Welcome! Here are the website rules, as well as some tips for using this forum.
Need to contact us? Visit https://heatinghelp.com/contact-us/.
Click here to Find a Contractor in your area.

Boiler pressure rising, new xp tank and new prv

Hi guys, figured I’d reach out because I’m getting progressively more and more stumped on this issue...

I have a boiler we installed well over a year ago. New trim kit, brand new everything (prv, backflow, Air scoop, xp Tank etc). Dunkirk Cast iron with radiators. And my boiler pressure keeps reaching 30 psi and tripping the relief valve. I’ve checked the Expansion tank and the diaphragm appears to be in tact (no water coming out when I check pressure). I shut the ball valve on my incoming water supply to see if it was an issue related to the pressure rising due to a bad reducing valve, but the pressure keeps rising regardless. Any thoughts feedback or input is appreciated. I didn’t see this thread anywhere because like I said I’ve already checked the Expansion tank unless I’m missing something (which I am obviously - the source of the pressure change)... thanks!


  • CTOilHeat
    CTOilHeat Member Posts: 56
    Does the boiler have an indirect water heater or a tankless coil? Either of these could be leaking slowly allowing house pressure to bleed over to the boiler side.

    High pressure always finds low pressure.
  • wasabi85
    wasabi85 Member Posts: 3
    One other piece of information... from room temp cold not firing it goes from 15 psi to 20, then creeps from 20-30 over the period of about an hour.
  • Zman
    Zman Member Posts: 7,542
    I would check the sizing of the expansion tank then check the precharge pressure with the tank isolated.

    The indirect or tankless coils are also likely suspects.
    "If you can't explain it simply, you don't understand it well enough"
    Albert Einstein
  • wasabi85
    wasabi85 Member Posts: 3
    4.4 Gallon / standard 30#, precharge 12psi, charge when running climbs but is at 22psi now matching the boilers pressure... no sidearm, this is a strictly heating only system... wondering if the Expansion tank is too small because it’s a radiator system with a lot of water in it?
    EBEBRATT-Ed Member Posts: 15,323
    Respectfully all your statements can't be true:

    Heating only, no tankless or indirect heater means the only way water can enter the system is your MU water and prv.

    You said from room temp cold and NOT firing that the pressure builds from 15-20-30 over an hour (this would show a leaking prv)

    You said the pressure keeps rising even with the city water is valved off. (this would show a bad or undersized expansion tank


    So not knowing here are the possibilities

    So, with no tankless or indirect:

    1. Prv leaks by

    2. Expansion tank is too small

    3. The expansion tank has lost it's air pressure. You must check the pressure on the air side of the bladder with the water side of the tank isolated and drained (0 pressure)

    Also, the air pressure in the bladder & the outlet set pressure from the PRv must match

    Do you have some type of check valve , flow check or zone valves that can prevent hot water from the system/boiler from getting to the expansion tank??

    Got any pictures??
  • Paul48
    Paul48 Member Posts: 4,469
    My house...........Gravity conversion, operated for many years with a #30 expansion tank. When the boiler failed, and I replaced it with a smaller boiler, I began having problems with pressure causing the PRV to discharge. I replaced the #30 with a #60 and the problem went away.

    Just one possibility, and may not be your problem.
  • Big Ed_4
    Big Ed_4 Member Posts: 2,700
    Radiators , pressure increases with temperature rise .. You need an larger expansion tank .. Twin in another #30 same location or replace with #60
    I have enough experience to know , that I dont know it all