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Pressure drop (no leak) when system is running, increasing when off.

Ciupaga_PL1 Member Posts: 7
edited January 2023 in THE MAIN WALL
I'm loosing pressure when my system turns on and is circulating water.
From gauge to top radiator is about 36 feet and I don't believe they're circulating (rental units) .
Gauge is on the suction side of the pump, expansion tank is on the outflow side (i know that's bad), all of this is at the feed side of the system.
It drops about 5psi when it's off.
How do I know if it's circulating properly or not? I have to menual bleed system also.one radiator on top floor doesn't want to purge, no water coming out just air, it hisses for about 20 sec and stops  no water, rest top floor okay. 

Picture 4 off
Picture 6 on


  • Jamie Hall
    Jamie Hall Member Posts: 23,635
    To be clear, the pressure seen on the gauge drops when the circulator turns on? That's what the title says, but that's not what you say in the discussion.

    If the pressure drops when the circulator turns on, with the gauge where it is and the expansion tank where it is, that's exactly what should happen. The circulator creates a pressure difference, and since the expansion tank is the point of no pressure change, that means the pressure at the inlet to the pump -- where the gauge is -- has to drop.

    Now as I read that gauge, it says about 12 psi. That's not enough pressure to reach the third floor -- you need an absolute minimum of 15 psi, and 20 would be better -- when the pump is running.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • hot_rod
    hot_rod Member Posts: 22,472
    The distance from where that gauge is located to the highest heat emitter in the building, what is it?
    Maybe 24' X .433= 10.3. add 5 psi do 15 psi at gauge is the static fill pressure

    In your case when the circ is running it subtracts the pressure differential from the discharge side.

    You would measure that with a gauge or calculate it knowing the piping circuit and pump curve.

    Ideally you would move the expansion tank and 15 psi static would work.

    Remove this air vent, since it is valved off, it is not doing anything. Connect the expansion tank above that valve. Now you are pumping away. Assuming you open the valve :)
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream