it is real cold out the boiler has a hard time keeping up the heat so I shut down the hw coil. My solar setup is a homebrew panel on the porch roof feeding directly into a hw storage tank with a pump on it. This tank then feeds into the gas fired hw tank when the hot water is turned on. Here is my question. Could I use non-toxic antifreeze in the panel in a loop to a heat exchanger, like a flat plate or typical domestic coil in a tank? How much  efficiency is lost?. Will it over heat in the summertime? Will it work well in the winter?

[strider3700]

I'm pretty sure that you will need to use a double walled heat exchanger even if using non-toxic antifreeze.  Over heating in the summer is possible so you will need a tempering valve to prevent possible scalding, also make sure the blow off valve is working correctly.   I can't answer the efficiency question, this is pretty common so I'd assume it works well enough to help out in the summer.  In really cold/cloudy locations evacuated tube collectors seem to be recommended. I'd assume a flat plate collector works but probably nowhere near as well.

[jonas302]

my 30 plate exchanger runs about 10 degrees cooler than the input water they come pretty cheap on ebay

[ptitchard]

Hi Mike,

Like most processes when you transfer energy you have losses.

Does you present system overheat? if not then it is unlikely that by fitting a heat exchanger you will suffer any problems.

I have been running a homebrew system for several years now.I have used an old domestic copper hot water tank with internal coil, as a preheat tank before going into the main boiler tank.

It is a thermal syphoning system so no pumps, which is filled with plumbers antifreeze.

The collector is an old house radiator suitable boxed etc.

It is an unpressurised system with small header tank and vent pipes.

The system does not produce much heat in the winter, just raising the temperature a couple of degrees. With a pump, a better collector and pressurised system this would be a different story.

If your solar tank is an open top you could just coil up some copper tubing and drop it in. Do not forget a filler pipe and vent.

You could probably pick an old domestic tank up from a local plumber cheap. A lot of people here in the UK are going to on demand boilers, so the plumbers are ripping the old tanks out and take them down to the local junk yard for the copper value.

[mikeyny]

The setup I have is this. Solar panel on the roof feeds directly into pre-heat 40 gal water tank. When hot water faucet is turned on cold water enters pre-heat tank and hot water enters existing gas fired hot water tank. I went from using 85 therms to 18 in the summer. In winter I close and open a few valves and the water comes in from the wood boiler. I have the top half (water jacket) of an old steel boiler with a domestic coil in it.

The boiler water feeds through the water jacket and the domestic water goes through the coil and back to the pre-heat tank. I was thinking about running antifreeze in the solar panel through the water jacket and draw hot water through the coil. I was a bit worried that it would over heat in the summer because of the small amount of antifreeze in the system. Probably about 25 or 30 gallons. If we are not drawing hot water off the tank during the day, it just may get too hot, but i am not sure.

                                         Mike