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steam boiler cutting out on pressure

BPO1222 Member Posts: 1
I have a 2 pipe steam boiler with a honeywell vapor stat that is short cycling. It is cutting out on pressure after the mains are heated. It runs for about 15 to 20 min fine until the mains are fully heated and the vents stop then it starts to cut out on pressure. Its off for about 1 min then fires for about 3 to 5. The vapor stat is set to 12OZ. Any ideas what could cause this issue. I though it might be the main vents but they seem to be working fine and shut off once the steam get to them.


  • Jamie Hall
    Jamie Hall Member Posts: 23,303
    All that is telling you is quite normal -- for a system in which the boiler can make more steam than the radiators can condense. Which is the case, to a greater or lesser extent, of almost all steam heating systems. In your case, with the boiler cutting out for 1 minute and then running or 3 to 5, the mismatch isn't great enough to be of concern.

    Now if it were doing that earlier in the cycle, then I'd say that the main vents probably aren't quite big enough. But again, in this situation, you're fine.

    Bottom line. The boiler makes steam, but a little faster than the radiators can use it. So it shuts down briefly to let the radiators catch up. Then it starts up again and makes more steam. And so on, until the thermostat is satisfied.

    Be happy. Don't worry about it.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
    EBEBRATT-Ed Member Posts: 15,527

    Check your boiler rating against the raring of all your radiation to see if it is oversized. If it is you may be able to have tour technician down fire it and do a combustion test
  • EdTheHeaterMan
    EdTheHeaterMan Member Posts: 7,838
    I agree with Jamie. I just wonder if you can down fire the boiler any? If it is Gas can you drop the manifold pressure from 3.5"WC to 3"WC I believe that is still a safe operating range? Oil heat can be done with a lower firing rate nozzle, as long as the combustion test indicates a high enough stack temperature and complete combustion. (assuming that is not already been looked at)

    The lower fire rate will make the initial 15 to 20-minute cycle a little longer and the short cycles a little longer. This will result in fewer cycles over the call for heat. As the outside temperature drops the lower fire rate may be closer in line with the system's ability to condense the steam and reduce the cycles even more.

    I was wondering if this is a new phenomenon, or has this happened before?

    Edward F Young. Retired HVAC ContractorSpecialized in Residential Oil Burner and Hydronics
  • dopey27177
    dopey27177 Member Posts: 887
    Based on what you have said on warmer days the radiators do not condense steam as rapidly as on colder days.

    The colder the outdoor temperature is the short cycle will be longer.

    The system is working just fine. (If you set the steam pressure to two PSI the short cycle will be longer)

    Leave the settings alone, you are working OK.