Replacing a Pressure Relief or T&P Valve

March 19th, 2011 by KTU | Filed under Notes on Approaches, Pennsylvania Church House.

This is not an interesting topic. However, I couldn’t find what I needed for this little project on Google, so figured I’d create a post that is likely to be found when people are searching for a solution to a leaking relief valve.

Your water heater has a T&P valve, which stands for Temperature and Pressure Valve. This valve opens when temperature and/or pressure exceed a pre-defined threshold. This is a safety feature so that your water heater does not explode if the pressure goes too high. I have a Weil Mclain Superstor indirect water heater and the T&P Valve is on the top, as shown below. It usually is designed to open at 210F and/or 150 psi, which is waaay above the safe operating temperature and pressure of a domestic water heater. Watts is the leading brand of these devices.

The T&P Valve on a Water Heater (the bronze thing with a 90-degree angle and shiny lever on top)

Your boiler (for your forced water heating system) also has a relief valve. However, this one is just a pressure-relief valve. It usually opens at 30 psi, which is well above the system pressure of most forced-water heating systems. So, while your water heater has a T&P valve, your boiler has a P valve. The valve on my Weil Mclain Ultra is shown below.  The valves in my system all are bronze with 3/4″ threads.

A Watts Pressure Relief Valve (with green tag) above a Weil Mclain Ultra Boiler.

In Pennsylvania, where I live, these valves seem to last about five years. They either corrode or the thermal expansion element seems to fail. Below you can see the inside of a pressure-relief valve and see that the spring-loaded valve seat is corroded.

Corroded valve seat on pressure-relief valve.

It’s not good when these things fail. Of course for safety reasons they are designed to fail open, which means that water starts to flow out the valve and onto your mechanical room floor. Sometimes they just weep or drip. This is typically the way the P Valves fail. Sometimes they spew violently (more typical of the T&P valves.) In a new house, you’ll have some nice floor drain right below the valve. In a 19th-Century Stone Church you have a lousy floor leading to a sump and you’ll probably have a big mess. Indeed, when my T&P valve failed open a few weeks back, I believe that hot water sprayed onto my floor for a day or two before I noticed dampness in the adjacent room. Anyway, replacing one of these is no big deal if you have basic plumbing skills. You buy the replacement (a Home Depot item) and you unscrew everything and replace the missing valve. On a water heater, the fittings may be sweated (soldered) copper, in which case you need a torch and soldering/de-soldering skills. Installed correctly, a T&P valve has an open length of pipe pointing at the floor so that hot water doesn’t burn a human standing nearby.

On a boiler, the installation is pretty much the same. I was lucky enough to have two shut-off valves on either side of the boiler, so replacing the valve took 3 minutes. The valve at Home Depot had a 3/4″ female fitting on it, so it required a 3/4 nipple to connect to the tee on the boiler outlet. Use teflon tape or pipe dope on the threaded fittings. The replaced part is shown below (towards the right side and attached to the galvanized pipe with the teflon tape showing).

Replaced Pressure Relief Valve on Boiler, installed with a nipple to get the genders to match. Technically this should have a piece of pipe on the bottom fitting pointing down.

So, when you have water on the floor of the mechanical room, some possible culprits are the T&P valve on the water heater or the pressure-relief valve on the boiler. They’re both pretty easy to replace once you know what to look for and what to ask for in Home Depot. (There are other potential culprits too, like leaking hot water heater tanks or leaking heat exchangers in the boiler.) Of course, if you have never done any plumbing, call your plumber. But, you may be able to save a return trip by telling him or her in advance what you think is dripping/leaking/spraying.

(Here’s an update to this post. This valve started to leak again, and I concluded that the reason was that the expansion tank in the hot water system was bad. I was correct. If the expansion tank fails, then when the water heats up in the system, the pressure will build up and cause the valve to open. That’s another issue to check if you have some leakage from your T&P or P valve.)

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2 Responses to “Replacing a Pressure Relief or T&P Valve”

  1. Jeff says:

    Good article! One thing I would like to point out is that MOST building codes these days require that the boiler pressure relief valves be mounted VERTICALLY, unlike the ones shown in your pics. There are reasons for this which I won’t elaborate on here, but if you want to do it RIGHT, when you re-install the new valve, purchase a STREET ELBOW and install that first, then mount the relief valve VERTICALLY and complete by piping to the floor. It is also good practice to routinely replace these valves every five years as you have stated.

    Thanks again for the good write-up!

  2. Jeff says:

    Forgot something… it’s a good idea to recommend to your readers that they SHUT THE BOILER OFF AND ALLOW TO COOL TO 100°F OR LESS before starting work. 180°F water coming at you at 30 PSI can KILL YOU! They should also drain the pressure in the boiler by first closing the manual water feed valve and opening a boiler drain to relieve the pressure. If the new valve is prepared for installation, one can usually work fast enough to not have to drain the entire boiler. It’s usually a pain in the you know what to re-fill a boiler. The less water you take out, the better!

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