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Hello,I am a retired mechanical engineer with 56 years in this business. I will go in depth with you, and up to a 6 hour time limit.
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try 20 lbs pressure on the extrol tank, let me know how it goes
25 lbs is standard for most systems
If that seems to help for a while, but later the relief valve starts blowing again, it is likely that make up water feed regulator valve is leaking.
Tell me if you are running the boiler as a sealed system with the make up water valve closed, or if you are running with the make up water valve open
Thanks, ***** ***** case your pressure regulator valve seat is most likely leaking slightly, so that the system will not hold steady pressure at say 25 psig for instance... but city water pressure is getting to the system via the leaking valve seat on the pressure regulator valve.
click here for some pictures
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A leaking pressure regulator valve seat in the make up water line is about all that would cause this problem, assuming that the expansion tank is working properly, (does not have a broken diaphragm) and has 25 psig of air pressure in it.
You can test the expansion tank by depressing the schraeder valve core stem on top, if water comes out, the diaphragm is broken. Replace the tank in that case.
Its time to locate the 'rate your expert..' link at the top of your page and rate my work, without a rating I loose my shirt on the question, rating does not affect your charges but is how I earn a living, in exchange I will lock the question so that it stays open and you can get back to me this summer if needed.