I want to install a solar water heater in my house in the future, so I've been thinking about how I could do it, something like this:

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  • Cold water enters the solar heater, hot water comes out.
  • If it's too hot, a thermostatic mixing valve mixes it with cold water to lower the temperature.
  • If the water is below X degrees (too much hot water used or in a cold winter) I want it to go to a tankless gas heater, from there to the house. If the temperature is above X degrees, it would go directly to the house.

I can't find a valve that does this last step, everything I search show results for thermostatic mixing valves. I want an on/off valve, 1 input/2 outputs. If temperature is below X degrees, all water flows to one output, otherwise all water goes to the other output.

I know that some tankless gas heaters can be configured to just let the water pass and not turn on the gas if the water is above X degrees, but I believe that would create unnecessary wear on the heater parts.

Does a valve that can do this exists? How are they called?

Preferably one that doesn't use electricity like the thermostatic mixing ones.

  • Do you want to use the solar purely for domestic water heating, or for space heat as well? Jun 17, 2021 at 3:07
  • There are electronic ones that work well and are adjustable. Other than that there are spring loaded ones like in a car but made for potable water the manifolds and thermally controlled valves are costly so I have used the electric ones they only consume about 5watts. The mechanical ones do allow some water to pass where the electronic ones measure at the manifold .
    – Ed Beal
    Jun 17, 2021 at 14:08

1 Answer 1


I'm designing such a system for a renovation in approx 6 months when I move back into my house after having it tenanted for a year.

An exterior mains pressure cylinder recieves a gycol circuit from solar panel on the roof. Cylinder also revieves electric and wetback from log burner (optionally). Electric is backup and used when gas is unavailable or too expensive or insufficient sun. This cylinder is the storage battery in a comparable (much less efficient) solar PV system.

Temperature probe in the cylinder controls solenoid diversion valve - which sends the water either to existing gas califont - or the house.

A small UPS ensures operation of the gas califont and diversion valve in a power cut. A solar PV panel may be added to charge this.

That's my plan!


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