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compression tank.

Jack_19 Member Posts: 16
On a hydronic system where the boiler is above the cast iron base board radiators when filling and pressurising the system to 12 lbs with the valve to the compression tank closed and the tank empty is better to precharge the tank with say 10 lbs. of air before opening the valve to the tank or will that let to little water into the tank.


  • Brad White_2
    Brad White_2 Member Posts: 188
    The pressures

    should be equal at a cold fill. (I assume you are using a sealed (diaphragm) tank, correct?)

    If you under-pressurize your tank, the pressures will eventually equalize but you will lose tank volume (by letting water in to the tank as you suggest). Water is supposed to get in there as a matter of course. But it will enter and compress the gas volume until the two are equal. As an extreme, if you had 1 psig in the tank, most of the tank volume would be taken up with water if at 12 psig. No room for the thermal expansion, its principal function.

    Rather than use what may be arbitrary pressures, I suggest that you have a top-of-system pressure at least 5 psig for positive air removal. Maybe a little higher, but I do not see you needing 10 or 12 psi (23.1 to 27.7 feet) at the top.

    Because the boiler is high and above the radiation, what I would really want to see is a low water cut-off (LWCO) though.
  • Jack_19
    Jack_19 Member Posts: 16
    compression tank

    Thanks Brad. I'm not using a diaphragm tank, and I have a LWCO.
  • Brad White_2
    Brad White_2 Member Posts: 188
    Good news on the LWCO

    The expansion tank then is the old-fashioned kind that we learned to love like a mother in-law. Always over your head, looking down and occasionally causing trouble, but in the end a heart of gold and did good things behind the scenes...

    With such a tank, I never pre-charged them, the pressure just settled where it did, a function of the altitude (system height), inlet pressure and so-forth.

    So as I see it, under the most controlled conditions of temperature, pressure and other variables, the tank will do as it damn well pleases :^)>
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