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Can you help? Boiler switches on and off intermittently.

gavinc Member Posts: 2

I am a homeowner and not a heating professional. I'd gratefully appreciate any feedback about this.

The problem:

The heat and hot water all work fine. But sometimes the boiler will switch on and then off almost immediately.

The facts:

The boiler is a cast iron Peerless. The house is a bi-level ranch. Heat is baseboard. It has 4 zones: one on lower floor, two upstairs, plus hot water.

I have been unable to determine if the problem is zone-specific, but I don't think it is.

Attempted solutions:

I called Peerless they thought it might be the hot water getting "topped off." This doesn't make sense because sometimes the boiler will switch on and off only about ten minutes apart.

My plumber switched out the aquastats for the three zones. We also changed the hot water min/max.

The problem remains.

A big shout out to Dan Holohan for his great books and wonderful site.



  • Paul48
    Paul48 Member Posts: 4,469
    Heat Anticipator

    Check for a bad heat anticipator setting on a thermostat.
  • chapchap70
    chapchap70 Member Posts: 139
    Have you been paying more attention to on off times?

    Did the on off times change or are you noticing on off times now when you did not before?  As, far as I know, you would only have one aquastat on a boiler so you must have had something else replaced for the zones.

    With a standard oil fired cast iron boiler, if you have a triple aquastat with the high limit at 170 and the low limit at 140, the differential at 15 degrees, when there is a call for heat, the burner will fire until the boiler reaches 170 then shut off.  At 155 degrees, the burner will fire again if there is a call for heat for any of the zones.  It may only take 5 minutes to get from 170 to 155.  If the burner fires and the call for heat is satisfied 30 seconds later, the burner will shut off even if the boiler is 158 degrees.  If there are no other calls for heat, the burner will not fire until the boiler reaches 125 degrees if the low limit differential is also 15 degrees. 
  • gavinc
    gavinc Member Posts: 2
    Can you help? Boiler switches on and off intermittently.


    It's been happening all along since we moved in.

    re: aquastats there is a Honeywell box on each of the three pipes. Each of these was replaced. Guess I got the term wrong.

    What doesn't make sense to me is how a call for heat could be satisfied so quickly when heating a large zone (like the three bedrooms on one side of the house). Five minutes I understand but not 5,10, 30 seconds. thanks 
  • billtwocase
    billtwocase Member Posts: 2,385

    problem. Either it's location, or CPH, anticipator setting like Paul said.  Make sure someone hasn't installed the thermostat over a baseboard. Pics will help. It is rare, but there could also be a bad end switch
  • dstroyr
    dstroyr Member Posts: 1
    short cycling

    Sounds like zone valves on this system..most likely a had thermostat.disconnect a wire on one thermostat at a time to find the issue, could be a bad aquastat too.disconnected wires to TT on it and see if the problem stops
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