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Thread: Presure in Hydronic heat system

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  1. #1

    Default Presure in Hydronic heat system

    I am a DIY homeowner, I installed pex pipe in my garage floor 4 zone loops. I dont have gas so I am using an electric water heater to heat the water. I have the zones connected to a manifold, a 3 speed circ pump, pressure reducing valv and double check on incoming water supply, expansion tank near water heater and check valves on the circ lines. I also have a combo temp/pressure gage on supple and return loops. the supply loop is 120 degrees F return loop 110 degrees F on low speed. My problem is the normal pressure of t he system when pump is not running is around 40 psi. when I run the pump the system pressure climbs to where it exceeds the readings of the guage and pegs at the center of the temp gauge.
    What do I need to do to control the pressure in the system?


  2. #2
    Master Plumber nhmaster's Avatar
    Join Date
    Jul 2008
    S. Maine


    Not sure what you used for a pressure reducing valve but it should have been something like a Watts S1156F or the Tack equivilant. These come factory set at 15 lbs which is about what you want for pressure. If you are getting a drastic increase in pressure when the circulator runs, you have the circulator mounted in the wrong place. It should be on the supply, pumping away from the tank (tward the zones) and past the expansion tank and fill valve.

  3. #3


    The pressure valve is f1201 25-75 psi .
    Pump is mounted on the verticle supply side of the zone loops
    The expansion tank is near water heater on supply side of pump
    I also have an automatic air relief at the high point of system

  4. #4


    Does orientation of expansion tank matter? I have it installed so the writing on the tank is correct, pipe fitting at top air valve at bottom.

  5. #5
    Retired Defense Industry Engineer jadnashua's Avatar
    Join Date
    Sep 2004
    New England


    The orientation of an expansion tank doesn't matter, and the way you have it makes it easier to replace when it fails without getting a showerbath. Note, if the pressure jumps while heating, it could be the tank has failed or lost its charge.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  6. #6
    Like an engineer alternety's Avatar
    Join Date
    Apr 2006


    When you say the pressure is 40 lbs, what exactly do you mean. You have two gauges. Are you reporting both of them at 40 lbs; the difference between the two is 40 lbs. If they are both showing 40 lbs with the circulator off, you have put in too much pressure and need to bleed some. With a sealed system you will not need to add water. Also check to see if you set up the pressure tank properly.

    If both gauges track each other but go off-scale when you turn the pump on, I am not sure what would cause that. Heating the water with a properly sized and operating expansion tank should not cause that kind of pressure increase. Again, investigate the tank. No valve turned off, pressure, size based on system water volume.

    If the gauges do not track each other and only one goes high, your pump is too big for the amount of tubing in the system and is causing a pressure drop across the load.

    If the latter, you can fix it by properly sizing the circulator or adding a pressure bypass to the system. This is done to stabilize pressure when zones open and close. Do you have separate control valves on the Zones? If not your pump is too big for the system. If you do run the zones independently (and since you say there are check valves in the loops, I assume you are) you need the bypass. A bypass is probably cheaper than a new pump. Ideally the pump should provide just the amount of flow needed to supply all loops at once. But even if it too big the bypass will deal with it.

    You may have both problems. Overfilled system and too large a pump.

    A side note. Since it is in a garage, if you are in an area that freezes, you should have properly formulated antifreeze in it. Not automobile stuff.


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