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Plumbing in the Home/leaking T & P Valve on hot hater heater


I have replaced the t & p valve twice in the last week, and it still leaks (opens when in heating cycle). I have checked the inlet pressure and found it to be at or about 50 psi. I* have checked the temp setting and measured the temp and it is 120 - 125 deg F. The heater is 2 years old and a gas fired heater.

What else can I check to solve this problem.


expansion tank
expansion tank  

TET installed
TET installed  
Hello Gary,
What is probably happening is called "thermal expansion". While the inlet pressure may be only 50 psi static, it is obviously exceeding that from time to time causing the TPRV to leak.

This is what's going on:

The hot water piping circuit is what is called a closed system. When you open a faucet and draw some of the hot water, cold water enters the water heater through the inlet valve and supply line to make up for the volume loss.

Once you close the hot water faucet, the system becomes a closed system at that point.

The drop in temperature sensed by the thermostat of the water heater causes the burner to light to bring the temperature backup to the setpoint.

When water is heated, it expands. Since it is doing this inside a closed system, the pressure inside that system rises, sometimes significantly. When it raises past the 150 psi release point of the TPRV, it does what it is supposed to do and bleeds off the excess pressure to protect the water heater and the connected piping system. So, technically, the TPRV is not leaking, it is simply doing his job.

The solution to this problem is to install an expansion tank. This is a small metal diaphragm tank that is designed for just this purpose. These are a small cylindrical tank that is divided in the center by flexible diaphragm. One side of the tank has pressurized air and the other has water under normal system pressure. When the water expands when heated, the additional volume is absorbed by the pressurized air side of the expansion tank and compensates for the pressure rise without causing the TPRV to leak.

This is a common problem and a common solution that should actually be included in every water heater installation.


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Dana Bostick


Pretty much any residential plumbing questions. For "item specific" details such as a specific model of fixture, I will need to research and there may not be any useful information available. Note: I live and work in Southern California. We do not, as a rule, use hot water or steam heating systems, oil fired boilers or private water wells so my knowledge in those areas is pretty limited. There are others here on AllExerts that can probably answer those questions better.


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