The Function of a T & P (Temperature and Pressure) Valve on a Water Heater

T&P Valve on a Water Heater

Lee Wallender 

On the side or top of your water heater tank is a valve connected to a metal or plastic discharge tube leading down. 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 how a T & P valve works will help you keep your water heater in tip-top shape and prevent possible damage to your home.

How a T & P Valve Works

Mandated by all plumbing codes, the T & P valve relieves excess temperature and pressure in a water heater when either reach critical points. The term temperature- and-pressure valve is therefore slightly inaccurate since this device really functions as a temperature and pressure relief valve. A water heater is a closed system, and thermal expansion is an inescapable fact of both normal and abnormal water heater functioning.

In a standard water heater, the water is heated by a gas burner or an electric coil located at the very bottom of the tank. As that water reaches temperatures between 120 and 140 degrees Fahrenheit, 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 degrees or the pressure reaches 150 psi (pounds per square inch), a properly functioning T & P valve opens and expels hot water and steam through the discharge tube.

How a T & P Valve Is Installed

When you purchase a water heater, the T & P valve generally comes pre-installed. A threaded inlet is welded onto the side of the tank, and the T & P valve is screwed clockwise onto that inlet. However, the discharge tube running downward along the side of the tank is not pre-installed.

To install the discharge tube, run a strip of Teflon tape clockwise around the threaded end of the PVC discharge tube. Screw the discharge tube onto the T & P valve, first by hand, then tightening it fully with a wrench. Be careful not to strip or break off the T & P valve or discharge tube by tightening it too much.

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 catch emergency outflows of water.

Warning

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

Test the Valve to Prevent Problems

Water heater manufacturers recommend regular checking of the T & P valve:

  1. Make sure that the discharge tube is firmly attached. Wear closed-toe shoes to avoid scalding.
  2. Place a bucket below the discharge tube.
  3. 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.
  4. Make sure that the valve snaps back to its original position. If the lever does not snap back into place, it will need to be replaced.

Common Problems with T & P Valves

There are really only two problems you might have to address with the T & P valve: a valve that sticks and doesn't open and close properly; or a valve that leaks by constantly dripping.

Dealing With a Leaky T & P Valve

When a T&P valve is leaking, it may be a simple matter of the valve not being properly seated in the threaded opening. This is especially likely if the valve leaking occurs immediately after you replace it. If this is the issue, then it can be remedied by shutting off the water heater, then removing and rethreading the valve into the tank's opening.

Leaking can also be one symptom of a T & P valve that is sticking.

Warning

A water heater that periodically discharges hot water and steam from the T & P valve may be set to a water temperature that is too high. Make sure that the water temperature setting is in the normal recommended range—no more than about 150 degrees Fahrenheit.

Fixing a Sticky T & P Valve

T & P valves can also become stuck either in a downward (closed) or fully extended (open) position. Either condition is a potentially serious problem. When stuck downward, the valve cannot provide relief if the system reaches maximum pressure; as a result, the water heater tank might rupture. When stuck in a fully extended position, the T & P valve will continually leak water down through the discharge tube, leading to a considerable waste of water and energy.

The stickiness of the valve can sometimes be remedied by simply opening and closing the lever several times. 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.

Safety Considerations

In most cases, the T & P valve will last as long as the water heater itself and fixes are easy if you perform annual testing to catch problems early. However, some caution is necessary whenever working with a water heater, because the T & P valve can become damaged if the water heater has exceeded maximum pressure or temperature levels. If you suspect a pressure-related problem with your water heater tank, consult a licensed plumber to examine the water heater.