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New Boiler Short Cycling on High Water Temp Limit...Thermostat issue??

cdvo871 Member Posts: 6
Hi Folks,

I recently had a new boiler installed at the beginning of the heating season, and over the past couple months of heating I am noticing a similar trend with the operation of the boiler - when the thermostat calls for heat, the burner kicks on and the boiler runs for about 15-20 minutes before it hits the High Water Temp limit setting (which I believe is set at 180F on my HydroStat 3250 aquastat). At this point, however, the temperature in the house has not yet been satisfied, so the burner will kick on and only run for about 2 minutes (at which point it hits the HIGH temp setting again) and then shuts off. In about 3 minutes after being OFF the burner will kick back on. I have confirmed that the circulator pump DOES run the entire time the t-stat is calling for heat. (I believe the temp differential for this aquastat is 10F, and is non-adjustable.)

This constant short-cycling of 2min ON/3min OFF, after the first initial long burn, will continue until the thermostat is satisfied which seems to range anywhere from 20 minutes to sometimes over an hour, depending on the weather conditions, or how much longer it will take to reach target temperature, etc.

Some basic specs...
- Boiler = Weil McLain WGO-2
- 86,000 BTU/hr
- Input: 0.7 USGPH
- Burner = Beckett AFG
- Baseboard radiators; copper pipes with fins
- Oil-fired

For some more specifics about my thermostat I am using the NEST thermostat - which I realize has caused many people issues with their heating/cooling systems. I usually keep the desired temperature at 69F. I do notice that the time in between heating calls seems to be in most cases about 2-3 hours! at which point the entire system has mostly gone cold (i.e. boiler water temp, baseboards, etc.) by the time the boiler kicks on again. I think I read somewhere that the default (i.e. non-adjustable) Nest temp differential is +/-0.7F? It seems with my situation with t-stat set at 69F, the boiler gets called at 68F but then will not shut off until temp reaches close/at 70F, and since I have baseboard heating the residual heat often takes the temp over 71F!

This is nearly a 3F degree swing (68F-71F), and I am wondering if this large setback is what's causing most of my issue?

There is a feature on Nest thermostat called "TrueRadiant" that is supposedly designed for hydronic baseboard heating systems. It says on the app that this feature "learns how long your system takes to heat up and uses that info to create a predictable schedule with even heat". I've had this setting turned ON since the new boiler was installed, yet based on my situation it doesn't seem to be working or doing anything! If it did I would expect the boiler to be called earlier, or shutting down earlier, or both to keep the temp closer to an even 69F.

Additional note: my boiler was sized by about 3-4 different companies I received quotes from, so I'm not sure if/how much oversized the boiler could be. 2 of them proposed the same WGO boiler which I believe is the smallest unit WM offers, and another company proposed a Columbia Oil Boiler at 103,000 BTU (which probably would have been way oversized). This third company said they calculated my heat load to be about 60,000 BTU.

Looking for advice on what my options are here. Could it be the thermostat creating too much of a setback that the heating system takes too long to recover from? I checked all baseboard rads and all seem to be giving off good heat, so I don't think there's any issues with non working sections of rads. I was thinking about picking up a "dumb" thermostat that has an adjustable temp differential between 0.25-2.25F and playing around with the different settings to see if that would help at all?

Thanks in advance for any insight!


  • Jamie Hall
    Jamie Hall Member Posts: 23,452
    Sounds like a combination of two problems -- partly the wide swing on the Nest, but the boiler cycling itself suggests rather strongly to me that the boiler may be too big and, being oil-fired, not being able to modulate.

    Boilers in hot water heating systems are designed on two interacting bases: first, the boiler should be capable of meeting the design heating load of the structure -- obviously (you quote a design heat load of 60,000 BTUh). There is no need for them to be any bigger than that. Second, the radiation installed must be capable of transmitting that heat output from the boiler to the building at some reasonable water temperature. Now a boiler which does not modulate requires that the heat output from the boiler be less than the capability of the radiation to radiate -- otherwise the boiler will have to cycle. Since your boiler is cycling, even at a high water temperature, it means that the installed radiation cannot handle the heat output from the boiler.

    The fact that the radiation can bring the building temperature to where you want it suggests that the radiation is adequate for the structure, at least at that high a water temperature.

    So... Having wandered all the way around the barn to get there, the conclusion is that the boiler is too big. In fact, based on the firing numbers you give, it's about half again as big as it needs to be to heat the house.

    Honestly there isn't all that much you can do about that. Changing the thermostat settings isn't going to help the short on/off cycling. What might help would be a good big buffer tank which the boiler would heat and which would then heat the house.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England