Solar storage tank and on-demand backup
I live in the bay area of northern California and am about to install some solar panels for water heating. This will feed a large storage tank. The tank will provide both domestic hot water and radiant floor heating.
I am concerned that if we have a sustained period of no sun, we won't have DHW or heat!
Using an on demand propane heater between the solar storage tank and the DHW system would satisfy the needs of DHW, but not those of the radiant floor heating system.
My thinking is that if I installed a small on-demand heater with a pump, then if the water in the tank gets too cold and I want to boost it, I can turn on the pump and circulate the water in the tank thru the on demand, and in this way heat up the water in the tank and simultanously meet the needs of DHW and radiant floor heating.
The pump would probably be manually activated (or possibly on a timer) with a temp switch that turns off the pump once the inlet to the ODH reaches a set temperature.
My questions are:
1) is this feasible and sensible?
2) I was thinking the smallest Rinnai unit - thoughts?
3) I propose sizing the pump to provide the maximum flow before the ODH temp rise curve starts to drop off - caveats or extra inputs?
Any advice much appreciated.