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Boiler has me stumped....

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kane0139
kane0139 Member Posts: 1
in THE MAIN WALL
I turned on my boiler for the season and I thought all was well until I realized the upper bathroom radiator was not heating. I had already bled the system so I decided to check the pressure. It was sitting at around 10psi cold so I decided to add water to the system as it has been a long time and I figured this was the issue. I added water to bring the system to 12 and turned it back on. The radiator heated and I was proud of my work. The next day, my wife called and said water was on the floor in the basement. Upon checking, it was coming from the pressure relief valve on the boiler. When the boiler kicked on in the morning and heated for over an hour, the pressure must have exceeded 30. Later in the day I powered on the the system and the pressure continued to build and it hit 30 again. I thought maybe it was the expansion tank so I was planning to drain it later. Once the system had cooled, I returned to the basement and the pressure had gone back down to near zero. Now the radiator in the bathroom is cold again and the system ranges from near 0 to 30 when it is running full. How is this possible? Could it get that much pressure if I have a leak? Would it still help to drain the expansion tank? I’m perplexed. Any expert advice is appreciated. 

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  • Ironman
    Ironman Member Posts: 7,376
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    If the expansion tank is actually a compression tank (no bladder), then drain it and and replace the relief valve. Then set the fill valve to 15 psi and bleed the radiator(s).

    If it's a bladder tank, replace it.

    You may also need a new fill valve.
    Bob Boan
    You can choose to do what you want, but you cannot choose the consequences.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,834
    edited October 2020
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    As you already know, water expands when it is heated and contracts when it cools. The expansion tank needs to contain enough air to compress, in order to let the water expand to that tank.

    That said, Draining an expansion tank may be misunderstood by many. You don't just want to let water out so the pressure gets to zero. Draining the expansion tank should more correctly be called "Adding Air" to the expansion tank.

    Picture an expansion tank where all the Air was replaced by water. "Yes, it could happen". Now there is no room for the water in the boiler to expand. Now you remove the only air in the system, the air in the bathroom radiator. The boiler is cold and everything is full. You turn on the boiler and the water heats and expands and there is no place to go, SWOOSH The relief valve releases the excess pressure. all over the basement floor.

    The first step is to Get Air Into "Drain" the expansion tank. See if that solves your problem.

    I have a unique way to do this task. I have taught it to everyone who worked for me and I have explained it here
    https://forum.heatinghelp.com/discussion/comment/1620527#Comment_1620527

    Hope this helps.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    MaxMercy
  • MaxMercy
    MaxMercy Member Posts: 508
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    EdTheHeaterMan said:


    Picture an expansion tank where all the Air was replaced by water. "Yes, it could happen". Now there is no room for the water in the boiler to expand. Now you remove the only air in the system, the air in the bathroom radiator. The boiler is cold and everything is full. You turn on the boiler and the water heats and expands and there is no place to go, SWOOSH The relief valve releases the excess pressure. all over the basement floor.

    Excellent explanation!


    EdTheHeaterMan
  • PerryHolzman
    PerryHolzman Member Posts: 234
    edited October 2020
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    While water does expand and contract with temperature - its not a large amount - and normally should not vary your pressure by more than a few Lbs.

    While it may be just the expansion tank as per the previous posts; it may be another issue.

    Trapped air in the system pipes that gets heated and cooled by the actual circulating water expands and contracts a lot more.

    I'd eyeball your piping system and look for high points that don't have vents that could have trapped air in them.

    I'm actually dealing with this issue myself as I just installed a 0-30 PSI pressure gauge on the system (with a new expansion tank) and I could see the pressure varying by several PSI based on a 20 F temperature change on the system. It is also obvious that the air scoop was installed in the wrong place by the contractor years ago (on the low point system supply (after the pump) - and the high point on the return piping would obviously trap air when the system is filled. Note my monoflow T system pipes are gently slopped upward about 1/2" for each side of the house. So the return pipe is about 2" higher when its piped back to the Low Loss Header.

    I wish you the best on this,

    Perry

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