What is a Water Heater TPR Valve?
A Temperature/Pressure relief or TPR valve, also called a T&P valve or popoff valve is a safety device required to be installed on all water heaters. Its purpose is to open and relieve pressure from the water heater in the event that the water heater becomes over-pressurized. Without a relief valve, an over-pressurized water heater can explode causing serious injury, property damage, and even death. Multiple things must go wrong with a water heater in order for it to become over-pressurized, so it is a rare event that requires the TPR valve to actuate, but in order to increase the safety within our homes, TPR valves have been required since the 1960’s.
What Does a TPR or T&P Valve Do?
A TPR valve is installed in the upper six inches of the water heater where the temperature will be the highest. Most TPR valves are designed to open and relieve pressure in the water heater if either of two conditions occur: if the pressure reaches 150 pounds per square inch (PSI) or if the temperature reaches 210° Fahrenheit. Most water heaters are designed to withstand pressures up to 150 PSI, so the TPR valve is designed to prevent the water heater from rupturing.
Why is a Rupturing Water Heater So Dangerous?
A rupturing water heater may not sound so dangerous. All it’s going to do is allow water to run all over and damage flooring and walls. Right? This may sound bad enough, but that is only a minor danger of a rupturing water heater.
The BIG danger is that you have a tank full of 30 – 50 gallons of superheated water. Superheated water is water that has been heated above its normal boiling point (212° F), but because of the higher pressure in the water heater, it does not boil, but remains a liquid. If you have ever camped in the mountains and cooked or boiled food, you know that cooking times are different in the mountains than they are at sea level. This is because the atmospheric pressure is lower at higher elevations which makes the boiling temperature of water lower in the mountains.
Higher boiling temperature of water at higher pressures is the concept behind a pressure cooker. The lid is sealed tightly onto the pressure cooker, so as the temperature rises, the steam that is produced is trapped in the pressure cooker. The trapped steam causes the pressure to increase. The increased pressure then causes the boiling temperature to increase. This higher boiling temperature causes your food to cook much more quickly.
Below is a table showing the boiling point of water at various pressures. (Data taken from www.engineeringtoolbox.com) From the data below, you will see that in an over-pressurized water heater, the water temperature can be as high as 359° F. This is more than 100 degrees above the normal (at atmospheric pressure) boiling temperature of water.
Pressure (psi) Boiling Temperature
14.7 212° F
20 228° F
40 267° F
60 293° F
80 312° F
100 328° F
120 341° F
150 359° F
Here is what makes a rupturing water heater so dangerous. When any liquid, including water, changes from a liquid to a vapor, it expands many times its original volume. In fact, water will expand and fill a volume about 1600 times its original volume. So, when an over-pressurized water heater ruptures, the pressure immediately drops to normal atmospheric pressure (14.7 PSI). This will cause the superheated water, to instantly change from a liquid to a gas (from water to steam). When this happens, 40 gallons of water will become 64,000 gallons (40 x 1600 = 64,000) of steam in less than a second. There is another word for this – Bomb! (Watch this video to see a rupturing/exploding water heater.) Here is a news story about a home that was destroyed by an exploding water heater.
The explosive power of expanding steam is why we don’t want our water heaters to rupture, and this is why we need a properly functioning TPR valve installed on our water heater. I am not trying to scare you. It is a very rare event that a water heater ruptures and explodes, because several unusual circumstances must occur in order for this to happen. However rare, it is still important (and simple and cheap) to protect against this happening.
Requirements for TPR Valve Discharge Piping
Discharge piping is required to be installed on the water heater relief valve in order to carry the hot, discharging water and steam safely away without causing damage or injury to someone who happens to be standing nearby. There are many requirements for the discharge piping on a TPR valve. Here is a brief summary of the requirements.
- It should be made of material designed to carry hot water so that it will not melt or be damaged by the water flowing through it. Appropriate materials include copper, CPVC, polyethylene, or galvanized piping.
- The discharge line should not be any smaller that the outlet of the relief valve at any point along the discharge line. This is normally ¾ inch.
- It should be as short as possible so as not to impede flow. It also cannot have any valves or “T” fittings.
- It must not be threaded on the end so as not to allow a cap to be screwed onto it.
- The discharge line must terminate safely. This means that it should terminate near (within 6 inches) the floor, into a waste receptor, into a floor drain, or to the outdoors (within 6 inches of the ground). It should not discharge where it can cause injury to someone.
- The valve also must discharge where it is visible to the home’s occupants. This allows the homeowner to know that it is discharging or leaking, and that corrective action needs to be taken.
- It must not be “trapped” (have a section of pipe that can hold water) or be connected to the home’s drainage system. Both of these conditions could allow for a cross connection which could cause contaminated water to be sucked backwards into the home’s potable water system. (See my blog about cross connections). The photo to the right shows a discharge line with a “trap”.
- It should be installed so it can drain by the flow of gravity. It must be able to drain completely of all water. (It cannot be sloped upward at any point which would allow a portion of the pipe to continue to hold water after the discharge is complete.) The reasoning is the same as for number 7 above. The water remaining in the pipe could sit there for months, becoming contaminated. If the home’s water pressure is lost and the TPR valve is leaking, then this contaminated water could be sucked into the home’s water supply.
The complete requirements can be found at (www.nachi.org/tpr-discharge-piping.htm)
How to Test the TPR Valve on Your Water Heater
It is important to test your TPR valve regularly to ensure that it is working properly. Testing it is a simple process. It is recommended that you test your TPR valve at least quarterly to ensure that it is working properly. To test it, simply raise the lever on the TPR valve for a few seconds and ensure that you hear and see water flowing freely out of the discharge line on the valve.
If do not hear water flowing when you test it, or if you only get a trickle of water, then the valve needs to be replaced. If you have some basic handyman skills, this is a simple thing to do. If you have any doubts about your ability, please contact a licensed plumber to do this for you. (NOTE: If there is no discharge line on the TPR valve, do NOT test it until you have had a proper discharge line installed. Testing the valve without a proper discharge line could allow you to be burned by steam or very hot water. A discharge line will safely direct the hot water and steam away from the valve.)
Sometimes, testing the TPR valve can cause it to leak. Corrosion, mineral deposits, or other buildup in the water heater sometimes get in the way and prevent the valve from seating properly after testing. Often simply opening and closing the valve a few times will clear the blockage and allow it to seat and stop leaking. (Keep in mind that some water may continue to drip out of the pipe for several minutes after the test just because the interior of the pipe is wet, so give it a couple of minutes to make to make sure it is or isn’t leaking.)
If it continues to leak after opening and closing it a few times, here are a couple of other things to try.
- Shut off the water to the water heater and then operate the lever on the TPR valve a couple of times. Turn the water back on to the water heater and check for leaks.
- If #1 did not work, tap lightly on the TPR valve with something solid such as the handle of a screwdriver, wrench or hammer. Remember, I said tap LIGHTLY.
- If the leak still doesn’t stop, then the valve will need to be replaced.
Here is a great link to more information about leaking TPR valves. https://structuretech1.com/leaking-relief-valve/
So now you know what a TPR valve is, what it does, why you need one on your water heater, and how to test it to ensure that it is working properly. Testing it occasionally to ensure that it is working, and replacing it, if necessary, is a small and simple thing that you can do to ensure that your water heater does not become a danger to your family. If you have any questions about TPR valves or about this article, please leave a comment or feel free to contact me.
© 2020 Mike Morgan
This article was written by Mike Morgan, the owner of Morgan Inspection Services. Morgan Inspection Services has been providing home, septic and well inspection services throughout the central Texas area since 2002. He can be reached at 325-998-4663 or at firstname.lastname@example.org.
No article, or portion thereof, may be reproduced or copied without prior written consent of Mike Morgan.