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Varied Formulas For Static Fill Pressure Calculating

D107 Member Posts: 1,814
edited August 2019 in Gas Heating
Will have a chance Thursday to check/adjust/correct boiler static fill pressure and diaphragm expansion tank air pressure setting. I have already had tank size discussed in a prior thread. Pressure setting is primary focus here. I want to make sure I have a good grasp of this:

1-Static fill pressure and expansion tank pressure settings --which should match--are based totally on the distance from the expansion tank connection to the top of the highest radiator in the house. As far as I know water volume doesn’t enter into pressure calc, though Wessel's calc below does seem to factor that in. (?)
2-The size of the expansion tank must take into account the total volume of system water.
3-The higher the pressure setting of the expansion tank, the less room for expansion.
4- There are various pressure-calculating formulas:
A. From Caleffi Seminar: Height x .433 + 5. In our case 16 x .433 +5 = 12psig
B. Wessel: with 75 gallons total heat/dhw system, low water temp of 50º, 16ft height, Initial pressure recommendation is 17psig. Tank size recommended 5.4 gallons diaphragm tank. If I raise low water temp to 70, tank size should be 5 gallons. Our existing tank is Amtrol Extrol EX-30 which holds 3.4 gallons.

5-On sizing, our situation is a little unusual in that we have these considerations, each of which factor in varying ways:
A. Atmospheric Boiler, 70mbh, 50mbhNet. 5 gallons boiler plus 20 gallons system = 25 gal.
B. Heating water temp range from 70º to 160º. Boiler set to cold start—at least for summer.
C. 48 Gallon Turbomax Reverse Indirect (not used as buffer) DHW priority water temp range 140-160. So less expansion concerns.
D. The pros have recommended calcing for the worst case scenario, where there is a heating outage on the coldest night of the year and all 50 DHW gallons and and 25 heating gallons have to be heated from 50º.
E. 2/3 of the radiation is cast iron radiators which will throw some relatively cool return water back into the boiler which I’m guessing slows down the rate of expansion a bit.
F. Also mitigating expansion requirements may be the small firing rate of the 70mbh boiler?

Our heat loss is about 35K, just north of NYC. Current static pressure is set to about 16, which agrees with Wessel but not Caleffi. Aside from the worst case scenario it would seem our Amtrol-30 will work 99% of the time. The Amtrol-Extrol EX-60 has 7.6 gallons which we could install if necessary, or add another EX-30 if need be as a prior thread advised. This summer season system pressure gauge runs 20-22 psig on DHW cycles.

The key issue for me is whether to have the static pressure lowered to Caleffi’s figure of 12psig, which seems adequate for building height and would also leave more room for expansion.


  • STEVEusaPA
    STEVEusaPA Member Posts: 5,966
    All the standard stuff:
    First think is to make sure the gauge is accurate. Your pressure shouldn't run up that high if you have a properly sized tank and no air in the system.
    The tank pressure needs to be checked isolated from the system. Hopefully you put on a Webstone expansion tank valve to easily accomplish the task.
    If all your numbers are correct, the 30 is/should be fine.
    Don't forget a lot of air can hide in those cast-iron rads.
  • D107
    D107 Member Posts: 1,814
    @STEVEusaPA Thanks, and pressure setting 12 or 17psi? Based on height alone should be 12.

    When they flush, refill can they purge without operating the burner? We’re in a heat wave. Prefer just to run the circs.
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,966
    edited August 2019
    I’d set it at 12. You don’t purge with the circs running. With cast iron rads, I’d purge all radiators cold, keeping the pressure up, run the boiler up to temp with circs on, shut everything off, go around and check/bleed all cast iron rads once more, making sure pressure doesn’t drop below 12psi. I doubt your house will warm up much if at all.
    Or do initial bleed/purge, then do it again like above when it’s cooler out.
    EBEBRATT-Ed Member Posts: 13,087
    I think your overanalyzing a bit. The only thing that determines the fill and expansion tank pressure is the height of the building at the highest pipe. You need to maintain 4-5 psi at the high point....that's it. Weather you arrive at this by calculation or by filling and checking matters not a bit. Once you know the fill pressure set the prv and the expansion tank air pressure to this determined psi. The expansion tank must be isolated and drained or removed from the system to set the air pressure. An oversized expansion tank is ok. If your tank is undersized the temp/pressure rise could cause the relief valve to open.

    with a 30lb relief valve I wouldn't want the pressure rise to get over 25 psi, I would rather see 20