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Thermostat vs. Boiler Short Cycling


I am trying to perfect my steam heating system, and I have read in this forum quite a bit about boiler "short cycling".  And I am trying to figure out if my boiler is short cycling or not.

When my system is in heating mode, it sometime happens that the boiler goes on and off several times over a period of let's say 30 mins, until the cycle ends.  However, based on what I have observed, I think it is the thermostat that is turning the boiler on and off, not the pressuretrol.  My reason for thinking this is that I clearly see and hear the thermostat click on and off every the boiler goes on and off.  So, I have two questions:

1.  If my boiler is short cycling, would this cause my thermostat to go on and off, or would the thermostat stay in the "on" position during the "short cycling" process?

2.  If it is in fact my thermostat that is causing the boiler to go on and off, why is it doing this?  There is no radiator in close proximity to the thermostat, and no sunlight or other external factor, so I don't understand why the theromstat would be shutting off quickly.  Is it possible that the heat is rising from the boiler room in the basement through the sheetrock wall to the thermostat and causing it to reach the target temperarture quickly and therefore shutting down?  It's just a theory of mine, but I have no other obvious explanation.

Any help would be appreciated!


  • Jamie HallJamie Hall Member Posts: 6,653
    If you hear the thermostat click...

    and the boiler promptly shuts off or starts up, as the case may be, it's the thermostat.  Much too coincidental to be anything else!  Although, of course, a quick check is possible -- turn the thermostat much higher and see what happens.  Then you might see the boiler cycling on pressure, with the thermostat still calling.

    As to why might a thermostat do this... what kind of thermostat is it?  If it's one of the newer ones with a cycles per hour setting, how many cycles per hour is it set for?  It should be one.  Does it have an anticipator instead (like the older thermostats and some new ones)?  What's that set for, it it has one?

    It is possible, I suppose, for heat to come up from the basement behind the thermostat and upset it that way -- but I have to admit that I'd be kind of surprised if you had a big enough passage for the air to do that (you really shouldn't!).

    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England.

    Hoffman Equipped System (all original except boiler), Weil-McClain 580, 2.75 gph Carlin, Vapourstat 0.5 -- 6.0 ounces per square inch
  • wanttolearnwanttolearn Member Posts: 39

    Thanks for the feedback.  It's a Honeywell digital thermostat.  I looked through the entire manual, and it doesn't make any mention of cyle per hour settings, so I assume there is no such setting.

    what do you mean by anticipator setting?  I know that the thermostat tries to anticipate how long it takes to reach a temperature, and start turing on accordinly, but this setting only applies when I program different times of the thermostat.  In my case, I set it fixed at 72 degrees at all times, so the anticipator should not really come into play.
  • Jean-David BeyerJean-David Beyer Member Posts: 2,530
    Honeywell digital thermostat manual

    I have one Honeywell digital thermostat that has a terrible manual. It says nothing about the parameters that can be adjusted by diddling the buttons on the front. You have to get their installation manual for that, and it does not come with the thermostat. Honeywell seem to think that the homeowner need not know that kind of thing.

    Grope around the Honeywell web site, and if you are lucky,  you may find the installation manual for yours.
  • JshineJshine Member Posts: 19
    model number

    What is the model number of your honeywell thermostat ?
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