But, you can do the solar hydronic plus propane system.
In most of these systems, there is some storage -- I'm not sure if you could do it with no storage.
One common scheme is a common storage tank that is heated by solar and also heated by a backup heater like a propane boiler (probably a proane tankless water heater could be used).
Thanks, Gary. If a solar/propane hydronic heating system has a 50 gallon propane water heater, why can't the water heater be the storage for whenever the solar system is in operation?
I guess this means the hot water tank has one circuit that pumps water out to hydronic radiators, and a 2nd circuit that receives heat from the solar collectors?
I don't claim any great expertise in this, but here are some thoughts on a 50 gallon hot water heater based system:
- 50 gallons is not much storage. If you heat it to (say)140F, and use the heat in it until its down to 80F, the heat energy is (50 gal)(8.3 lb/gal)(140F - 80F)(1 BTU/lb-F) = 25K BTU -- about equal to 1/4 gallon of propane. Maybe not enough to do a lot of good in heating the building up?
- 50 gallons is not enough storage to support a whole lot of collector area -- the usual rule of thumb is about 1.5 gallons per sqft of collector, so about 33 sqft of collector. This is not likely to be enough solar collector to put a big dent in the shop heating unless the shop very very well insulated. I think that 3 to 5 times that area would be more typical for a garage size shop in a cold climate with typical insulation.
- The collectors have to be protected from freezing, so the circuit out to the collectors has to be antifreeze and unless you want to make the whole 50 gallons in the hot water tank antifreeze, then there has to be a heat exchanger between the collector circuit and the hot water tank. This could be a coil embedded in the tank (kind of pricey), or heat exchanger outside the tank that tank and collector water is circulated to -- this means two pumps.
- You would have to work out how the heating element in the tank is controlled. If you let it just maintain 140F (or so), then there is no water available for the solar collectors to heat up -- its already hot.
You would have to do something where the solar heated the water in the tank from a low temperature, then the solar heat in the tank is used, then the burner on the tank comes on and heats the water after the solar is used. I've not heard of anyone doing this, but maybe its possible.
You might take a look at Guy's system:
http://www.arttec.net/Solar/BarnHeat.htmlI don't remember all the details, but its a shop with hydronic heat and propane backup. I think he uses a tankless heater for the backup, but don't remember for sure.
His hydronic heat is in floor radiant, but hydronic baseboards should work the same way, other than not providing any extra storage thermal mass.
I may be missing something here -- anyone else have ideas that might be simpler?
Gary