Temperature and Pressure (T&P) Relief Valves

How They Work

The last line of defense for pressurized equipment, Temperature and pressure (T&P) relief valves are critical safety devices that protect equipment from excess pressure and, ultimately, catastrophic failure.

T&P valves used on residential water heaters are typically designed and manufactured to relieve pressure at 150 psi and temperature at 210 degrees F. ASME, ANSI and CSA (AGA) approved relief valves protect the water heater from excess pressures and temperatures by discharging water.

Thermal Expansion

When water is heated it expands. In a 40-gallon water heater, water being heated to the thermostat setting expands by approximately 1/2 gallon. The extra volume created by this expansion usually pushes back into the water tower of public water supplies, or into the well tank in homes with a private well, resulting in negligible pressure increase. However, on public or shared water supplies were a backflow device is installed on the water main feeding the house, the pressure can no longer push back into the tower or  well tank. With no place for the expansion to go, pressure will dramatically increase.

A good indication of a thermal expansion problem are small, recurring discharges from the T&P valve (a thermal expansion issue can be confirmed using a Watts IWTG gauge). These recurring discharges can compromise the function of the T&P safety valve so it’s critically important that any thermal expansion problem is addressed! The preferred way to address a thermal expansion problem is with the use of an expansion tank designed for potable water applications, such as the Watts PLT series.


What You Need to Know

  • In normal operation of the water heater and T&P valve, no water should be discharged from the valve.     
  • A relief valve functions by discharging water, it is essential that a discharge line be piped from the valve outlet to carry the overflow to a safe place of disposal.
  • The discharge line must be the same size as the valve outlet, and must pitch downward from the valve with no traps and no more than 4 fittings.
  • The termination point of the discharge line should be observable to the occupants of the building so any discharge can be readily detected.

  • Discharge from a T&P valve may indicate an unsafe temperature or pressure condition exists, which requires immediate attention by a qualified service technician or licensed plumbing contractor.