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short cycling

SRC Member Posts: 8
Have two pipe system. Gas fired boiler. Original Thermostat for coal fired boiler, 1931.  Honeywell pressuretrol PA404A.


Have no complaints as far as this system to heat up this house, 4000sq/ft. However, once it reaches temperature and starts to cycle and maintain set temperature it comes on for 30secs off for 3 mins.  Is this a satisfactory boiler cycle?


If not is it the old coal fired boiler thermostat? Who makes a good thermostat to control boilers?

Perhaps a bad pressuretrol?

Could this boiler be that oversized???


Everything seems to work except this cycle maybe too short.


Thanks for any suggestions.


  • RAF
    RAF Member Posts: 65
    I would start by

    First see what is making it cycle on and off is the Thermostat of the Pressure control .
  • Eli
    Eli Member Posts: 42
    edited February 2010
    Me too! Any thoughts out there...

    Two pipe, vapor system (VECO mechanical traps). Cycles on for a minute/off for a minute (can watch the vaporstat bulb see-saw back and forth!). On and off on pressure once up to temp. The system is set for 10 oz. (calculated using TLAOSH), 6oz differential, subtractive on the vaporstat. The system has been working well for years, however I recently discovered the pigtail for the vaporstat was clogged. After cleaning it out, I noticed the short-cycling. The boiler is gas-fired, intermittent pilot. (Weil Mclain EGH-125-PIN, 550,000 BTU) The boiler is correctly sized for the radiation (calculated using TLAOSH). So it appears that when permitted to run to higher pressure (2 psi on secondary control/pressuretrol), there is no shortcycling, however when run to design (10 ozs), there is short cycling. Which is the better situation? Or should I split the difference and allow the high limit to 1psi? Would appreciate any input.
  • Jamie Hall
    Jamie Hall Member Posts: 18,689
    Let's see here...

    It isn't really a satisfactory boiler cycle -- one would really like to see a longer fire/shorter off relationship.  However, you gotta run what you brung, as they say.

    And yes, the boiler could be that oversized.

    It would be very unlikely to be the thermostat.  Thermostats don't usually cycle like that.  Old coal fired thermostats work just fine, usually; they just aren't quite as sophisticated about maintaining and even temperature as newer ones.  On the other hand, they're bullet proof.  Keep it.

    Probably not a bad pressuretrol, either -- just a big boiler in relationship to the size of the system.

    On Eli's question, again that really isn't that abnormal a behaviour for a system after running for a while -- although again, I'd like to see more of a two thirds one third split than half and half.  You could try running a little higher cutout, though I really doubt that it will make much difference.  You do lose some efficiency in cycling like that, but so long as the off time is relatively short the difference isn't much.  Someday we will all have modulating burners on our boilers -- oil or gas -- and it won't be an issue!  (I can dream, can't I?)
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • Eli
    Eli Member Posts: 42
    edited February 2010
    short cycling solved

    Jamie, thanks for your feedback. Regarding my earlier post, I went back and timed the on/off cycles and it was cycling one minute on, 1/2 minute off (at 4 ozs cut-in/10 ozs cut-out). I adjusted the cut-out up and discovered that my system wouldn't build pressure above about 13 ozs (22 inches h20 on my gage). So I set it up for 4 ozs cut-in and 14 ozs cut-out. 13 ozs at steady state (on this day at this external temp) seems to be the balance point of what the boiler puts out and what the radiators can condense. So I'm not fighting that!If you have a short cycle problem on a vapor system, I suggest as a test, setting your cut-out high (2 psi?) and see if you gage reveals a happy pressure (make sure you have a good, precise gage maxing out at a psi or two). Hope this helps someone out.
  • the roots of short-cycling

    i think that if your venting were more capacious [or your boiler smaller], the short-cycling would disappear.  most modern boilers can raise steam more quickly than most people's vents can let it out. until the air is out, the steam cannot be in the radiators, condensing.

    if you were to remove all the air vents in a one-pipe system, and fire the boiler, the steam would be pushing against the back pressure of the  trapped air, constantly short-cycling. as you replace the missing vents, the steam encounters less back pressure, and the short cycles will get longer. when you get to the maximum amount of venting, then the boiler will not short-cycle [unless it is over sized], as all the steam made can be condensed.--nbc
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