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Short Cycling

Jim_90 Member Posts: 6
I have a recently installed Weil-McLain WGO 3 oil boiler with a Honeywell L8148A 1017 aquastat set at 170. The system has an Amtrol indirect hot water heater and 4 heating zones with 3 Honeywell Chronotherm IIIs (T8602 series about 18 years old) and one T87F. My problem is that, apparently at random, many times a day, the burner fires for brief periods of time ranging between 10 seconds to a minute and then shuts off. This can’t be long enough for a call from a thermostat to be satisfied and never happened with the old boiler. During the short cycling I have confirmed that the boiler water temperature has not reached the limit of 170. I have seen this happen when the thermostats are at setback and no one is using any domestic hot water. The heating guy says he knows this happens but does not know the cause or solution. I suspect something in the new installation is wired incorrectly with all the thermostats, circulators and zone valves. Any help on how to proceed with troubleshooting this or what could be causing the problem would be greatly appreciated.



  • Al Letellier_9
    Al Letellier_9 Member Posts: 929
    short cycling

    It can be a problem and is difficult to pinpoint at times. Had the same problem with my boiler in our office zone. The boiler would come on and shut down even before the prepurge cycle was over. Sometimes it would fire, but then shut down immediately. We found the T87 bulb would "bounce" on occasion. Just before the call for heat, the vial of mercury would tilt and just come right back on the spring. we replaced the stat and no more problems. It could be the T87 or any of the stats. That's the only thing that makes the call for heat. IF you've eliminated the possibility of a intermittent short in the wiring, disabling each stat one at a time will find the problem one. Just do it on a milder day, OK?
  • Jim_90
    Jim_90 Member Posts: 6
    Short Cycling

    Thanks for the advice Al. The system has 3 Taco zone valves and also, since they are on the domestic tank loop circulator, a Honeywell F7-9832 or V8043F1036 valve. All of this was there before and worked fine. The circulators (2) are both Taco and are new.

  • Uni R_3
    Uni R_3 Member Posts: 299

    Are they set to a certain CPH value?
  • Jim_90
    Jim_90 Member Posts: 6

    The 8602s are set at 3 CPH (@ 50% on time) and as far as I can tell are factory set and not adjustable. I should point out that on some of these short cycles the purge comes on and the burner lights but only burns 5-10 seconds before shutting off. Other times, it runs for 1-1.5 minutes.

  • LarryC
    LarryC Member Posts: 331
    Flaky aquastat?

    HO here.

    What is the differential set for on the boiler aquastat? That is one of the "new" pieces in the system, and if I understand these things correctly, doesn't that turn the furnace on to bring the internal temperature back up to a desired level?

    Just my meddling $.02.

    Larry C
  • mark schofield
    mark schofield Member Posts: 153

    could it be the anticipator acting up on the older mecury T87.
  • Joe Mattiello
    Joe Mattiello Member Posts: 703

    the Anticipator on a thermostat should match the amp draw of the valve or it should be .9 when using Taco 570 series zonevalves.
    Joe Mattiello
    N. E. Regional Manger, Commercial Products
    Taco Comfort Solutions
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