Understanding Water Heater T&P Valves

You might be asking yourself the question of what exactly a temperature and pressure relief valve (T&P valve) on your water heater is. Sure, it sounds complicated but it's a vital piece of equipment for your water heater. If it fails to function properly, your water heater becomes susceptible to excess pressure which can ultimately lead to complete failure. If you're not sure what all this means and why it's important, read on. Below we dive into how these valves work, the purpose they serve, and how to make sure they are always in good working order.

How Do T&P Valves Work?

Just like anything else, when water heats up it expands. In a 40‐gallon tank, water being heated to a set temperature can expand up to 1/2 gallon. The expansion increases the pressure (pounds per square inch or PSI) and excess water has to go somewhere. In most residential water heaters, temperature and pressure relief valves are designed to open when pressure reaches 150 PSI and temperature reaches 210 degrees.

In this way, temperature and pressure relief valves act like an emergency shutoff valve. When water and pressure become too great inside the tank the valve is triggered and excess water is released in to a discharge pipe so that is can safely be disposed of. When water is released through the valve, both the temperature and pressure inside the tank are immediately lowered to safe levels.

Making Sure Your T&P Valve Is Working

If you think your temperature and pressure relief valve isn't functioning the way it should, the first thing to do is to check for water or dampness below the discharge line. If there is noticeable moisture, it's a sure sign that either the temperature or pressure of the water inside the tank is too high and/or the valve is leaking. When this is the case the best thing to do is turn down the temperature of your water heater for a day or two and see if the water disappears. If you don't find any more wet spots, then you most likely had the temperature set too high. If water is still there, however, you probably have a leaky valve that needs to be replaced.

The next thing to check is if the valve is stuck shut. To do this, simply lift the lever on the valve so that a small amount of water can escape and listen for a hissing sound. If no water is discharged and you don't hear the sound, then valve might stuck shut and it's time to replace it.

How to Replace a T&P Valve

While it's never a bad idea to call a plumbing professional for repairs, if you insist on doing it yourself there are some parts and tools you're going to need. At a minimum make sure you have pipe wrenches, a tubing saw, pliers, PTFE tape, sandpaper, PVC glue (if your pipe is PVC), a coupling, and the proper replacement valve.

Step 1: Turn off the electricity for electric water heaters and set the gas control valve to pilot on gas water heaters, then drain all the water out of the tank. Not only does this alleviate any messes, it should be done on a yearly basis anyway as routine maintenance.

Step 2: Open the valve completely to make sure all pressure is out and remove the discharge line. If the valve is on top your tank, you might have to cut the line into sections to unscrew it. If you have to cut your discharge line, save it as you'll be able to put it back together with a coupling and save money by not having to buy a new one.

Step 3: Unscrew the relief valve from the tank with the pipe wrench. If you're having trouble unscrewing the valve, try tapping the end of the pipe wrench with a hammer. Take your time and be gentle with this step of the process, as yanking or twisting of the valve could cause damage to the tank. With the old valve off, you're ready to screw in the new one. Before sticking the valve into the tank, wrap the end in PTFE tape in the opposite direction you'll be screwing it in. Once you have the valve secured, make sure it's facing in a direction that will allow you to attach the discharge line.

Step 4: Attach the discharge line. Just like the valve, you'll want to wrap the end you're attaching to the valve with PTFE tape. If you had to cut the line initially, reattach the second piece with a coupling once you've secured the first section. If your pipe is PVC, you'll need PVC glue and a PVC coupling to attach the sections. On the other hand, if your pipe is copper, a push fitting is best to put the sections back together.

Step 5: Perform some final checks to make sure everything is working properly. First, refill the tank with water, opening a faucet to get all the air out; then open the relief valve make sure it is working as it should. If you hear the hiss of water running, you are ready to restart your water heater. For gas water heaters, check that the pilot light is still on, then turn it to the previous operating setting. For electric water heaters, make sure the tank is filled to prevent element burnout, then turn on the electricity.

Final Thoughts

There's no doubt that the phrase "water heater temperature and pressure relief valve" is a mouthful. It sounds technical and like something that is too difficult for the average homeowner to understand. With that being said, knowing where your valve is located, what it's for, and how it works goes a long way in helping prevent potential damage to your water heater and your home. If you make it a point to check your valve every so often and happen to find a problem, you'll be armed with the knowledge to address the problem in a safe and cost‐efficient manner.


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