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hydronic system-air in lines

I installed my heating system about 10 years ago and it has worked great until last year when I began to get air in the hot water lines. I am able to purge the system but that lasts only a couple of months. Someone mentioned the expansion tank bladder could be not working or the air valve not working. Is there a method to check these for malfunctions or is necessary to replace?


  • Brad White_184
    Brad White_184 Member Posts: 135
    Check the relationship between the expansion tank and

    the circulator...No, not like Dr. Phil.

    The circulator should be downstream of the expansion tank. If it is upstream, there will be a tendency to pull a negative (sub-atmospheric) pressure on the inlet side of the circulator (aka- "pump").

    If you are not "pumping away" from the expansion tank you can draw in air from any automatic vent or any valve stem or joints. (Air can pass in areas that water, being more viscous and more dense, will not.)

  • Royce Gilbert
    Royce Gilbert Member Posts: 1
    Hydronic Heating System - Getting rid of air in the line.

    We recently purchased a 122 year old stone house complete with a hot water heating sytem. Last winter it operated quietly and kept all rooms warm. This year it is making gurgling noises and one of the radiators upstairs is hot on the bottom and cold on the top. I am assuming that there is air in the system.

    How do you add water and/ or get rid of air in a hot water system?


  • RJ_4
    RJ_4 Member Posts: 484

    Look on your exp. tank for a 1 inch plug unscrew this, you will find a tire gauge looking fitting. Push in stem with your thumbnail, if water comes out you have a defective bladder and will need to relace bladder or tank. If you have air check with a tire guage you want 12 psig. If it is low add air, you can use a bicycle pump. IF you have the old style compression tank look for pinhole leaks at top of tank. Both would cause the relief valve to open, causing fresh water to fill boiler through the PRV (fill valve). also check the fill valve strainer if this is plugged or if valve is corroded and pipng to boiler is plugged it will cause valve to overfeed. Make sure the exp. tank is on the suction side of system.
  • Weezbo
    Weezbo Member Posts: 6,232
    might consider ...

    where the fluid is going ,perhaps there is some high point vent creating a vacuum situation...
  • Dave_4
    Dave_4 Member Posts: 1,405
    air leak

    Mark, doesn't the pumping away from the ex tank also pump away from the air remover? If so could it create a negative at the air valve even with the pressure in the system greater than atmospheric?
  • Clyde Edmonds
    Clyde Edmonds Member Posts: 9
    air in line

    When I depressed the relief valve at the bottom of the expansion tank, water came out so there must be a hole in the bladder. I guess that when the tank fills with water, there is no room for expansion, therefore when the temperature goes back down the system so how sucks air into the system. Right?
  • jp_2
    jp_2 Member Posts: 1,935
    neg pressure

    it would have to go below system pressure, not really possible.
This discussion has been closed.