Guage Readings/Pressure relief valves not tripping


Old 06-06-12, 10:57 AM
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Guage Readings/Pressure relief valves not tripping

Ok, found this site and have gained a wealth of knowledge using the search function. Have a scenario that would like others to chime in on...sorry for the long post.

Have a oil fired boiler with tankless coil, expansion tank above boiler. about this time last year, I heard the boiler running for what seemed to be a long time (no call for heat as it was late May/early june) Came down stairs and the temp was reading 220 and the psi was showing appx 35 psi. Quickly shut down the boiler and called in a tech (before I found this site) Tech came out and said the aquastat was bad, so it was replaced and off I went. I asked him about why the relief valve didn't trip, and he indicated that over time they may corrode and not function properly. I asked him if it should be replaced as well and he indicated it was really not necessary.

Ok, fast forward to this year.....had a third zone installed on the boiler for my basement remodel, and I started paying more attention to the guage again. As part of adding the third zone, they installed a new zone valve, pressure reducer?, and also a new pressure relief valve as I had them out there anyway. Got the 3rd zone hooked up, and ran the heat...all was ok. I started looking at the gauge on the boiler, and it was now reading 200-210 and a pressure of almost 40 psi. So, I started to think that the original aquatstat was fine, as if I was at 40 psi then it was highly unlikely that both the original and new relief valves failed to trip. My new tech (same person that installed the zone, good guy) indicated that the guage was ok, but my expansion tank needed replacing. He indicated that when he went to check the rain valve on the bottom of the expansion tank, water began to leak out. I watched a few vids on the tanks, and when I tap the top it is more of a thud and the bottom is more of a, can even a small leak in the tank be causing the high pressure, or more likely the gauge is just shot. (Aquastat was set to 160L/180H and I have turned down to 155/175) After running some tests, it seems like the gauge is reading 170 when the boiler kicks on at 155 aquastat, and then shuts off when it hits 195 on the gauge. Just tired of replacing parts on the boiler. Any thoughts. (I read the sticky on checking the gauge, just not sure if I want to tackle this myself) So, my question....bad tank, bad gauge, both?

Last edited by myethreesuns; 06-06-12 at 01:04 PM.
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Old 06-06-12, 11:13 AM
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Welcome to the forum.
I had the very same thing with similar readings on my gauge and even turned the system down as you did.
I had posted a question about it previously and it was noted that those gauges (PSI and temp) tend to go out of whack. I have yet to replace mine as I am going to be making some adjustments to my system during the summer down time. I suspect that both our systems potentially have bad gauges.
Mine is also an oil burning boiler.
Old 06-06-12, 04:16 PM
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I would replace the expansion tank and, for good measure, the gauge. Verify that the nameplate on the new relief valve says 30 psi.

You are not replacing parts of the boiler, per se, just accessories.
Old 06-06-12, 06:35 PM
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You can verify pressure with a gauge like this. You can buy one or make your own.
2 1/2" Hose Bib Quick Home Water Pressure Test Gauge 0-160 PSI
Old 06-06-12, 07:51 PM
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Yes, you could buy one like that... BUT it would be pretty much useless for low pressure hot water system use. Better off rolling your own:
Old 06-07-12, 06:08 AM
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Thanks folks. Well, he is coming out this morning to replace the tank, and while he is at it I am going to have him replace the gauge as well. (I could make a test one for $25 or so, but a new gauge is only appx $35 so I'll just have him swap in a new one when he does the tank)

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