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Adjusting Pressure Reducing Valve (PRV)

JHMartin
JHMartin Member Posts: 40
edited March 28 in Domestic Hot Water
Hello,
I recently reconfigured the plumbing for one of my two boilers which included replacing the ½” inlet and outlet to the PRV with ¾”. The pressure now reads 28 psig (at only 160 degrees) while previously it read closer to 20. I would like to get it down to around 12 but turning the adjusting bolt doesn't seem to do anything. I have three questions:
1. The PRV manual says to set the pressure when the system is at ambient temperature, but the pressure has always read zero after the boiler is off for a while. What am I missing?
2. With the reconfiguration, I made it possible to bypass the PRV so that I can better purge air from the system. However, I did not put a ball valve on the backside of the PRV. Is it possible I have damaged the PRV by exposing the backside to ~30 psig?
3. I see there is a rebuild kit for this PRV. Is that worthwhile?
Thank you for your help,
Jay (boiler newbie)



Comments

  • Jamie Hall
    Jamie Hall Member Posts: 23,276
    Step one. Check the gauge. The built in gauges sometimes don't tell the truth. You can get a gauge which goes onto one of the drain fittings at a big box.

    That said -- the system should not read 0 psi cold. The cold pressure should, if this is a typical two story building, be around 12 to 15 psi, and the hot pressure maybe somewhere around 20. If that's not true, then the expansion tank may be faulty.

    Now all that said -- what happens when you drain a little water from the system? What should happen is that the pressure should drop to the PRV setting, and the PRV should act to hold it there. Turning the adjusting screw without draining water won't get you anywhere at all -- all that screw does is change when the valve will open.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    JHMartin
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,832
    edited March 28
    Boiler pressure at zero when cold (ambient) is easily fixed by increasing the pressure regulator by screwing the adjustment screw clockwise. That adjustment screw does not look like the original equipment. And there is a jam nut missing.

    Anyway, if you are going from zero cold to 28 PSI @ 160°, then you have a problem with your expansion tank.

    EDIT: Can you post a picture of the expansion tank and the pipe that connects the tank to the boiler system
    Here is the instruction sheet on the B7-12 feeder https://s3.amazonaws.com/s3.supplyhouse.com/product_files/Bell-Gossett-110196LF-Install-Instructions.pdf
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    JHMartin
  • Intplm.
    Intplm. Member Posts: 1,945
    Hello @JHMartin
    My suspicion is that something may have got caught in the valve. There should be a clean out plug on the bottom? Check there.
    If memory serves, turning the nut clockwise adds pressure, counter clockwise reduces the pressure. Adjust accordingly.
    Double check that it is installed in the right direction.
    As to the bypass valve/fast-feed pressurizing the discharge doing any damage, probably not.
    As to the rebuild kit? Best bet is to replace the entire thing. The chances for success are greater if you replace rather than rebuild.
    If the pressure has always read zero the gauge might be broken.
    Put a new gauge on and check the results. Adjust the pressure between 12-15 psi.
    JHMartin
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,832
    I would like to get it down to around 12 but turning the adjusting bolt doesn't seem to do anything.
    . Changing the adjustment screw does not lower the pressure in itself. All that adjustment screw does is change the pressure at which the valve will open to let water in. If the water pressure is already above the set pressure, the valve will stay closed.

    In order top get the water pressure to go down, the water must get cooler and contract. (water expands when heated, it contracts when cooled) OR you must open a valve to let water out to reduce the pressure.

    It appears that the water pressure is falling to zero when cold. That is because the pressure regulator is set below 12 PSI. When the boiler water is cold and you screw the adjustment clockwise, the boiler pressure will start to increase. You want to get the boiler water to 12 PSI when cold (ambient).

    Assuming that the expansion tank is operating properly, when the water temperature increases, the water will expand into the tank and the boiler pressure will not go much higher than 15 or 18 PSI. I suspect that the expansion tank is waterlogged (not enough air in it) and has no room for the water to expand. That will cause the pressure to increase from 12 PSI to something much higher. You may even get the pressure to rise above 30 PSI and see if the Pressure Relief Valve opens to release the overpressure condition. If that happens, then you expansion tank needs to be serviced so that it will accept the expanding water as the temperature increases.
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    JHMartin
  • JHMartin
    JHMartin Member Posts: 40
    Thank you all for the helpful replies. It's been a hectic day. Let me answer a little and then try to take and post pics tomorrow.
    I bought a liquid filled gauge and attached it to the hose bib on the boiler. Surprisingly the original gauge is tracking right along with it.
    The expansion tank was originally 1/2 to 2/3 full of water. During my reconfiguration, I pumped it up to 20 psi. But then it only was filling the top 2 or 3 inches, so I let a little air out, bringing it down to 17 psi.
    When I let a little water out of the system, the pressure drops. But then it climbs right back even though I have adjusted the PRV. I need to try setting it to 12 psi tomorrow when it is at ambient.
    Again, thanks all, and I'll be back tomorrow.
    Jay
  • Jamie Hall
    Jamie Hall Member Posts: 23,276
    The expansion tank needs to be adjusted to the cold system pressure when it is completely empty of water and isolated from the system. Only way to do it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    Intplm.JHMartin
  • Mustangman
    Mustangman Member Posts: 86
    Important point Jamie. You check the expansion tank pressure with the system at 0 psi. If you don't the boiler pressure and tank pressure will be the same. Isolate the tank, then check it. Was the fill valve replaced when the boiler was changed out? That is a B&G B 712 fill valve. Its all brass so that is a plus. At the bottom of the fill valve, there is a plastic or brass cap. With the water off and isolated from the system, you can take the cap off and there is a fine mesh strainer screen. They get clogged, especially if you are on well water. Probably should have a professional take a look at it. Like anything else, taking it apart is easy, its putting it back together and not having a leak is the hard part. We always change fill valves on an install unless the part was recently replaced.
    Good luck
    JHMartin
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,832
    edited March 29
    JHMartin said:

    Thank you all for the helpful replies. It's been a hectic day. Let me answer a little and then try to take and post pics tomorrow.
    I bought a liquid filled gauge and attached it to the hose bib on the boiler. Surprisingly the original gauge is tracking right along with it.
    The expansion tank was originally 1/2 to 2/3 full of water. During my reconfiguration, I pumped it up to 20 psi. But then it only was filling the top 2 or 3 inches, so I let a little air out, bringing it down to 17 psi.
    When I let a little water out of the system, the pressure drops. But then it climbs right back even though I have adjusted the PRV. I need to try setting it to 12 psi tomorrow when it is at ambient.
    Again, thanks all, and I'll be back tomorrow.
    Jay

    There is a major misunderstanding with the expansion tank pressure. You need to start at 12 PSI air pressure in the tank and set the boiler feed cold (ambient) temperature to 12 psi. They both need to match. Also, both the fill valve and the expansion tank need to be connected at the same place, or at least on the same side of the circulator pump.

    With that done properly, when the water starts to expand, from the heat, the 12 PSI expanding water in the system will start to move gradually into the expansion tank. As that water migrates into the tank, the air in the tank compresses gradually. That will cause the pressure to rise gradually. As the water keeps expanding the pressure will rise slowly about 3 to 5 PSI more that the cold pressure.

    Now think of this. If the air pressure in the expansion tank is set at 20 PSI, Then the 12 PSI boiler pressure will need to increase to 20 PSI before any water can migrate to the expansion tank. You may only get to 125° or 130° water temperature before the expansion tank can accept any expanding water. Now you have only a 10 PSI pressure difference in the boiler before you reach the relief valve pressure of 30 PSI. If you had 12c PSI in the expansion tank, you have a 18 PSI difference to accept expanding water.

    If you put 40 PSI air pressure in the tank, then you need to increase the boiler pressure to 40 PSI before the tank can accept any expanding water. But you will never get there because the relief valve will open to let the expanding water out of the boiler.

    Go the other way and put only 6 PSI air pressure in the tank. When you fill the boiler to 12 PSI the ambient temperature water pressure will fill the tank until it compresses the air in the tank to 12 PSI. That will add water to the tank so there is less room in the tank for the heated water to expand. That will cause the pressure to increase at a greater rate because there is less air, so less space in the tank to accommodate the water.

    So to sum up, You want the boiler “cold” pressure and the water feed pressure setting and the expansion tank air pressure to be the same. And that pressure is based on how high the tallest water carrying pipe is in the system. In a 2 story building that might be 12 PSI. and the folks who design the system will size all the components and the expansion tank based on the pressure.

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    JHMartin
  • JHMartin
    JHMartin Member Posts: 40
    I'm not sure if it's a "duh" or an "ah ha" moment, but of course my mucking with the expansion tank pressure has affected to overall system pressure. Makes complete sense. I will demount the tank, empty it of water, set its pressure to 12, remount the tank, set the PRV to 12 (at ambient), and report back.
    Thank you again to all,
    Jay
    Intplm.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,832
    You might find this illustration and the related podcast mentioned in this comment.
    https://forum.heatinghelp.com/discussion/comment/1747354#Comment_1747354?
    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    Intplm.
  • JHMartin
    JHMartin Member Posts: 40

    Step one. Check the gauge. The built in gauges sometimes don't tell the truth. You can get a gauge which goes onto one of the drain fittings at a big box....

    I have always been suspicious of the original gauges so bought this one and some fittings for it. Here they are, mine connected to the hose bib on the front of the boiler. I was thinking they were tracking pretty well, one to the other, but not today.
    Thank you,
    Jay




  • JHMartin
    JHMartin Member Posts: 40

    ...That adjustment screw does not look like the original equipment. And there is a jam nut missing.

    The jam nut is there only temporarily screwed up so high it's out of the picture frame.

    ...Can you post a picture of the expansion tank and the pipe that connects the tank to the boiler system

    Here is how I reconfigured things for now. I replaced all the 1/2" pipe from the city water to this 1/2x1/2x3/4 Tee.
    Thank you,
    Jay




  • JHMartin
    JHMartin Member Posts: 40

    The expansion tank needs to be adjusted to the cold system pressure when it is completely empty of water and isolated from the system. Only way to do it.

    Given the above pic of my expansion tank setup, do I need to physically remove the tank and dump the last bit of water out of it before setting it to 12 psi? Or, can I drain it well enough in place?
    Thanks,
    Jay

  • JHMartin
    JHMartin Member Posts: 40

    There is a major misunderstanding with the expansion tank pressure. You need to start at 12 PSI air pressure in the tank and set the boiler feed cold (ambient) temperature to 12 psi. They both need to match. [huge snip]

    And then,
    Zman said:

    ...system height divided by 2.34 +5 PSI is a good place to start....

    So this formula is for when the system is at ambient and not when operating? (I was thinking it was the other way round.)
    Thanks tremendously,
    Jay
  • JHMartin
    JHMartin Member Posts: 40
    At the request of @EdTheHeaterMan for full system pics, here are two shots of Boiler #1. Boiler #2 is partially visible. I'm leaving him for a separate thread. :#




  • Larry Weingarten
    Larry Weingarten Member Posts: 3,289
    Hi @JHMartin , Regarding your question about needing to remove the expansion tank to set it to the right pressure, you have installed it correctly with a shut-off and drain so it can stay in place when you pressurize it. Just close the shut-off and open the drain when you add air. 👍

    Yours, Larry
    JHMartinEdTheHeaterMan
  • neilc
    neilc Member Posts: 2,703
    just throwing this out there, cuz why do it like the directions read?

    what happens if you lower boiler pressure down to 10, 8, something, keeping pressure on the system, but under the 12 psi air fill,
    then adjust air pressure up to 12,
    if boiler pressure rises back up to 10, 12, or higher,
    then drain a bit more water, under the 10, 8,
    and adjust air back to 12.

    only downside I see is if tank bladder is bad,
    which you should determine first,
    water at air stem ? = tank bad
    known to beat dead horses
    Intplm.
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,832
    JHMartin said:

    The expansion tank needs to be adjusted to the cold system pressure when it is completely empty of water and isolated from the system. Only way to do it.

    Given the above pic of my expansion tank setup, do I need to physically remove the tank and dump the last bit of water out of it before setting it to 12 psi? Or, can I drain it well enough in place?
    Thanks,
    Jay

    What @Larry Weingarten said.

    If there is any water left in the tank, the air pressure will push it out. Then when you put the boiler pressure back on the tank, with air at 12 PSI, as the water expands, there will be lots of room for the water to go. And the pressure will rise in much smaller increments as the air gets compressed.


    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,832
    edited March 30
    At the request of @EdTheHeaterMan for full system pics, here are two shots of Boiler #1. Boiler #2 is partially visible. I'm leaving him for a separate thread

    OMG, we may have another @Jamie Hall, Calling boiler #2 Him! Is Boiler #1 a Her? or perhaps, Non-binary? You may want to have your boilers meet Cedric. Have you named them yet @JHMartin?

    I guess I could name the boiler at my old home BUD (My sons home now). It's a Buderus from the 1990s. LOL :D

    Edit:
    Or "RELRUS" RiELlo budeRUS


    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
    JHMartin