Understanding Relief Valves

understanding relief valves

You may be a bit confused as to what a temperature and pressure relief (T&P) valve on your water heater unit actually is.

Though it may sound complex, it’s really an essential component of your water heater unit.

If the T&P valves malfunctions, then your water heater will be exposed to extreme pressures which may eventually result in a total breakdown.

Continue reading to find out the meaning and importance of all of this, as we explain the workings, function, and maintenance of these valves.

Understanding Relief Valves: Operation

Similar to all else, water expands when heat is applied. In a tank of 40 gallons, water can expand to about ½ gallon when heated to a preset temperature.

This expansion in turn raises the pressure (PSI [Pounds per square inch]) and the surplus water must go somewhere.

Many water heater for residential use have their T&P valves set to open whenever temperature and pressure reaches 210 degrees and 150 PSI respectively.

Here, the T&P valves functions as an emergency shutoff valve.

When the tank is filled with excess water and pressure the TP valve is triggered, releasing excess water into a discharge pipe to be safely disposed of.

This in turn lowers the tank temperature and pressure to safer levels.

Understanding Relief Valves: Function

If your temperature and pressure relief valve has malfunctioned, you should first of all inspect under the discharge line for the presence of water or dampness.

When moisture is noticed, then it indicates excess water pressure or temperature within the tank and/or valve leakage.

If this happens, it’s best to lower your water heater temperature for one or two days and check if the moisture fades.

When there is no more moisture, then it means the temperature was set too high. Otherwise, if there’s still water, it’s likely that the valve is leaking and should be replaced.

You also need to check if the valve is stuck shut.

This can be done by simply lifting the valve’s lever to let out little water and listen for a hissing sound.

If you do not hear a sound and no water comes out, then the valve may be stuck shut and needs to be replaced.

Understanding Relief Valves: Replacement

Contacting a plumbing expert for repairs is a great idea, but if you want to do it on your own you’ll need to get some tools and components.

At least ensure you have a tubing saw, pipe wrenches, PTFE tape, pliers, coupling, sandpaper, PVC gum (for PVC pipes only), and the right replacement valve.

Step 1

For electric heater units, turn off the power supply, and for gas heater units, set the gas control valve to pilot.

Now drain out all the water from tank.

This should be done at least once a year as routine maintenance, as well as lessens confusion.

Step 2

Turn the valve wide open to let all pressure out and disconnect the discharge line.

If your valve is on the top of the tank, then you may need to cut it into sections to unscrew.

If this needs to be done, then have it kept in a way you’ll be able to reassemble.

Step 3

Use a pipe wrench to unscrew the relief valve from the heater tank.

If this is challenging, you can try to use a hammer to tap the wrench’s end.

Ensure you do this gently so as not to damage the tank by twisting or yanking the valve.

Now that you’ve taken out the old valve, the new one is set to be screwed in.

First of all use a PTFE tape to wrap the end in a direction opposite to the screwing.

As soon as you secure the valve, ensure it faces a direction which will let you connect the discharge line.

Step 4

Connect the discharge line in a similar way to the valve by wrapping its end with PTFE tape.

If you initially had to cut the line into sections, use a coupling to reconnect the next piece as soon as the first one is secured.

For PVC pipes, you will be needing a PVC gum and coupling to reattach the sections. However, for copper pipes, you can best reattach the sections by a push fitting.

Step 5

Do some last inspections to ensure everything is in place and works correctly.

First of all, while keeping a faucet open to remove all the air, refill the tank with water; then turn open the relief valve and ensure it’s working properly.

If you can hear running water hissing, then your water heater is ready to be restarted.

For electric heater units, ensure the tank is filled so as to avoid burnout of the element, then turn on the power supply.

For gas units, ensure the pilot light is still on, the set it to how it was previously.


No doubt the water heater temperature and pressure relief valve may seem technical and very difficult to the normal homeowner to deal with.

However, by knowing the position of your valve, its function, and its operation, you can prevent likely damages to your water heater unit and even your home.

If you practice regular checks on your water heater and encounter an issue, you’ll know just how to resolve it in a manner that is cost-effective and safe.

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