Boiler pressure 30 psi!, What do I do??

fm1976February 13, 2010

I have a four-unit building with an older ack-o-matic hot water boiler, 4 taco zone valves, 1 B&G 100 series pump. A month ago, I noticed when the boiler was fired up, the pressure would get up to 25-30 psi and the pressure relief valve would start to drip quite a bit. When the burners turn off, the pressure comes back to 12-16 psi and the relief valve stops dripping. This happened no matter what heat temp the boiler was set at (150 - 180). Fortunately, all 4 zoned units were getting good flow and heat. I first drained the expansion tank (closed the valve to it first and then opened it when I was done). That would make the pressure stabilze for a few days, and then the same thing would happen. There is no hole or drip in the expansion tank. I called an HVAC pro, who did what I did and said it looked good and left, but of course a few days later the problem was the same. I did the same thing 3-4 times, same outcome and eventually even after the expansion tank drainage, the pressure would still get too high a few minutes later. I then drained each unit separately and there was no gurgling and the flow was good. Then I checked the feeder and it would always fill up to 12 psi when I drained some water out of the system. The HVAC pro returned and he tried draining separately from the expansion tank, relief valve, and the bottom side of the boiler itself and in each case no air but the high pressure remained when the burners come on. We also tried bleeding the baseboards in all four units (each room) and there was no air. He kind of gave up and left. Since then, it has been about the same but now there is a small knocking sound and with that I can see the guage on the pressure meter jump every few seconds. I also tried to let the boiler cool, drain some water, let the feeder get it up to 12 psi, close of the fresh water supply before the feeder and let the boiler run. It would still go up to 18-20 but then when the burners were off the pressure would come back down to 6-8 psi. Right now, I left it the way it is but I am very nervous about the high pressure and I am sure it will destroy the boiler eventually. I've has a few HVAC pros and they don't know why it is happening and are looking to replace any and all parts. Anybody have any ideas about what might be wrong or what else to try? Thanks...

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The pressure won't destroy the boiler if the pressure relief valve works, but it adds fresh water to the boiler which will kill it from rust. So, you want to stop that.

It sounds like you have a ceiling hung expansion tank, or multiples. In many cases, opening the drain valve of the tank will not drain it. It becomes vacuum locked because there is only one exit. In stubborn cases, a small hose is inserted through the valve or hole and up to the top of the tank to break the vacuum. The water in the tank must be cold and it makes a mess. The tank must be completely emptied. If it's a continuing problem, I would install an Extrol tank, sized properly. Many times a ceiling hung tank will never give trouble.

You don't seem to have a fill valve problem or a pinhole in your domestic coil, if you have one. Since it responds to the heat/cool cycle, I would completely drain the expansion tank or replace with Extrol(s).

    Bookmark   February 14, 2010 at 6:56AM
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Ditto, change to a extrol expansion tank, also check the fill and see if its clogged, there is a small filter in it that has to be cleaned. Same with your back flow preventer.

    Bookmark   February 14, 2010 at 9:37AM
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