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Pressure swings on hot water system

<i>"Last winter as an experiment I shut off the water supply to the system and within a couple of days the pressure dropped until the system shut down."</i>

From this, I surmise that you have a leak somewhere. Isolate each segment and find that.

This of course goes contra to your stated continued rise in system pressure when heated (and drop when it cools). If you had a leak, that would be a constant pressure reliever too, hence the contradiction.

When you stated that the expansion tank goes to 17 PSI while the system is 25 PSI, that puzzles me because the pressures should be just about equal. The diaphragm tension being the difference usually and that is not much.

Regardless of how you sized your tank, I suspect that the tank is still too small, perhaps you underestimated your volume? Piping and radiators both count. That kind of rise in system pressure, so rapid with relatively modest temperature gains, says so to me. I would add a second #30 tank in parallel to the same connection line and tell me what happens. I would expect to see a leveling off or at least a flattening of the pressure curve.


  • Marc_19
    Marc_19 Member Posts: 11
    Pressure swings on hot water system

    I am experiencing pressure swings on my hot water system. On coldest days the pressure is rising and the pressure release valve is discharging water.

    My system:
    Munchkin 199k BTU with Vision 1, 4 years old.
    Also making domestic hot water.
    Old four BR home with radiators.
    Sytem worked with no problems for 3 years.
    System set for max water temp of 170 on coldest days.

    Last year system began discharging water during coldest days. Bad PRV? Replaced it--still discharged. Then checked Watts-30 expansion tank. Pressure low, some moisture released. Adjusted pressure in tank--then still discharging from PRV. Bad tank? Replaced last month with Extrol 30, sized for my system volume and temps, according to their charts. Adjusted tank pressure to 12 psi.

    Now: I am watching manifold temperature and pressure like a hawk. No water releases yet--no extreme temperatures yet.

    System off: Temp 70, pressure 15 psi.
    Cold morning:Temp 140, pressure 25 psi, expansion tank 17 psi. (Outdoor temp 20.)

    It seems that with my pressure this high on a 20 degree day, the pressure is going to blow the PRV (over 30 psi) again on coldest winter days. Can you help me with what is going on? Does it have anything to do with the system fill valve (Watts 1156F--tag reads "set 12-15 PSI, range 10-25 PSI)? Last winter as an experiment I shut off the water supply to the system and within a couple of days the pressure dropped until the system shut down.

    I appreciate your help. All you pros really gave me some great advice in helping my choose my system 4 years ago.

  • Jim_64
    Jim_64 Member Posts: 253

    My 1st question would be where is your prv located,if it is wrong location you can have a pressure drop in your piping and have the feeder do what it is supposed to do when there is low pressure add water.Now as to why the system lost pressure when you closed valve could be that you had a blob of air in system and automatic air elimanation removed it and there you are no pressure. And if that doesnt do it,I was taught when pressure and temp go up at same time its expansion problem double check your systems water volume again. A picture is worth a thousand words
  • Marc_19
    Marc_19 Member Posts: 11

    Thanks for responses and ideas. Here is a photo of the system. The PRV is inside the unit cabinet, near the "n" in "Munchkin". You can see the copper downpipe. I don't think there is a location option on the Munchkin. Gray tank hanging vertically is Extrol-30. Small white horizontal tank is for DHW recirculation.

    I will also try to re-estimate system volume. Thanks again,
  • Jim_64
    Jim_64 Member Posts: 253
    just a thought

    If you are right about system volume you might want to make sure the line to the expansion tank is clear cant see what it ties to above but i see a old steel supply or return in the picture which means you might have some crap floating around in system. good luck
  • mark ransley
    mark ransley Member Posts: 155

    What happened to me but may be irrelavent, is when we had a new boiler put in they disconnected the expansion tank on the 3rd floor and put one in the basement of correct size. But as the temp went up so did the pressure blowing the boiler relief every day, this never happened before that the pressure increased with temp. My installer a boiler pro could not understand it, I demanded a bigger tank, he put in a second one, and all is well for many years.
  • Rich Kontny_3
    Rich Kontny_3 Member Posts: 562
    Process of elimination

    You are doing everything that I would by replacing the PRV and then the expansion tank. Did you inherit this system? I would look at the highest piping in the system and convert it to PSI (1 ft = .434 #) you may have elevation affecting system pressure especially when heated up. Also a restiction in piping could create inflated system pressures.

    Install an accurate pressure guage and start the system cold,then start monitoring the pressures until the system goes all the way up to high temp. Watch or partially closed valves and pull out zones to isolate the problem.

    Good luck as these can be head scratchers. Let us know what you find! Great post!

    Rich Kontny
  • kpc_40
    kpc_40 Member Posts: 62
    You mention

    the system has radiators... Standing CI rads? If the heatloss is around 125k (you have a 119 Munchie)... you may very well need a bigger EXTROL ie, #60 EXTROL....
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