How the boiler relief valve works.

This is a description of the the relief valve used on hot water boilers designed to relieve excessive pressure.
This video is part of the heating and cooling series of training videos made to accompany my websites: and to pass on what I have learned in many years of service and repair. If you have suggestions or comments they are welcome.
If you are a homeowner looking to repair your own appliance, understand that the voltages can be lethal, the fuels are highly flammable and high pressures are used. Know your limits.

20 Replies to “How the boiler relief valve works.”

  1. If your going to test the boiler safety valve, make sure you have a new one with you & I know it's common sense but never plug the valve to stop the dripping, always replace it!

  2. hey I relieved water from my pressure relief valve now the water wont stop running? what should I do

  3. Hi, Do you think can I used hot water PRV to my live steam engine? If its not. What will happen to hot water PRV or to my live steam engine? Ty

  4. I open the relief valve in my boiler and a lot of water it's coming out, outside my house. It's that bad or good??

  5. Well Water coming out hot from hose…?.. Any idea why..?. Water drops from very hot to warm after two minutes or so.

  6. I tested mine and its been weeping like crazy, and the psi on my tank is 0 when cold and my 2nd floor radiators are not bleeding and beginning to fail. Could this be the problem?

  7. I have also come across them. As for myself, I will not leave a plugged relief valve. So far I have not had anyone object when I told them the dangers.

  8. Unfortunately yes. If the ex tank gets waterlogged, the valve will weep. When you get the tank refilled, it will oftentimes will not reseat. You may also want to replace the pressure regulator.

  9. HI Gary. Great video. Wondering if you would answer a quick question. We've been in our house for 5 years and fired up the boiler to 120F for the the time today and the relief valve started weeping. I emptied the expansion tank and brought the system PSI to under 10 but the valve kept weeping. I've tried bleeding it manually at full blast a couple of times, but still weeping. Seems like it needs to be replaced, does this sound right?

  10. Yes. The relief valve must relieve at the rating for the boiler. If, as in the case of a 50# valve on a 30# boiler, the boiler could rupture. Thanks for the comment.

  11. Also what would happen if the relief valve keeps on weaping if you dont replace it?

    Exellent video and info.

  12. Are relief valves supposed to be the same pressure as the pressure rating of the boiler?
    for example if you have a boiler that is rated at 30 psi and you install a 50 psi relief valve.

    Best regards

  13. I have never cleaned one. These are high liability parts and disassembly is not recommended. I generally do not test these valves because of the weeping problem. I know manufacturers recommend testing, but the design of the valve is such that it is almost impossible to have a failure to relieve unless the valve is damaged when installed. Certainly when first installed it should be tested. Hope this helps.

  14. From your great presentation, it would seem that one could open this valve up and clean the seat–could this be recommended? My valve looks the same and has the four corner screws. It periodically drips for awhile after testing.

  15. The sizing of steam boilers depends on what you are doing with the steam. If its space heating, there is a formula of 1.33 times the loss of the structure. I would not hold a lot of credence to this because there are a number of factors involved. I dont really think I can answer that with any real accuracy. Sorry about that.

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