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Show short is a \"short cycle\"?

Tom Hopkins
Tom Hopkins Member Posts: 552
When you set the pressure so low on the pressuretrol, you can expect the boiler to come on and off more often. since you are setting it so low, I would recommend a vaporstat instead of a regular pressuretrol as this will take a little longer to come on and off with a vaporstat. the vaporstat is much more sensitive to such low psi. If you have any doubt that a radiator valve (main vent) is leaking, I would change it.


  • Tombig_4
    Tombig_4 Member Posts: 45
    How short is a \"short cycle\"?

    I have a single pipe steam system, and have recently cleaned and replaced some air vents, changed the settings on the pressuretrol to 0.5-1.5, and also cleaned out the pigtail the pressurtrol is attached to. Now, the system is cycling due to the pressuretrol getting activated, which is good... I think. Here's what it does:

    From a cold start, the system runs until all the radiators are good and hot, and the vents close. So far so good.

    The pressuretrol triggers, and turns off the burner, for about a minute, maybe less. Then the pressuretrol turns the burner back on.

    The burner runs for a few minutes, then turns off again. Repeat.

    The radiators are hot, so this seems reasonable, but... my question is: how long should I expect my system to stay off due to pressuretrol cycling? What's a reasonable length for a normal cycle for a system in a single family house after the radiators are hot?

    I notice that my main vents (and maybe some radiator vents too) seem to hiss slightly as the furnace runs and after the furnace goes off, perhaps they're not sealing completely -- I can imagine that this might be lowering the pressure quicker than ideal? Is it worth replacing main vents to try to address this?

    Thanks for any help, reading through this forum has already helped me a lot!
  • Jamie Hall
    Jamie Hall Member Posts: 22,892
    Doesn't sound like a problem

    Or at least I hope not; that's about the way mine works and has for years!

    That type of cycling is, as you say, to be expected; the short 'off' section of the cycle is due to the steam condensing, which is just what it should do, and dropping the pressure. Not a venting problem. Since the boiler is staying nice and hot, you aren't losing much in efficiency.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Tombig_4
    Tombig_4 Member Posts: 45
    Sounds like my system is pretty normal...

    though I'll keep the vaporstat in mind. Given the price for a new Honeywell vaporstat ($200+), I suspect I'll do without for at least the rest of this winter... thanks for the feedback!
  • Lot of factors.

    Your boiler might be oversized, and you might want to check your thermostat’s heat anticipator as well.
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