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Bad expansion tank. replace or not

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STEAM DOCTOR
STEAM DOCTOR Member Posts: 1,968
in THE MAIN WALL
Good afternoon. I have a customer with a converted gravity hot water system. The (undersized)m expansion tank is shot. The circulator is on the return(i.e. wont raise system pressure). At the highest point of operation the pressure in the boiler doesn't go above 22psi. Is it necessary to replace the tank? The home is two stories with fill pressure at 12 psi. The system has been blead. Thank you.

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  • Paul48
    Paul48 Member Posts: 4,469
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    OK

    The high volume in that system is finding some place to expand. That situation may change, and probably will, this winter and you'll be getting a call that the boiler is discharging. When my boiler was replaced (same type system as that), with no means of removing air automatically, there were no issues other than bleeding radiators occasionally.When a spiro-vent was put on, the boiler began going over-pressure. The #30 expansion tank was fine, just too small. I put a #60 on, and no more problems.
  • icesailor
    icesailor Member Posts: 7,265
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    Closed Systems:

    On a closed heating system, the pump does not add pressure to the system, no matter where you put it. The water in the system is in equilibrium. Water expands when heated. That's why you need to know how much water is in the system, and the range of water temperature that the system to be run.

    The pressure gauge usually has an altitude section on it. That tells you how high the water is above the gauge. System pressure is controlled by the pressure reducing valve setting.
  • Gordy
    Gordy Member Posts: 9,546
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    Radiators

    May have air absorbing some of the system expansion. And yes you need to replace the tank.



    The circulator creates pressure differential to move water. By moving the tank to the return on the circ, or by moving the circ so the tank is on the inlet side the differential will not drop below the tank pressure on the return side of the circ. simply put pumping away from the point of no pressure change.
  • hot_rod
    hot_rod Member Posts: 22,143
    edited October 2012
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    here is how a circ moves the fluid

    From Modern Hydronics Third Edition. I believe it was also an old B&G training diagram.



    The red line indicates the 10 psi fill pressure, pump off. The green line shows the pumps added pressure and how the pipe and fittings in the loop "scrub away" that added head.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • Roland_18
    Roland_18 Member Posts: 147
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    Bad or not?

    Just curious, how did you determine that it's bad? If this is a bladder type tank, maybe it lost it's charge of air. Just dealt with this same issue myself. Tank seemed to be full of water. When I removed it, I found out that the initial charge had leaked out.  I am not going to re-use it as the bladder may have been weakened.
  • Paul48
    Paul48 Member Posts: 4,469
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    If

    the charge leaks out, it's scrap. IMHO
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