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Thread: Adjusting Expansion Tank Pressure

  1. #1
    DIY Member Soapm's Avatar
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    Default Adjusting Expansion Tank Pressure

    I had to change the fresh water pressure reducing valve on our boiler at the church. Thanks to reading this forum, the boiler was staying at 30lbs with constant seepage from the PRV. I found I could shut off the fresh water supply and the system would stay where ever it was set for days. Open the fresh water supply and the system would slowly rise over the course of a few days until it hits 30lbs and the seeping starts.

    While I had the system down I changed the reducing valve, expansion tank and PRV. However, not knowing what I was doing I let the air out of the expansion tank trying to check the level. I did it with the system pressurized so it was reading high and I kept trying to reduce it to 12lbs and now it's out of air (my bad).

    I would hate to have to open the system and purge it again since that took about 4 hours to get all the air out of all 5 zones.

    If I turn off the fresh water supply then open the PRV and let the system go down to zero, is that sufficient to adjust the expansion tank? Does opening the PRV introduce air to the system? I don't know the normal process to adjusting the expansion tank but taking it off the system means completely draining the entire system since there is no shutoff valve going to the expansion tank.

    Also, the new reducing valve holds the boiler right at 20lbs. It used to sit about 15lbs. We have two floors, basement and upstairs with 5 zones. Is there any reason to adjust the reducing valve down to 15lbs or can I get by leaving it at 20lbs. I prefer to leave it at the factory setting if there is no potential harm.

    Lastly, if the new pressure reducing valve holds the system is at 20lbs, where do I set the expansion tank? That's why I was looking at it, my logic was if it's set at 12lbs then it will stay full since the system will always be at a higher pressure. My logic is it needs to be set the same (20lbs) or slightly above the system pressure (22lbs). Am I on the right track?

  2. #2
    DIY Senior Member kcodyjr's Avatar
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    1st of all, I think you might have your mords wixed. The reducing valve and PRV (pressure reducing valve) are the same thing. Do you mean the TPR (temperature & pressure relief) valve is the one that seeps above 30psi? That would make sense.

    2nd, no, I don't see why you should have to drain the system down, just get it to zero pressure. Turning off the supply and briefly opening the TPR should do the job just fine. No need to even remove the tank.

    Setting the expansion tank pressure is indeed confusing, but high school physics says it doesn't need to be exact. If it's set slightly lower than system pressure, there will indeed be an immediate influx of water, which will compress the air in the bladder, raising its pressure. Flow will stop when the bladder pressure matches the system pressure. If it's set slightly higher than system pressure, a bit of expansion will occur before any flows into the tank. No big deal either way.

    Edit: brainfart. Only use the TPR to release pressure if it's the only option. Safer to open an ordinary valve if one is available.

  3. #3
    In the Trades Tom Sawyer's Avatar
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    Is the expansion tank a bladder type or just a tank?
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

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    DIY Senior Member kcodyjr's Avatar
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    Good question, Tom. Do they really make expansion tanks without bladders, or are you thinking some other type being misused?

    Wouldn't the air in the tank be absorbed by the water, just like in an old-school air chamber?

  5. #5
    DIY Member Soapm's Avatar
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    Quote Originally Posted by kcodyjr View Post
    1st of all, I think you might have your mords wixed. The reducing valve and PRV (pressure reducing valve) are the same thing. Do you mean the TPR (temperature & pressure relief) valve is the one that seeps above 30psi? That would make sense..
    I did my best, my pressure relief valve doesn't sense temperature, just pressure (I think) which is why I thought it would be a PRV or pressure relief valve. But yes that is the one that would seep but she's rock solid at 20lbs now. The other one is a pressure reducing valve which would also be a PRV. It makes more sense the way you put it (TPR).

    Ok, so if I set the expansion tank around 20lbs then I should be cool. It'is a bladder type.

    The other valves on the system are used to control flow and one to purge the system so I was afraid to open the purge valve for fear of introducing air into the system. The other ones wouldn't reduce the pressure??? If the TPR works then I can use it. I took an old (probably 15 or 20 yo) bicycle pump to the church today and if I had any air in the tank I sure don't now. Never could get it to pump any air into the tank so I'll have to get a new pump Thursday and try it again.

    Thanks...

    Ps... Any reason to change the pressure reducing valve back down to 15lbs or is it cool to leave the system at 20lbs?
    Last edited by Soapm; 12-31-2013 at 03:34 PM.

  6. #6
    In the Trades Tom Sawyer's Avatar
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    Water pressure should be between 12 and 15 lbs

    Expansion tank at 15 lbs
    [B]No, plumbing ain't rocket science. Unlike rocket science, plumbing requires a license[B]

  7. #7
    Retired Defense Industry Engineer jadnashua's Avatar
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    The autofill valve and therefore system pressure is determined by how high the water must be pumped. If this is all on one level, or up to two, 12-15psi is fine. The ET pressure should be set to the system operating pressure. Note that the air pressure you read in the ET is always the same as the system pressure UNLESS the system pressure is removed. Because you need the air to expand, when adding water, you must have the system open, and some water will be released as you add air to the ET. Once you have it at the desired pressure, then you can close the valve, and maybe add a small amount of water to restore the pressure - it won't take much...water does not compress, and the pipes don't expand at 12-15psi pressure, either.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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