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point of no pressure change

guys im working on a large water soure heat pump job.
its an old school building ,three stories high
I was sent there to do a start up on some new boilers.
I noticed some problems right away.the main circ pump is
pumping to the air seperator and expansion tank instead of away and the boilers are on the discharge side of the pump.
so as you can imagine there are and always have been air issues.I was rereading my PUMPING AWAY book. and it clearly states that the pump cannot affect the PONPC so the pumps diff pressure will show up on the suction side.
when I shut the pump down to see what the static fill pressure was I see abt 15 to 20 pounds on the boiler pressure gauges ,which I dont think is enough to fill this system to the top floor. but im here to tell you. when I turn that big circ on it is adding pressure to the discharge side.THe expansion tank seems small for the volume of water that would be in this system,do you think maybe the expansion tank is full of water? this system has a new pump that is bigger than the old one. the contractor that did the piping installed suction and discharge gauges but neglected to actually drill holes in the pipe IM gonna do this on monday. any help or comments would be great
thanks folks
mike terry


  • jim lockard
    jim lockard Member Posts: 1,059
    You said Boilers.

    Like more then one in this system? So is there one expanison tank location for the system or 1 for each boiler? Mike you can only have one X-tank location for the system, the one thing you cannot have is too many X-panison tanks just keep them at the same location. If you have too small of an expanison tank you will blow the relief valve. System pressure should be about 1/2 psi per foot of height to the top of the highest rad plus 5 psi. Make sure you have the correct size relief valve you may need a 50 psi valve. Best Wishes J.Lockard
  • michael terry
    michael terry Member Posts: 30
    it has

    50 psi reliefs on the two on the two lockinvar boilers
    system has one 60hp circ pump that moves 65 to 85 degree water through out the building to a bunch of water source heat pumps there is also a closed loop cooling tower.
    there is one expansion tank that sits on the floor of the mechanical room all the piping (4inch mains} is about 12 feet above the x tank. I know that the best thing to do is move the air seperator and x tank and the boilers to the suction side of the pump ...an expensive propisition.
    Im seeing the pump add to the PONPC so im wondering if im correct in my assumption that the x tank is full of water or too small or both
    thanks for your reply
  • Aidan (UK)
    Aidan (UK) Member Posts: 290

    The pump should not affect the pressure at the PONPC. To change the pressure of the air in the expansion tank, you'd have to expand or compress it & this would require a change in the volume of the water in the system. Water is incompressible, so should only expand or contract due to temperature.However, this is based on an assumption (NB) that the water-side is full of water.

    There is a history of air problems with this system; I would think that there is air trapped in the system. The assumption is wrong in this case. This would be similar to having another expansion tank on the top floor. When you switch the pump on, the pressure on the return side drops, the trapped air on the top floor expands and the air trapped in the expansion tank is compressed. You're seeing a change in pressure at the expansion tank because the volume of the water-side increases(although not the volume of the water).

    You could check the expansion tank but I think you need to sort out the air problems a.s.a.p. since it will be causing corrosion problems. I think moving the expansion tank (or just the connecting pipework) would be the first step; maybe just calculate the correct size of tank and buy & fit a new one.

    This link might be useful.
  • Tundra
    Tundra Member Posts: 93

    The bigger the structure the more problems you will have with pumping towards. In a one bedroom home it is not right but of little consequense. On a three story building it is a bit more of a problem. My customers pay me for fixing the problem. I correct the mistakes of my competitors and take away their customers. Check out the Xtank, move the pump, and do what else you know to be right. No customer wants the bill but they would rather pay for having a problem fixed Vs. paying a lesser amount to not fix the problem.
  • michael terry
    michael terry Member Posts: 30

    to all that have replied. the more I think about it the more I think theres not enough water in the system to begin with,thats in addition to the components being in the less than ideal location. before I opened valves to the boilers the system had a cavitation or air noise sound that would come back to the pump about every 5 minutes or so.the guy that works for the city has been fighting this thing for a while and I still have not been able to find the person who decided it needed a bigger pump and why.
    maintenance guy made a comment about using pump to get water up to the third floor when they filled it. there is supposed to be a unit on the roof that taps the loop. Im going up to the roof and see if i can open a pipe or valve during the static fill and see if this helps my problem until I can convince someone that these piping changes will benefit everyone
    mike terry
This discussion has been closed.