Everything to Know About Water Heater T&P Relief Valve

What is a Water Heater Temperature and Pressure Relief Valve?

A water heater is designed to supply a home with ready hot water at all times. It does so by heating a constant volume of water using either natural gas or electricity.

When water is heated, it expands. Expansion of water means that it is exerting more pressure inside the water heater. Water heaters being closed systems needs a way to discharge excess water, should the pressure and temperature inside exceed their rating.

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A water heater temperature and pressure (T&P) relief valve is a lever-actuated valve near the top or at the top of the water heater. It automatically opens and closes to release water when the pressure and temperature inside the water heater exceeds its rating (150 psi and 2100 F), preventing it from bursting.

All water heaters are required by code to have a functional temperature and pressure relief valve, also known as T&P or TPR. This valves however fail with time and hence the need to keep testing them, ideally twice a year.

In most homes, a vertical pipe known as a discharge tube is connected to the T&P valve for safety and also to direct water to a pan or bucket, therefore avoiding messing the floor.

In other houses however, the discharge tube is connected to a pipe that drains the excess water outside the house. Take time to check the type of discharge system you have in your house.

How Does a Temperature and Pressure Relief Valve Work?

A T&P valve is one of those home accessories that should only come into effect/use when something is wrong. It is usually installed for safety purposes, but it is not something you want to see being triggered on and off all the time.

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A T&P valve has 2 sensors, one for the pressure and the other for temperature. When the temperature and pressure inside the water heater exceeds the valve’s rating, the water pushes a seat/disc in the valve which in return compresses a spring, allowing water to drip out of the valve until safe conditions are restored.

Different home plumbing systems work differently. If you have a closed-loop system, your water heater has the potential to explode when the valve fails to open. On the other hand, a house with an open system will be safe even when the T&P valve fails to open.

A closed-loop system describes a condition where water in the house can only flow in one direction, due to the presence of a check valve or pressure reducing valve (PRV). The opposite is what describes an open system.

With an open system, expanding water from a water heater can flow back through the cold-water supply pipe thereby bringing down the pressure and temperature of the water in the water heater. If you have a closed-loop system and unfortunately the T&P valve fails, the excess pressure and temperature will have nowhere to go, unless there is an immediate demand for hot water.

If you have a closed-loop system and want to have a back up for the T&P valve, consider installing a water heater expansion tank, also known as a thermal expansion tank.

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A water heater expansion tank is a small tank (about 2 gallons in size) with an air bladder inside, installed on the water heater’s cold-water supply pipe, between the shut off valve and the water heater.

The tank has 2 sections. The upper part is a balloon-like bladder, whose air pressure should be equal to that of the incoming cold water. Its lower compartment is filled with the incoming cold water.

When water in the heater starts to heat up and expand, it flows back to the expansion tank, a move which compresses the air in the bladder instead of leaking out through the T&P valve. In some areas a water heater expansion tank is required by code.

Please not that if you have a hot water recirculation pump/system in your home then you have a check valve which makes your plumbing system close-looped. You will therefore need to install a thermal expansion tank.

People who use water from a well and have a check valve between the pressure tank and the water heater also have a closed-system. A water heater expansion is therefore needed in the house as well.

How to Test a Water Heater Temperature and Pressure Relief Valve

So, how do you know if your water heater temperature and pressure relief valve is bad, or if it is working as designed? It is actually very easy.

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Lift off the lever on the P&T valve gently and check if water drains from the discharge pipe. If water is discharged from the tube when you lift the lever then the valve is working properly. On the other hand, if no water flows out the valve is faulty and will need to be replaced as soon as possible.

What causes a water heater temperature and relief valve to fail? The number 1 cause for a failed T&P valve is minerals deposits especially in areas with hard water which causes it to get stuck. A sticky valve is very unsafe, especially if you have no water expansion tank and have a closed-loop system.

The main reason you need a discharge tube installed on the T&P valve is to direct the hot water/steam downwards and hence safely, instead of it blowing straight on your face. As you test the valve make sure that you are wearing safety boots to prevent your scalding your feet.

Why is my T&P Valve Leaking?

There are 3 scenarios which can lead to a leaking water heater temperature and pressure relief valve:

  • The water temperature and pressure in the water heater are too high, and the valve is just doing its job.
  • The valve is faulty or not properly installed.
  • The water heater expansion tank is waterlogged.

A waterlogged expansion tank happens when the bladder/diaphragm is raptured and as a result the tank fills with water. Since unlike air water cannot be compressed, the water heater will have nowhere to discharge the extra water pressure other than via the P&T valve.

To check if the water heater expansion tank is waterlogged or not, remove the cap at the top and try to bleed off air from the air inlet valve by pressing it down with a finger. If air flows out then the tank is ok.

When you attempt to bleed out air from the tank but instead water flows out, the expansion tank is waterlogged and will need to be replaced. A water heater expansion tank lasts for 5 to 10 years or even longer depending on how well it is maintained.

After confirming that the tank is not waterlogged, the other thing you can do is to check the pressure of the air in the bladder. Connect pressure gauge and compare it to that of the incoming cold water.

If the air pressure in the tank is too low then water is bound to leak from the T&P valve as there will be minimal compression. Use an air compressor or bicycle pump to add air into the tank until it is equal to that of the incoming cold water (usually between 40 and 70 psi).

Also Read: Why is my water heater leaking from the bottom?

How to Replace a Water Heater Temperature and Pressure Relief Valve.

If you have come to the conclusion that your water heater T&P valve is faulty, replacing it is easy and you don’t even need to call a plumber. You just need to buy a new replacement valve which costs between $20 and $30.

The one thing you need to be sure of is the size of valve you need to buy. There are usually 2 sizes of T&P valves; ¾ and ½-inch valves. If you are not sure which size you have, look for a tag on your old valve and you will find all of its specifications there.

Here is how to replace a water heater T&P valve:

  • Turn off the cold-water pipe shut off valve.
  • If you have an electric water heater, turn off its power at the circuit breaker. Turn off the thermostat to the pilot light if you have a natural gas-powered water heater.
  • Attach a garden to the water heater drain valve and direct its other end to a floor drain or out into the driveway.
  • Open the drain valve using a flathead screwdriver.
  • As the water heater drains, keep lifting and releasing the T&P valve to see if water is still flowing out. When water stops flowing out of the T&P valve, the water lever in the water heater will be lower that the valve, and you should then close the drain valve.
  • Disconnect the discharge tube from the valve using a wrench. Put it away.
  • Use the same wrench to also disconnect the valve from the water heater.
  • Inspect that status of the water heater T&P valve threads. If there are mineral deposits or any other type of dirt scrub it off using a toothbrush or wire brush.
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  • Wrap Teflon tape in a clockwise direction round the new T&P valve threads (4-6 wraps) to help create a good seal.
  • Thread the valve on the water heater, being careful not to cross the threads until it is hand tight. Tighten it more using the wrench, but make sure the drain is facing down.
  • Connect the discharge tube to the valve and tighten it as well.
  • Turn on the cold-water supply.
  • Dash to the nearest hot water faucet and turn it on. This will help in flushing out air from the water tank, preventing an airlock.
  • Watch the flow of water from the faucet. At first, water will be sputtering as air is being flushed out but as soon as all the air has been expelled a smooth flow of water will start. Turn off the faucet.
  • Turn on the pilot light or electrical power at the circuit breaker.

And basically that is how to replace a water heater temperature and pressure relief valve. Remember that you should drain your water heater at least once or twice a year. As you do so, test the P&T valve as well to make sure it is working properly.

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