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I've recently installed solar hot water and as expected it gets extremely hot. I would like to keep the tank as hot as possible (>180°F) and simply mix down the water to 120°F / 50°C as it's used so the tanks charge lasts longer. The sun doesn't always cooperate however and it can obviously get much colder than that.

The solar company installed a thermostatic mixing valve, however it doesn't hold the temperature at 120°F. Depending on how hot the tank is (120°F - >180°F) the temperature at the faucet fluctuates greatly.

Is there a valve that will simply keep the temperature at 120°F no matter what the source tanks temperature is? While it is at least >120°F of course.

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A thermostatic mixing valve is the same device as a thermostatic shower, the mechanism is the same but the body & fittings different.

So, have you tried adjusting the existing valve? Is there any chance it has got some debris in it causing it to fluctuate. Is the cold supply to the mixing valve restricted?

It may be a faulty valve which needs replacement.

  • Should it be adjusting to an exact temperature, or a wide range of temperatures depending on the current temp of the tank? I'm not sure if I have a faulty valve or misunderstand how it is supposed to be working I think. – Citricguy Aug 19 at 9:42
  • They usually work well as long as they get a cold enough supply as if the hot is not then they cannot heat water only cool it down. So if the cold fluctuates wildly that may cause an issue... – Solar Mike Aug 19 at 9:54
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Check that the valve installed is rated for very hot input water. I have a working setup like this, regardless of how hot the solar tank is the water is always cooled down to a specific temperature which I can change by turning a knob on the side of the valve. In Australia it's called an anti-scald valve.

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