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Low Pressure Indication in Weil-Mclain Boiler

geller Member Posts: 3
My Weil-McLain boiler supplies hydronic heat to two zones. It does the job - all radiators get hot, including the ones in the attic. However, my pressure gauge on the side of the boiler reads zero, both when the boiler is cold or hot. The water supply valve is set to 15 psi, and even when I let it fill to cutoff, the pressure gauge is zero. The expansion tank gets hot on top when the boiler is running, and is cold on the bottom. I suspect that the problem is the gauge. Is the proper way to check by measuring the pressure in the expansion tank, which should be equal to system pressure?


  • Jamie Hall
    Jamie Hall Member Posts: 21,109
    Won't be the first of those gauges to quit. Provided there is some water and some air in the tank, the Schrader valve on the tank will indicate the system pressure -- but a better choice would be to get a cheap gauge and fit it up so yo can hook it to a drain.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    HVACNUT Member Posts: 5,025
    edited February 2020
    If the attic rads are heating then the pressure is probably ok and it's a faulty tridicator gauge. You can get a 60 psi gauge with a hose adapter to check system pressure.
    A little tip of you don't already know. Is that 4" up there? Holy Cow! If you dont want to have to purge, get a 5 gal bucket and a 6 ft washing machine hose. Isolate the boiler from the system by closing necessary valves and the auto feed. Cap or tape (electrical tape works well) off the Spirovent. Have the new tridicator ready to go. Connect hose to boiler drain and open into bucket. The second the water stops flowing, shut the valve. Replace tridicator quickly. Open the PRV and then quickly remove tape from Spirovent. Wait for any air to vent if there is any. Open valves. Cycle and check.
    With minimal air displacing the water in the boiler, it makes for a quick swap. And tephlon tape is fine and dont go gorilla on tightening it.
  • STEVEusaPA
    STEVEusaPA Member Posts: 6,132
    Screw this or one similar onto any hose bib (boiler drain, purge valves)

  • geller
    geller Member Posts: 3
    Thanks for these suggestions. Will do. I will isolate the boiler and seal spirovent. Only issue that I predict is that the old indicator will not come off without some effort, as I am sure its been there 30 years.
  • unclejohn
    unclejohn Member Posts: 1,830
    You'd be surprised I have changed a bunch they normally come out pretty easy.