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pressure fluctuations & dripping backflow

zacmobile Member Posts: 211
I'm having some pressure fluctuations in a heating/chilled water geo system I recently commissioned, when the system is switching from heating to cooling the pressure seems to drop proportionately to the temperature until it hits about 5 psi then goes back to 15 when the system nears the chilled setpoint.  while the pressure is dropping the backflow preventer is dripping from the vent.

the expansion tank & combo feed are tied into the main system return header which is not exactly the PONPC however the header is quite large (3") and has very little pressure drop. the system volume is approx. 300 gal. and the expansion tank is an extrol SX-90V  which @ 44 gal is quite oversized, combo feed is a caleffi 573, system temperature varies between 42-120° F.

Any thoughts?


  • heatguy
    heatguy Member Posts: 102
    check valve

    install check valve after back- flo problem   solved
  • Jean-David Beyer
    Jean-David Beyer Member Posts: 2,666
    Dripping backflow...

    N.B.:Bear in mind that I am not a professional.

    I have trouble visualizing your system, but it seems to me that if your Caleffi 573 is leaking out the vent, then the 573 needs repair or replacement. The backflow preventer is basically two check valves in series with a vent between them, and is built so that the vent is closed provided that the supply pressure is sufficient. I can easily make it close with the pressure of my little finger. If there is some supply pressure, but not enough to close the vent, then some of the supply water would dribble out the vent, but that is unlikely, and the vent would leak all the time.

    Now if the pressure inside your system exceeded the supply pressure, the check valve at the delivery end should close. My guess is that that check valve is leaking when the system pressure exceeds the supply pressure. If that happened, then the water pressure from the system would cause the vent to open to release the excess pressure of the system and not allow the system water to go back into the supply. I do not know if rebuild kits are available, but if not, replacing the valve could be considered.

    Now I have no idea why the system pressure should exceed the supply pressure.
  • zacmobile
    zacmobile Member Posts: 211
    incoming pressure

    I forgot to mention that I already have changed the backflow preventer, I intitially thought it was a bad unit as I have replaced 2 other caleffi ones recently that were a few months old but I think something else is going on here, good call on the incoming water pressure JD that should have been the first thing I checked they are on a well system, however it seems to only happen when the system is transitioning from heating to cooling.
  • Slimpickins
    Slimpickins Member Posts: 327
    on a well, ahhh

    If it's on a well system the problem is more than likely you've lost the charge on the air side of your pressure tank. Easy enough fix, don't waste your money on another check valve.c

    First, shut power off to well, remove lid of pressure switch. Note what pressure rating the switch is. Usually they are 20-40 psi, 30-50 or 40-60. Next, drain off water to pressure tank and then check pressure at schrader valve, I bet it's below the static pressure of your geo system. Next add air to the pressure tank to 2 - 4 psi below your cut in pressure, which is the lowest number. Also a  good idea to put the lid back on the pressure switch with the power off.  Turn water back on and it should stop dripping if the seat is clear. I've seen this often especially with 20=40 switches since it's close to hydronic operating pressures.
  • EricAune
    EricAune Member Posts: 432
    Great advice on the switch

    Unless I am missing something, why not isolate the water supply and leave it off unless needed.

    The assumption would be your system is closed and the water supply is primarily used for filling, therefor, why not isolate it?  That would solve the dripping without any settings changes.
    "If you don't like change, your going to like irrelevance even less"
  • Slimpickins
    Slimpickins Member Posts: 327
    well pressure tanks

    Sure you can shut the feed off if you have a LWCO. Well pressure tanks need maintenance just like expansion tanks on heating systems. I would suspect there are water pressure problems on the house side when taking showers or flushing toilets. 

    Would like to hear from the original poster if he solved the problem.
  • zacmobile
    zacmobile Member Posts: 211
    i'll check it out

    I'll be going back there next week, will post update then. thanks for the advice all!
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