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 Shutting Off and Back on in Frigid Weather

Jeff_H Member Posts: 37
With the temperatures in the single digits, I am noticing something that I have not experienced. After the boiler is running for a certain amount of time (I have not timed it but am guessing more than 20 minutes), my boiler is shutting off for a minute or two even before getting to the temperature that I have set on my thermostat. Then it starts back after a minute, runs for a minute or so and shuts back off. And the process continues. Is this short cycling? Should I be concerned so long as I am getting heat? Could this be due to the extent to which my boiler is running in this cold weather?


  • BobC
    BobC Member Posts: 5,479
    In very cold weather the boiler has to run much longer. It sounds like your boiler is hitting the cutoff setpoint of the pressuretrol. That shuts the boiler down until the pressure drops below the pressuretrol cut in setpoint. Do you know wht pressurethe guage displays just as it's about to cycle like this?

    Smith G8-3 with EZ Gas @ 90,000 BTU, Single pipe steam
    Vaporstat with a 12oz cut-out and 4oz cut-in
    3PSI gauge
  • jch1
    jch1 Member Posts: 200
    Like BobC said, it's probably either short cycling due to pressure, but it could be your LWCO that's causing the boiler to shut off. If you have a Hydrolevel CycleGard low water cutoff, it is programmed to shut off intermittently to check the water level.
  • Jamie Hall
    Jamie Hall Member Posts: 23,441
    This is perfectly normal behaviour for longer boiler runs. It is not "short cycling" in the sense of something of concern -- we try to limit that to problems early in the cycle caused by inadequate venting.

    What it most likely is, given the timings, is that the boiler is slightly oversize for the radiation, and the way it modulates to adjust its output to the radiation is to shut off (on pressure) for a short time, then come back on to feed more steam, and so on.

    You should check the pressure settings -- you should be cutting out at 1.5 psi or less. changing the settings won't affect it much, and raising them is a real no-no. If your settings are good, it's OK. Hope for summer...
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • nicholas bonham-carter
    If the oversized boiler is short-cycling with such frequency, then you could explore the installation of a two-stage gas valve (for an atmospheric burner), which would drop down the flame 25-40% as the pressure rose. This would give you longer burn times, which would be more efficient. If this is a large building it would be worth the cost.
    How long does it take for steam to fill the mains? What size is the boiler?--NBC
  • Fred
    Fred Member Posts: 8,542
    At the end of the day, and if it only happens in frigid weather, it is simply the boiler saying "I've delivered all the steam my radiators can handle (Condense) so, I'll take a little break and let them catch up". It makes us a little nervous but it is the way the system is suppose to work. Much better to have the boiler stop while the radiators condense and make more room for new steam that to have it continue to try to pump more steam into them, increasing the pressure. When the boiler stops, it hasn't stopped producing heat (that's the job of the radiators), it has simply said I need to let those rads do their thing until they are ready for more.
    Of course, it may mean the boiler is somewhat oversized but until it has more significant problems and you have to make a decision to replace it with one that is better sized, we work with what we have.
    As NBC said, if the problem is excessive, a 2 stage gas valve will certainly help. If it just happens in frigid weather, most of ours short cycle, to some degree, in those conditions.
  • simmons_plumbing
    Thermostat is calling for heat but has not heated up the living space enough to stop the call for heat but the boiler pressure is going to get up to pressure and start to cycle while the thermostat is calling for heat still it is normal in a system to be oversized moderately or have more than one zone or type of heat transfer and cycle rather than be undersized and take too long to satisfy thermostat. Having someone tell you to adjust the pressure settings that does not know your system is dangerous.

    A system that is set up correctly should not have to run continuously, and should cycle!
  • nicholas bonham-carter
    Just not every minute on and off, as it is hard on the boiler.
    if it were cycling every 10 minutes, it would be more normal.--NBC