System is a AO Smith power vented 50 gallon gas water heater about 8 years old. 2 gallon Amtrol ST-5 expansion tank (charged to 65 psi) on cold water supply side was installed in December 2022 to replace 8 year old 1.25 (?) gallon tank. House pressure is currently set to 40 psi. Temp set to 125F.

Last year, I replaced the hot water heater's TPR valve after it started dripping/partially opening. I drained and flushed the tank while I was at it. After a few days, I remembered to check it again, and I noticed it was still doing that. After some internet "research", the symptoms pointed me to the expansion tank, which was probably installed at the same time as the heater. I put in one a size bigger due to wanting the temp higher for therapy reasons. I installed the new one (it was precharged to 50 psi). Again, I checked it after a few days, and the drip had resumed.

This drip gradually increased to a steady flow of water from the TPR valve over the course of a month. I increased the precharge on a new tank from 50 psi to 65 psi. It returned to a drip. I replaced the TPR valve again, and the tank stopped dripping for a week.

So I started paying closer attention to both the peak and current water pressure as measured by a dial gauge on the tank valve on the bottom of the water heater. Although I have the house pressure set to 40 psi, I was recording spikes every day over 130 psi. The spikes are present on both cold and hot water side. I noticed with further inspection that the spikes are coinciding with the last few minutes of the heating cycle, and the TPR valve discharges are occurring then as well.

Is this pointing to a bad PRV for the house, a bad water heater (maybe scaling inside?), or something else?

EDIT for clarity: this is for the potable water for washing and such and not in a closed loop heating system. The supply from the street has a check valve installed by the city to prevent backflow.

  • See this answer for a great explanation of how the fill valve, relief valve, and expansion tank work together and how to precharge. That answer is about a heating system, but I think your question is about domestic hot water? I don't have experience with the latter but I think the same principles would apply.
    – jay613
    Commented May 23, 2023 at 18:33
  • Some good reading
    – jay613
    Commented May 23, 2023 at 18:41
  • Sorry for the lack of clarity; this hot water is for consumption (showers, dishwasher, etc) and not in a closed loop home heating system.
    – justin
    Commented May 24, 2023 at 13:37
  • Why do you have a whole house pressure regulator? Are you sure you need it? If you can get rid of it you can also get rid of the expansion tank. If you keep them, disconnect the tank and preset it to 38 PSI.
    – jay613
    Commented May 24, 2023 at 13:46
  • I think based on your earlier link, the expansion tank pressure should be reduced to just under "steady state" pressure if I am measuring at the hot water heater, or otherwise pretty much the supply side pressure if measuring at a cold spigot. Is this a correct reading of your information?
    – justin
    Commented May 24, 2023 at 13:46

2 Answers 2


If I had to guess: Turn the heat down on your hot water heater. The bottom may have scaled up or not been flushed recently, and the bottom is not conducting especially well. Towards the end of your heating cycle, the bottom has gotten extremely hot and is flashing some of the water to steam, which goes up to the top and blows out the TPV.

You should also check for crap in the bottom of the heater -- turn everything off, let it cool down, and drain it a bit and look for chunks. You might also have to replace the sacrificial anode.


It sounds like your hypothesis is that the TPR is being bumped to leak by what’s called “water hammer”, a pressure impulse caused by the mass of water in motion when a valve is closed rapidly. The expansion tank on the cold water line is presumed to be ineffective to control the pressure impulse.

The expansion tank is presumably ineffective because the cold water inlet is a tiny fraction of the surface area of hot water heater tank. The rate at which water moves into the expansion tank to relieve the pressure impulse is limited by the inlet area. A pressure impulse from the outlet acts on the entire surface of the hot water heater, including the TPR valve, and the inlet pipe. The net effect is that the expansion tank limits the duration of the pressure impulse, but not the magnitude of the pressure impulse.

The most obvious answer is to install a water hammer arrester at the outlet of the hot water heater. It’s a tiny version of an expansion tank but it only takes a small volume to reduce the magnitude of the impulse. The important difference is that it reduces the magnitude of the impulse before it reaches the hot water heater tank.

If there are any known sources of water hammer on the hot water line it would be prudent to install water hammer arresters at those points of use. Anything that can make pipes rattle , e.g. a Dishwasher, or clothes washer, or a sink with single handle valve all close rapidly enough to cause water hammer.

There is of course a possibility that an external pressure pulse lasting seconds if from the supply to force the TPR to open. If your neighbors are having the same problem you are that would be a reasonable assumption. It would be necessary to install a water pressure regulator at the point where the supply enters the building to block such a pulse.

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