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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.

Rohit Gupta
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justin
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  • See [this answer](https://diy.stackexchange.com/a/218771/65210) 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 May 23 '23 at 18:33
  • [Some good reading](https://www.hpacmag.com/features/expansion-tank-dos-and-donts/) – jay613 May 23 '23 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 May 24 '23 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 May 24 '23 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 May 24 '23 at 13:46
  • The PRV is installed on the supply as the pressure from the street usually exceeds 90 PSI in my area; I am in a low spot in the city. But all homes I have seen in my area (southwest Ohio) have PRV valves. – justin May 24 '23 at 13:49
  • What is the relief valve rating? Mine is 150PSI. If yours is also 150PSI, nothing you have written would explain its constant leaking. – jay613 May 24 '23 at 13:53
  • It is rated at 150 PSI. I am not using a calibrated gauge, so it could be that it is not entirely capturing the high end of the spikes occurring at the end of the heating cycle. – justin May 24 '23 at 13:58
  • I'm stumped. Wild guessing now. Expansion tank incorrectly installed? Verify by putting a gauge (bicycle pump?) on the air side after correctly precharging, connecting, and with a cold tank. Should see precharge pressure. Begin heating tank, should see pressure rise. Turn on hot shower, should see pressure fall .... etc. Make sure the air side is doing what it should. – jay613 May 24 '23 at 14:12

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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.

gbronner
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