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Pressure Reducing Valve adjustement question.

Nick246 Member Posts: 11
I need to lower my prv valve setting to its lowest level (currently it's set at 15psi and my Xtank is set at 12psi), which for my prv is 10psi. My question is that based on my understanding of how the prv adjustment mechanism works (the adjustment bolt either applies pressure to, or relieves pressure from a spring compressed upon a diaphram), I'm concerned that since there is no exact guidance on how many rotations equates to what exact pressure setting, that in lowering the pressure setting if im not careful the bolt will pop out of the valve. Will this happen, or are there built-in in stops that will prevent this?

How about when adding pressure? Are there stops there, for example if i want to increase the pressure to the prv maximum?

Any danger to setting my prv at 10psi with my Xtank at 12psi? I'm inclined to do it simply because i suspect that my system runs at somwhat higher pressure than normal, maybe because my circ pump is on the return and my Xtank is on the fill.

Also, since im not interested in draining and bleeding the system, i figure between the 15psi remaining in the radiator part of the circuit, and the new 10psi in the local part of circuit, it might average down to something a bit closer to 12psi anyway.

Thank you!


  • Jamie Hall
    Jamie Hall Member Posts: 23,433
    edited November 2018
    The static pressure in your system -- your whole system, radiant, boiler, radiators can't be different for different parts of the system, unless you have some valves closed for some reason. The ideal pressure, generally speaking, measured in psi is half the elevation (not distance, elevation) from where the gauge or tank is located to the highest point on the system, plus 5 psi. So if you have a fairly typical two story house, that would usually be 16 feet or so, so 13 psi. Which could happily be rounded off to 15 psi.

    The expansion tank should be set to the same pressure. So should the pressure reducing valve -- and the pressure reducing valve connection and the tank should be close to the inlet side of the main circulating pump.

    I presume from your description that the expansion tank and the pressure reducing valve connection are both on the outlet side of the boiler (you mention "fill" -- but that isn't clear) while the circulator is on the return. This is far from ideal, as the circulator may be running at very low inlet pressures, depending on where the head losses in the piping are, and can lead to a short and miserable life for the circulator. It would take replumbing to fix that, though.

    There is no point to setting the pressure reducing valve to anything other than the tank precharge pressure, assuming that they are reasonably close together. If your expansion tank is set at 12, that's where your pressure reducing valve should be set -- but, honestly, if your circulator setup is as I mention above, I'd be happier seeing the precharge in the tank -- and the pressure reducing valve - both set to 15. The only reliable way to set either one is with accurate gauges.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Erin Holohan Haskell
  • Solid_Fuel_Man
    Solid_Fuel_Man Member Posts: 2,646
    edited November 2018
    You have to isolate and drain a diaphragm type expansion tank to set air pressure. It was 12 psi from the factory, but that was long ago, just like a new car had 30psi in the tires...

    There is nothing wrong with 15 psi operating pressure, in fact a bit higher helps with the less than ideal circulator on the return. But you need to set the ex tank to 15 psi first.
    Serving Northern Maine HVAC & Controls. I burn wood, it smells good!
    EBEBRATT-Ed Member Posts: 15,616
    If your expansion tank is set to 12 psi then the prv should be set to 12 psi.

    12 psi is good for 20 feet so measure from the basement floor to the highest radiator is 20' or less your fine as is. If you want to check the expansion tank pressure you have to isolate it from the system and drain any water pressure from it or remove it from the system.

    Shut off the water feed to the boiler.Then drain some water from the boiler until your down to 10 psi. Then loosen the locknut on the prv and back off the adjustment 2-3 turns. Open the water feed and listen for the water to feed & feel the pipe to see if it gets cold, check the pressure gage on the boiler. Turn the adjustment screw in 1/2 a turn, feel the pipe check the gage and listen for water. Keep adjusting and checking these 3 things until you get to the pressure you need 12psi unless you change the tank pressure.

    Do the adjustment and checking slowly a little at a time 1/2 a turn at a time until the pressure is adjusted. I wouldn't worry about the bolt backing out. may take you 15-20 min to adjust. Let the prv adjust and settle out between adjustments.

    Don't drain too much water out you may have to bleed radiation
  • hot_rod
    hot_rod Member Posts: 22,256
    are you sure of the tank precharge?
    Might be best to start from square one.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream