Solar storage tank and on-demand backup

pogobongoNovember 6, 2011

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.



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The type of hot water heater that you are proposing is a costly investment for something that you hope you will rarely use. It will eat up the savings you expect from solar.

What do you heat with now? Why not keep it and use it for back up?

    Bookmark   November 6, 2011 at 1:53PM
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Ditto to ionized. Lots of other issues besides upfront costs using on demand anything as a backup to solar although lots of people do it. At least two good independent studies find that replacing existing hot water heaters with on demand is not cost effective and it would be even more so as solar backup.

I personally would be tempted to go with electric resistance heaters(or existing backup infrastructure) and a resistance tank for the dhw.

Here is a link that might be useful: article about on-demand issues for solar backup

    Bookmark   November 6, 2011 at 8:11PM
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