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.

Pressure on cutout and venting

Hello. I’m new here and this forum has been excellent in helping me get my dad’s boiler running better. 

Just a question on pressuretrol settings: The system seems to vent right at .25 psi (stays there for a while before building pressure) and cuts out at .5 psi. The boiler drops below .25 psi during the delay and builds back up again until Cut-Off (1.5psi). Does this have any negative impact on venting of the radiators?

I’m curious if there is any negative impact of allowing the system to drop below the venting pressure. 


  • Jamie Hall
    Jamie Hall Member Posts: 23,324
    Actually this is what you want it to do. This gives the vents a chance to open again, if there is still air in the radiator, and let it out when the boiler fires back up again, so the radiator can heat up more.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Ldallas_87
    Ldallas_87 Member Posts: 18
    Got it! Thank you for the reply. 
  • AdmiralYoda
    AdmiralYoda Member Posts: 629
    The standard 0-30psi are notoriously inaccurate, especially near the min or max readings.

    If you are really interested and want to fine tune the pressure, or at least observe it in finer detail....find yourself a low pressure gauge. 0-3psi should cover it. You have to keep the 30psi gauge to be code compliant, but you can T off of the pig tail and have dual gauges.

    Long story short....for three winters I ignored this advice. Bought a new 0-30psi gauge and was certain I never exceeded 1psi. I installed a 0-3psi gauge and realized I was closer to 3psi, not 1psi. Not ideal at all.

    I now have a 0-20oz gauge and have tuned the system to not exceed about 10oz of pressure.
  • SteamingatMohawk
    SteamingatMohawk Member Posts: 1,007
    As mentioned many times in other discussions, most low pressure gauges cannot handle a pressure surge to 15psi (relief valve setpoint).

    If you insist on keeping a low pressure gauge installed, either get one (like a Magnehelic) that can handle that pressure or install an isolation valve at the inlet to the low pressure gauge.