Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Boiler drops pressure when circ runs

Brad White
Brad White Member Posts: 2,393
The circulator is pumping toward the expansion tank (through the boiler incidentally) and that is the root of it.

Move the expansion tank to the suction side of the circulator (make it an Extrol or other diaphragm type) as you said, and be done with it.
"If you do not know the answer, say, "I do not know the answer", and you will be correct!"

-Ernie White, my Dad


  • mike faust
    mike faust Member Posts: 58
    Boilerdrops pressure when circ pm runs.

    Customer called with water on the floor. Oil fired boiler with domestic hot water coil. Boiler was at140 degrees. When circ ran on call for heat boiler pressure dropped to zero or below. Expansion tank seemed to be full. When heat call ended, boiler heated to 160 degrees, pressure rose to 30 lbs., and blew off at safety valve. Component arrangement: Circ on return, water fill 2" down stream of circ, boiler, 5" of riser pipe out top of boiler supply, combo flow check and 1/2" copper expansion tank line to tank overhead. Seems like th circ should be pumping away from that tank?? And that the tank and water fill should be hooked up at the same place??? Can I use this same expansion tank??? Or should I go with spiro vent and extrol 60??
  • ALH_4
    ALH_4 Member Posts: 1,790

    In addition to pumping away from the expansion tank it also sounds to me like the expansion tank has failed. If this is a compression tank, as Brad said, replace it with a bladder tank on the suction side of the pump.
  • chapchap70_2
    chapchap70_2 Member Posts: 147
    What happened when you drained the expansion tank?

    You did drain the expansion tank and run another cycle, didn't you?
  • mike faust
    mike faust Member Posts: 58

    Adding the diaghram type tank and piping it and the fill line in the proper orientation is what I proposed to do.
  • mike faust
    mike faust Member Posts: 58

    How does a compressipon type expansion tank fail, by leaking correct?
  • mike faust
    mike faust Member Posts: 58

    Wellll; The homeowner claimed he emptied it, but it was full when I got their. Someone had recently replaced this boiler; homeowner just purchased the home ( I believe this happened when they began using the heat.)
  • Bet there was no way

    for air to re-enter the tank when he "emptied" it.

    I sometimes wonder if switching to a diaphragm tank is really the best way to go, given that the diaphragm is a moving part and will eventually fail. A plain steel tank fed from an air separator and thru an Airtrol fitting with proper piping between the two, won't waterlog. And it has no moving parts.

    Something to think about, eh?

    To Learn More About This Professional, Click Here to Visit Their Ad in "Find A Professional"
  • Weezbo
    Weezbo Member Posts: 6,232
    Go along with Brads thoughts...

    He is saying it clearly.
This discussion has been closed.