Mastertemp 400 Thermal Regulator Corroded


Well-known member
Aug 24, 2017
Long Beach, CA
Was getting noise in the area of the inlet and outlet to the heater and opened up the area with the thermal regulator.

The thermal regulator was badly corroded and the middle of it pretty much fell apart when I tried to clean it off under running water.

Have a new one on order, can I put the cap back on and run the circulation pumps without this part?

I wouldn't run the heater until the part was replaced.



TFP Expert
Mar 2, 2011
The main thing that it does is block water from going through the heat exchanger when the heater is off so that the heat exchanger doesn't constantly drip condensation.

As long as you're not getting condensation on the equipment, you probably won't get condensation on the heat exchanger and it will be ok.


TFP Expert
Mar 2, 2011
If you remove the thermal regulator, you should be able to reach up and feel the round disc that is the manifold bypass.

Or, you can open the top union and look inside to see if the bypass is intact.

The internal bypass is a spring loaded check valve that opens based on the water pressure in the inlet portion of the manifold.

At high pressure/flow, the disc is pushed open to allow water to go directly from the inlet to the outlet without going through the heat exchanger.

This prevents too much water from going through the heat exchanger.

Sometimes, the little disc will break loose and then too much water bypasses the heat exchanger.

I suspect that the bypass is broken. If you have to replace it, you will need to remove the manifold and then you will be able to check the condition of the ends of heat exchanger tubes.

You will also see if the combustion chamber is full of water when you remove the manifold because water will pour out of the bolt holes.

Mastertemp bypass valve
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