Old water heater was leaking at the inlet pipe. Installed a new heater and the PRV leaks a small amount about every other day. Why does the new one relieve when the old one never did?

  • 1
    They could be set differently. The new one might not be tightened quite enough. Care to provide some details like make/model? Maybe a picture or two? Without that it's like me asking you why my car idles rough every 3rd day - doesn't give you much to go on...
    – FreeMan
    Dec 17, 2020 at 17:54
  • When you refer to a PRV do you mean the pressure relief valve on the water heater or the pressure reducing valve on the water line coming in?
    – HoneyDo
    Dec 17, 2020 at 18:35
  • It is a Reliance 40 gallon electric water heater. The 150 psi/ 220 degree Pressure Relief Valve relieves a very small amount of water about every other day.
    – Marty
    Dec 17, 2020 at 19:48

2 Answers 2


PRVs are designed to release pressure. Heating water increases the size of the water and the pressure. Your new water heater is likely heating the water quicker due to newer elements, less sediment in the bottom or other factors. Heating the water quicker will cause the pressure to increase over a smaller amount of time which might trigger the PRV. Do you have an expansion tank?

Your old PRV might have been defective or less sensitive or the new PRV could be more sensitive. Do you have a pressure gauge can you tell what pressure it is releasing at? There are pressure gauges that you can screw onto hose bibs so you can attach them to the tank drain and monitor.

Homedepot has some for $10. Water Pressure Test Gauge.


  • I do not have an expansion tank or a pressure gauge. I'll have to get a guage. Is there any harm letting it relieve a little like it is?
    – Marty
    Dec 17, 2020 at 19:51
  • Over time the tpr valve will start to build up debris and leak more often. If you see any discoloration like white on the brass that is a desiderata that over time could cause the valve to fail from 6mo to 6yr
    – Ed Beal
    Dec 17, 2020 at 20:45

The valves are both pressure and thermal usually referred to as TPR or TPRV If you are on city water and there is a check valve or a regulator on the inlet the water pressure is fed to the set point let’s say 60 psi. If the water in the tank is at a minimum whatever it is then the heater kicks in the water cannot back flow out the supply because of the regulator or check valve. The water in the tank expands as it is heated creating a high pressure causing the tpr valve on the water heater to leak. You can see it takes just the right conditions to create the leak , the tank cooling , incoming pressure at its peak then the heater kicking on.

If this is the case (quite common) a small pressure tank can be added to the system usually on the incoming cold after the PRV(pressure reducing valve) or regulator and this stoops the leak by providing a small expansion space.


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