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PRV Not Opening

woodshed
woodshed Member Posts: 36
I'm using an expansion tank for boiler feed make up water with a pressure reducing valve. The system expansion tank is set at 15 psi and the PRV is set at 15 psi. I have 55 psi of water in the feed water expansion tank (white). When I lower the system psi to 10 psi or less, the PRV does not open to bring the system back up to 15 psi. I have to increase the PRV to about 25 psi before it would open. Any ideas why the PRV is not opening at 15 psi?

Comments

  • hot_rod
    hot_rod Member Posts: 21,882
    As the pressures get closer together the flow into the system decreases. At startup with 0 psi in the system the Autofill will flow 5.3 gpm as long as you have 30 psi inlet pressure. As the system pressure increases the flow slows. The last few lbs may take a few minutes to show up.

    Here is the actual flow test of the Autofill and several competitors. Notice how the curve flattens out as it approaches stepping pressure.

    We have had cases where then top plastic cover has come off that Autofill, if the gear under it is not placed back in the same position, the indicated pressure may not match the number you are setting it at. See below. Rough shipping caused some to break or pop off. Sometimes the supplier re-assembles them if they find on that came off. Has the top been off that you know of?

    If you set it to 25 psi does it go to the 15 psi and stop? Or will it continue to 25psi? That would indicate if the two are matched.

    Under the hood it is just a spring and diaphragm, the tension applied to the spring determines the shutoff psi.
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • woodshed
    woodshed Member Posts: 36
    hot rod said:

    Has the top been off that you know of?

    If you set it to 25 psi does it go to the 15 psi and stop? Or will it continue to 25psi?

    HR - The top was not off when I installed it. If I set the PRV to 25 psi, the auto fill will continue to 25 psi and stop.

  • hot_rod
    hot_rod Member Posts: 21,882
    Are you sure the expansion tank is in fact exactly 15 psi?
    Bob "hot rod" Rohr
    trainer for Caleffi NA
    Living the hydronic dream
  • woodshed
    woodshed Member Posts: 36
    hot rod said:

    Are you sure the expansion tank is in fact exactly 15 psi?

    HR - The system expansion tank air pressure is 15 psi , the system pressure gauge reads 15 psi and the pressure gauge on the make up water expansion tank reads 55 psi.