My hot water tank cold water is always flowing and it's losing water out of the temperature valve relief tube on the cold water intake side (see attached photos). I just noticed this, but it could have been going on for weeks. The tank natural gas and is 10 years old and I've never tested the TP valve until recently. I've never pulled the TP valve out to check it. I've tested the thermostat and the burner is firing fine. We have no problems with the hot water in the house. The house water pressure is sitting at around 60psi

Home Depot guy said that the pressure in the city has gone up due to check valves being installed, however I'm not so sure. He said to install an expansion tank on the cold water intake side, however, I'd like to understand what the problem is better before doing this. Another potential clue is that there was a broken water main at a house down the street, however, I don't think that this is contributing to the problem as our house pressure is 60 psi.

Is this pressure, temperature, tank related, or perhaps a failed temperature control valve problem (on the cold water intake)?

What should I do to better diagnose this problem? Should I replace the temperature control valve on the cold water intake? Install a expansion tank? If so where would I install the expansion tank on my cold water side?

edit: That "temperature control valve" is actually a thermal expansion valve.

From the website: https://www.mmcontrol.com/apollovalves/plumbing.php

Apollo® 78RV Series Combination Water Heater Isolation Valve/ IAPMO approved Thermal Expansion Relief Device. This combination water heater shutoff and thermal expansion relief valve is an ideal alternative to expansion tanks in residential plumbing systems.

Update 2: I have run the cold and hot water tests for both at 5 minutes each and neither have stopped the flow of water out of the thermal relief valve. When I closed the valve completely, the water still flowed, however it turned to hot water after around 3-4 minutes. Does this mean I have too much pressure inside the tank? Replace valve? Install expansion tank? Replace water heater? Call in the pros? :) Many thanks again all.

Please see attached photos for details.

Thanks E

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  • Check to see if the water is still running out the valve after running water at a faucet for 10 seconds. Oct 24, 2018 at 18:45
  • Thank you will test this when I get home and report back. Sorry if this is a stupid question, run hot? cold? or both? many thanks.
    – Dolph
    Oct 24, 2018 at 20:14
  • Adding expansion tank is a good advice. You sould also add a 'heat trap' to your piping (a reversed U pipe from hot water exit that prevents heated water from rising 'naturally' in your system)
    – DDS
    Oct 24, 2018 at 20:52
  • 1
    Either or. The idea behind this test is to see if the pressure is high and causing the value to leak. If it stops leaking, then it's likely the pressure and you should install a expansion tank. If it still leaks, I'd start with replacing the valve. Oct 24, 2018 at 20:54
  • Thanks guys, please note that the cold water pressure in the house is 50psi. This was taken directly above the tank in the laundry room. Additionally I should have noted that when I turn the valve off, the water stops flowing out of the tube. I will run that test later.
    – Dolph
    Oct 24, 2018 at 21:35

1 Answer 1


I'd try replacing the relief value that is screwed into the shut off valve to see if it solves the water flow issue. Sometimes once they are activated, they don't like to stop for varous reasons.

As for the expansion tank, it should be installed before the water heater. They are normally installed near where the water comes into the building.

  • Update: Leak stopped. Replaced the 80psi valve with a 100spi (apparently they stopped making them). I had a look into the valve and it was full of brown sludge (rust?) meaning that IMO that this WH is on it's way out soon. It's still working and producing hot water, so I'm going to keep checking it and the TPV as well as this new PV to make sure that it's running ok. WH sits on a drain pan with another drain in the room so (hopefully) I'm good with any small leaks that form. Thanks all. E
    – Dolph
    Oct 27, 2018 at 22:10

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