After writing a blog post about leaking temperature and pressure relief (TPR) valves at water heaters, I learned that this particular issue seems to completely befuddle folks. In the two years that I allowed comments, that post received 245 comments, including my own. The majority of the comments were questions from readers who were trying to troubleshoot their own leaking TPR valves.
I spent so much time responding to questions on that one blog post alone that I finally disabled comments on blog posts over 90 days old. Responding to reader comments on old blog posts was turning into a part-time gig for me.
The good news is that while answering reader questions, I ended up doing a fair amount of research to help myself understand problems that people were having, and to make sure that the advice I was giving was correct. The purpose of this blog post is to give some troubleshooting advice to people with leaking TPR valves.
A Quick Primer
All water heaters are equipped with a temperature and pressure relief valve. This valve will allow water or steam to escape from the water heater if the temperature or pressure gets too high; these valves are set to open when the pressure reaches 150 psi, or when the temperature reaches 210 degrees fahrenheit. This prevents water heaters from exploding or turning into missiles.
Troubleshooting
The rest of this blog post is going to be about troubleshooting a leaking TPR valve. If a TPR valve leaks, either it's defective or it's not. If it's not defective, it's leaking because the temperature was too high or the pressure was too high. In other words, a leaking TPR valve indicates one of these things: a defective valve, excessive pressure, or excessive temperature.
1. Verify the right valve is installed.
As I mentioned earlier, the TPR valve on a water heater is set to go off at 150 psi, or 210 degrees Fahrenheit. These numbers will be printed right on the valve, or on a tag attached to the valve. See below.

If a pressure relief valve for a boiler is accidentally installed on a water heater, it will leak like crazy from the start. These valves may look identical, but they're set to go off at 30 psi, not 150 psi.
If the proper valve is installed and it leaks, go on to step 2.