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Boiler issues

mjwag300 Member Posts: 1
When I first fired up my boiler this year I noticed some water leaking but it seemed to have stopped so I assumed the system had purged some water but self corrected.  

As the weather got cold I went and checked and the leak had happened again.  After some review I noticed it was coming from the pressure release valve.  It was old and heavily corroded so I thought maybe it just had some buildup and wasn’t working properly so I replaced that.  I gave it a few more days and the leak persisted but only when the heater is on.  I’ve looked at the gauges and pressure seems
to be high while it’s operating, usually 30-40 PSI.  

I’m wondering if the high pressure is due to a faulty expansion vessel and how I could tell before just replacing it.  It’s not an expensive part but I’ve never done that before and feel I’m starting to reach the limits of my knowledge so don’t want to make things worse.  

Also, there is some air in the system that I haven’t bled out of the baseboard heaters as I was waiting to finish the repairs to avoid having to do that multiple times.  However now I’m wondering if the air in the system could be causing the increased pressure.  

Any thoughts or advice are welcome. 

Thank you! 


  • Steamhead
    Steamhead Member Posts: 16,723
    Could be the expansion tank or something else. You really need a pro. Where are you located? We might know someone............
    All Steamed Up, Inc.
    Towson, MD, USA
    Steam, Vapor & Hot-Water Heating Specialists
    Oil & Gas Burner Service
  • Jamie Hall
    Jamie Hall Member Posts: 22,916
    Actually any air left in the system is probably helping, not hurting...

    It probably is a failed expansion tank. Replacing them isn't all that hard -- sometimes -- but if you are concerned about your ability to do it, it's something a good plumber or heating person should be able to do fairly easily.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    HVACNUT Member Posts: 5,744
    It's usually not hard if the boiler is piped correctly. But it could be something else. 
    Can you post pics of the entire system?
    EBEBRATT-Ed Member Posts: 15,317
    Chances are it is one of 3 things:

    1.Bad expansion tank
    2.Make up water pressure reducing valve leaking by
    3.Leaking coil from a tankless heater or an indirect tank (this one is only possible if you use the boiler to make Domestic Hot Water)

    To check #2 manually shut off the make up water to the boiler and dain some water out until you get to 15 psi.

    1 If it is the expansion tank you may get water out of the schrader valve (looks like a tire valve) on the EX tank.

    If you change the tank yourself be cautious they weigh a ton when they have failed they will be full of water.

  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,563
    So you decided the relief valve was bad because it was letting water out when the pressure increased above 30 PSI setting. You did that because you did not know to look at the gauge and see if the water pressure in the boiler was higher than 30 PSI that the relief valve operates at.

    So with that information, you believe the expansion tank may be the problem. Good guess, but you don't want to spend money on a new tank if that is not going to fix the problem. Another good choice.

    There are 2 different types of expansion tanks and they are fixed and tested differently. Tanks that come pre-charged with air have a bladder or diaphragm that separates the air side from the water side. They must be checked with no water pressure on the boiler side of the tank. First lower the water pressure in the boiler below 8 PSI. (work with me here professionals) then using a tire gauge see what the air pressure is inside the tank. Must be between 12 and 15 PSI. If you have only 8 PSI, then you are measuring the water pressure in the boiler. Add air to the tank until it gets to 12 PSI. If you get any water from that air valve or the pressure does not go up to 12 PSI without all the air bubbling into the boiler and pipes, then you need a new tank. If the tank holds 12 PSI air pressure when the boiler pressure is at 8 PSI (or less) then the tank is good and you can let the boiler operate normally.

    The other tank is the compression tank usually found in the rafters above the boiler somewhere. If that gets waterlogged then you must remove all the water until the tank is empty of all water. If there is a valve between the boiler and the tank you can usually turn that valve off so the boiler water will not go into the tank while you let ALL the water out. Not just the water pressure, all the water must be removed so the tank is full of AIR. That is the air cushion that you need for your boiler to operate properly. To let all the water out you need to open a valve or remove a plug so the water drains out, and let air into the tank through the same opening or a different opening. Some tanks have a special valve that has a tube running up to the middle of the tank. The valve lets the water out and the tube lets the air in.

    SO, What type tank do you have?

    And to rule out a problem with a DHW heat exchanger coil... What makes your hot water? The heating boiler or a separate water heater.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics