pumped solar water heating


New Member
I am about to build my first solar water heating system my design will be a pumped sytem solar powered.I would like to heat the water to the maximum temp possible with a cut out below boiling and would have the system pump when the temp in the collector is above the storage tank temp and stop when tank temp is above collector temp.Because the system is not passive (syphonic) I can put my tank anywhere may be under the house and because I live in a frost prone area the collector would have a thermostatically switched pump to feed water into the collector when the collector temp aproaches freezing.
My problem is I need a temperature differential controller (when collector is above tank temp pump when tank temp is above collector stop) there are no set temps as they would float up to a max temp preset as a stop to prevent boiling. Does anyone know of a unit of this type could they point me in the right direction to buy one.

ken morgan

Solar Addict
probably be easier to build one using off the shelf temp controllers I would think. what voltages are you looking at? AC? DC? the whole temp differential relay is readily available in 12 volt format where the controller has the ability to measure with a temp probe I am sure they exist in 120 ac format as well as they are used to control greenhouse vents and fans.


New Member
Hi Ken not that easy have temp controllers but they won't do it, I think it would have to be processor controlled maybe (arduino). The reason is there are no presets except max temp, so long as collector is hotter than storage,pump runs


New Member

Steca 0301U Solar Hot Water Control: LCD Display, Power Cord, and 2 sensors​

This is he one I have been using for 15 years. The max storage temp is 160. I would not want it any hotter my self.


Solar Enthusiast
I considered just continuously circulating the water around below the hot water tank inlet ( pump controlled by solar). Water will only rise up into the inlet if it is warmer. I already have a 600w 12v element at that position.


Science Penguin
I would probably engineer it myself using arduino and sensors. Happy to link parts that I think would be appropriate for this application.

For myself, I would probably try to design two thermometers at each important location and trigger an alert if any pair diverges.

Good luck!


Solar Addict
I built one for grins with two resistive temp sensors in a bridge. when one is more than the other the pump turns on. Personally, I would never have solar thermal heating.


Solar Enthusiast
I have 80sq ft of collector that thermosiphons into a heat exchanger within a 30 gal tank suspended at the peak of a finished attic. When the water in the suspended tank reaches a given temp a small pump transfers the hot water to a storage tank in the basement. Cold water replacing the hot water turns off the thermoswitch. The thermoswitch was stolen from a 200w box fan and the 25w pump is from an instant hot water at faucet system. Except for the occasional replacing of the nontoxic antifreeze in the collector I've done nothing for 13 years. Note: The polycarbonate glazing needs replacing.
PS There's an extended ball valve handle mounted thru the bathroom floor that allows me to tap the upper tank directly if needed.
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