What Does a T&P Valve Do?

T&P Valve on a Water Heater
Water heater T&P valves are emergency pressure relief valves for safety. Lee Wallender 

On the side or top of your water heater is a valve with a tube leading downward. If all goes well, that valve never gets used except during testing. But in the event of an emergency, this valve is highly important. It can save your water heater from exploding, your home from flooding, and it can protect your safety. This valve, called the T&P valve, is an essential part of your home's water heater. Understanding what a T&P valve does and how it works will keep your water heater and your home in tip-top shape.

T&P Valve Defined

Mandated by all plumbing codes, a T&P valve relieves excess temperature and pressure in a water heater when either reach critical points. The term T&P valve literally stands for temperature and pressure valve. More accurately, it is a temperature and pressure relief valve, because a water heater is a closed system and thermal expansion is an inescapable fact of both normal and abnormal water heater functioning.

What a T&P Valve Does

Water is heated by a gas burner or an electric coil located at the very bottom of the water heater. As that water reaches temperatures between 120 and 140 degrees F, both the water and the water heater's metal tank expand. Some expansion is normal, but too much expansion is abnormal. When the temperature reaches 210 F or the pressure reaches 150 psi (pounds per square inch), the T&P valve opens and expels hot water and steam through the discharge tube.

T&P Valve and Discharge Tube Installation

When you purchase a water heater, the T&P valve will come pre-installed. A threaded inlet is welded onto the side of the tank, and the T&P valve is turned clockwise onto that inlet. The discharge tube will not be pre-installed.

To install the discharge tube, run a strip of teflon tape clockwise around the threaded end of the PVC discharge tube. Turn the discharge tube onto the T&P valve first by hand, then harder with a wrench. Be careful not to strip or break off the T&P valve or discharge tube by turning too hard. 

The discharge tube should point straight downward, terminating a few inches above the floor. It is recommended that you place a water heater pan below the water heater to collect slow drips or to control emergency outflows of water.

Common Problems and Troubleshooting

Because the T&P valve is rarely used, mineral build-up can cause it to stiffen over time. This is a significant safety hazard. In the event of a temperature or pressure spike, the T&P valve may not open and the water heater might explode.

Safe Test Procedures

Water heater manufacturers recommend regular checking of the T&P valve. Whether making a scheduled or occasional test, it is important to follow proper test procedures.

To test, make sure that the discharge tube is firmly attached. Wear closed-toe shoes to avoid scalding. Place a bucket below the discharge tube. Briefly pull back on the T&P valve's metal lever to cause a small amount of water (about a quarter cup) to discharge into the bucket. Make sure that the valve returns to its original position.

T&P Valve Is Leaking

When a T&P valve is leaking, it may be a simple matter of a valve that is not properly seated. Jiggle the T&P valve by gently pulling the lever towards you. Much like jiggling a toilet handle, this action may be enough to unstick the T&P valve. If this does not fix the problem, replace the T&P valve.

T&P Valve Is Stuck

A T&P valve that is stuck either in a downward (closed) or fully extended (open) position is dangerous. When stuck downward, the valve will not provide relief if the system reaches maximum levels; as a result, the water heater may burst. When stuck in a fully extended position, the T&P valve will continually leak.

Follow safe test procedures before dealing with the valve. The T&P valve might be in extended position because the water heater had exceeded maximum pressure or temperature levels. If you believe this is the case, consult a licensed plumber to examine the water heater.