How do we prevent temperature spikes with our on-demand water heater?

Asked by Jack Herndon
Seattle, WA

We have a gas Rinnai on-demand heater in series with a solar hot water system. We purchased the 150K BTU Rinnai because of our low hot water demand. The water temperature spikes when the preheated water reaches the Rinnai, then the Rinnai shuts down. I can understand the shutdown, but how do I deal with the spike to scalding temperature? It makes things uncomfortable in the shower. We tried setting the delivery temperature too.


You will need to install a Thermostatic mixing valve prior to the fixture and just after the Rinnai flow through hot water heater.

  • This valve will automatically mix cold water with heated water to the preset temporature and elliminate the temperature spike in water temperature.
  • Depending on type of solar hot water system, you may be able to install a circulation pump and loop into the hot water system to constantly circulate and provide even temperature hot water at all fixtures.

Look into the following manufacturers of mixing valve: Taco, Honeywell, Caleffi and Cash Acme.

Tagged In: tankless water heater

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