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Thread: Boiler getting overpressurized -- reaches 30 PSI and PRV releases water

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  1. #1
    DIY Junior Member BrianW's Avatar
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    Default Boiler getting overpressurized -- reaches 30 PSI and PRV releases water

    I just bought a house in August that has a boiler. Iíve never had a boiler before Ė what little I know, Iíve learned from reading the excellent posts on this forum, as well as a few other websites. I have a problem with pressure, and would really appreciate some help.

    Hereís the brief description of my problem: My boiler runs at a constant 16 PSI during the day when the heat isnít on. At night, when the heat kicks on, the boiler runs up to 30 PSI and triggers the pressure relief valve, draining water out. It also makes a fair amount of noise Ė some of the pipes are making loud knocking sounds, and thereís a much quieter knocking coming from inside the boiler. Based on what Iíve read, Iím guessing that most or all of the symptoms Iím experiencing are because the system is becoming overpressurized. Iím trying to figure out why.

    Other information: Itís a Burnham P209. Itís installed in a closet in our finished basement. A note written on the wall in the closet has the date 9/12/88 and indicates that it should be run at 23-25 PSI. I presume 1988 is when it was installed, although I donít know that for sure. The house was expanded significantly around the same time Ė 7/20/88 is when the building permit for the expansion was granted. It would make sense that a new larger boiler would accompany the expansion. The house is 3600 square feet, three floors, and has a four-zone heating system.

    I have an Amtrol Extrol 30 diaphragm expansion tank. From what Iíve read, overpressurization when the water is being heated seems to most often be related to a problem with the expansion tank. I did a simple test on the tank, touching the outside of it while the system was under high pressure. Roughly the top 1/4 of the tank was hot to the touch, and the bottom 3/4 was cool, indicating that the bladder is still holding air. I donít know of any other quick tests I can run on the expansion tank.

    Thereís a pressure and thermometer gauge on the outgoing pipe, right after the water comes out of the boiler and just before it goes through the expansion tank. Thereís also a pressure and thermometer gauge on the left side of the casing around the boiler. These two gauges give very different readings Ė the gauge on the pipe reads higher for temperature, and the gauge on the boiler reads higher for pressure.

    When the boiler heated up tonight, the gauge on the pipe got up to 31 PSI and 210 degrees. The gauge on the boiler reached 38 PSI and 185 degrees. When the gauge on the pipe hits 30 PSI, water starts getting released by the PRV, which is rated 30 PSI, so that seems to confirm the PSI reading from the pipe gauge. I donít know which temperature reading is correct.

    Around 200 degrees on the pipe gauge, the boiler shut off, but the temperature continued rising to 210. After that, it dropped down to about 160, and the boiler went back on, taking the temperature back up to 195 (175 on the other temperature gauge). The pressure again went up to 30 PSI, releasing more water. Itís not a lot of water getting released each time Ė Iíd guess less than a cup.

    There are two things I suspect may be going wrong here:
    1. Primary issue: The system is getting up to 30 PSI, triggering the PRV. My quick touch test seemed to rule out a complete failure of the expansion tank Ė the tank isnít full of water. What could be causing the pressure to rise so much?
    2. Secondary issue: Temperature probably shouldnít be reaching 210, so it seems like thereís a problem there too. Could be that the temperature gauge on the pipe is reading too high, or it could be that I need to lower the high temperature limit on the aquastat. (The aquastat is a Honeywell L8148E.)

    Iíd really appreciate any help on what to try next to diagnose and solve this problem! Thanks a lot!!

  2. #2
    In the Trades Tom Sawyer's Avatar
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    210 is way too high for the limit. Drop it to 180 or 190. Touching your expansion tank means nothing. Tank pressure needs to be tested by dropping the system pressure to 0 and then checking the tank. It should be around 12 - 17 lbs.
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  3. #3
    Retired Defense Industry Engineer jadnashua's Avatar
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    Unless you have an unusual situation, most residential boilers run around one atmosphere - 14-16 pounds...running it nearly double, so close to the relief valve setting, is counter productive. IF the expansion tank is intact and sized properly, it should easily keep the pressure within the static, cool pressure range. If your pump is not sized properly, or there's a restriction, it's possible they set it up that high (in pressure) to keep the boiler from flashing to steam because the water isn't passing through the boiler quick enough and getting too hot on the pass. If that's the problem, it needs to be addressed.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  4. #4

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    The banging and noise you notice could be a result of the boiler making steam. As mentioned above, lower the temperature to 180.

    Another check of the expansion tank is to tap the top of the tank with a wrench, it should have a dead sound. Hit the bottom of the tank and it should ring just a bit. If you get a dead sound from the bottom part of the tank it means that the tank is full of water and the bladder has failed.

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    DIY Junior Member BrianW's Avatar
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    Thanks a lot for all the replies!

    The aquastat is wedged in between the boiler casing and the wall, so it was hard to get to. With a headlamp and a mirror, I can now see that the aquastat has a high temp setting that can range from 180 to 240 degrees. It was already set at the minimum, 180. Despite that, I've seen the temperature on the gauge read as high as 210, so I don't know if the aquastat or the gauge is faulty.

    I tried the "tap a wrench on the expansion tank" test. The tank has a dead sound on top (as expected), a hollow sound if I tap it on the side near the bottom, and a dead sound if I tap it right on the bottom. Does that mean it's likely that the tank has failed? If so, it sounds like the next step to take the system pressure down to zero and measure the tank pressure? Any other suggestions?

    Thanks again for all the help!

  6. #6
    Retired Defense Industry Engineer jadnashua's Avatar
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    Assuming it is a screwp-in temperature gauge, when you have the presure drained off, get a new one and install it. If you take an instant read meat thermometer and wrap some insulation around it to hold it right on a pipe, after a few minutes, it should give you a close approximation of the temp. An IR thermeter would work, but kind of expensive for a one-time use!

    The aquastat may be defective. On the expansion tank, it's probably shot. If you turn the autofill off, then momentarily open the valve on the bottom of the tank, if you get any water out, it's shot. A really shot burst of air won't change things much, but I think you'll probably get some water. Also, after you get the cap off, it may be obvious if it is all rusty underneath, another indication (not always, though).

    There are only a few reasons the pressure would rise on its own: expansion tank shot or non-existant; a defective fill valve; a bad indirect WH or tankless coil. The easy one is probably the expansion tank, and the most common. You can often replace it without tools. All of the other choices require tools and would probably cost more to fix.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  7. #7

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    Your tank sounds normal. I just went down and checked my tank (which is only two months old)...

    Top 1/3 I get a dead "clank"
    Bottom 1/3 I get a ring
    Bottom of tank near the air valve I get less of a ring, but a ring none the less

    I think it would be odd to get a ring from the bottom 1/3 and no ring at all at the bottom. A failed bladder is a failed bladder.

    Regarding the zero pressure test, turn off the boiler, close the auto fill ball valve, and then isolate the zones with the ball valves. From there you'll need to open the bleed valves for the zones. You aren't looking to drain the zones, just release pressure. The water will flow for a few seconds at a rapid rate and then drop-off. Close the valve when it drops off. After the pressure is released from the zones the pressure gauge on the boiler should read zero.

    Now you can check the bladder pressure. If you get water from the valve, you've got a bad bladder.

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