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Boiler pressure climbs even when system is shut off

B2161 Member Posts: 3
First, here is a brief description of my sistem and how the problem started. I have an oil fueled hot water boiler hydronic heating. The boiler has a conventional expansion tank, no autofill valve, and two shut off ball valves - one which feeds the hydronic heating and the other feeds DHW.

One day I added a kickspace heater in a return loop. I used monoflo tees spaced about 12" apart. The kickspace heater I working fine. After doing that work, the boiler pressure slowly climbs until the PRV opens. If I drain some water out until the pressure reads 15psi, it takes about a week until the pressure creeps back up to 30psi and the PRV opens. The more i drain the water, the more the pipes seems to gurgle with air in them.

All valves are shut off and the expansion tank is not waterlogged. I put a screw driver to the shut of valve with my ear against it and hear no water moving through.

I attached a few pictures. Any ideas are much appreciated.


  • Zman
    Zman Member Posts: 7,328
    Either your tankless coil is pinholed or your fill valve in not holding pressure.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • NY_Rob
    NY_Rob Member Posts: 1,370
    Is your fill valve an old style valve that uses a washer?

    Could be the washer has deteriorated over time.
  • B2161
    B2161 Member Posts: 3
    When you say full valve, are you referring to an automatic feed? Or the ball valve that I open to add more water to the loop? I do not have an automatic fill valve
  • NY_Rob
    NY_Rob Member Posts: 1,370
    We were both referring to the valve (which you have now told us is a ball valve) that you use to add water to your system heating loop.

    Water can come from that source- if it's leaking when shut off. The other source of new water is the domestic water coil inside your boiler. They sometimes develop leaks and that adds water to the heating loop.

    You have to isolate one or the other to determine the source of the leak. Shutting the make up water supply ball valve is no guarantee it's not still leaking some water into your heating loop.

    Ball valves while better than washer type valves can still leak, and it's easier/cheaper to replace a $10 valve vs a domestic water coil. Since the make up water valve is hardly ever used, you could even use a SharkBite ball valve as a replacement if you don't feel like soldering.
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,687
    Before you go to bed you could isolate the coil, close off cold, open a hot faucet. Reset your boiler pressure, check it in the morning. If it rose, it's air (or your boiler feed ball valve).
    If it doesn't rise, and you open you ball valve to the coil and it rises again, its the coil.
  • B2161
    B2161 Member Posts: 3
    Thanks for your help. I will try replacing the shutoff valves. Could there be any cause of increasing pressure besides water being added? It seems like the ball valves are not letting water through since they are totally silent.

    Steve, if it does not rise, you say it could be air. How would you go about determining that and solving it?
  • neilc
    neilc Member Posts: 1,804
    If you're changing that ball valve fill valve,
    you should really consider adding a backflow preventer there also.
    You really don't want the boiler water that close to your domestic.
    known to beat dead horses
  • STEVEusaPA
    STEVEusaPA Member Posts: 5,687
    B2161 said:

    Steve, if it does not rise, you say it could be air. How would you go about determining that and solving it?

    The usual way. Turn off system, close valve to exp tank and drain. Purge entire system of air.
    Then you have to re-charge expansion tank, opening valve to it and the feed to keep the pressure up.
    Being you have a valve and a union to the tank, I would install this:

    There's even a video.
    Or course, as @neilc said, back flow preventer too.

    EBEBRATT-Ed Member Posts: 11,985
    Check or replace the ball valve that you use to add water to the boiler system. If it is not that it is likely your tank less water heater has a pinhole in it
  • Jamie Hall
    Jamie Hall Member Posts: 18,903
    There is no way that the pressure can rise in a system at constant temperature without something being added to it. Further, whatever is being added has to come from something at a higher pressure.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England