Boiler Short Cycle Mystery

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Kudzoo83

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Hello everyone.

I have a 10-year old oil-fired boiler for hot water baseboards (and domestic hot water via coil). We had to replace the aquastat last winter with an entirely different model (Honeywell L7224U). Since then, I've noticed it short cycling multiple times a day (only for heat, never hot water). The company that changed the aquastat has been out several times to diagnose, and can't seem to find anything amiss. Hoping someone here might have an idea.

I'm defining short-cycling as less than a minute. Sometimes it only runs as little as 10-15 seconds.

Boiler high is set to 180, low to 160. Both diffs are set to 10. Our house is one-level, 1300 sq ft, and has 3 zones on the main floor. Thermostats are Honeywell programmables, all set to hot water boiler (not steam) in the settings. I can't seem to adjust the swing on this model, but they're all installed on interior walls and away from windows/doors.

The short cycles always occur in the same manner: no zones are calling for heat, circulator is off, and the boiler temp is in the 168-170 range when it shuts down. I'd estimate it does this maybe 1 in 10-15 cycles. A normal runtime is about 5-8 minutes.

Thank you for reading, and any and all thoughts are appreciated.
 

Fitter30

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Here is a manual if you don't have one. Page 5 has a anti short cycle setting enable it run it out max time. Without knowing all your setting need to know all of them. When boiler was service was the nozzle size the same or did they go larger? What is model of t stats for the zones. Outdoor reset would save you money varying boiler temp by outdoor air temp.
https://www.google.com/url?q=https:...EQFnoECAgQAg&usg=AOvVaw2iGuyY2eJHlOKVI6BSinvP
 
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WorthFlorida

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Here is a link to the manuals, Short cycling explanation is on page 7 of this manual. Low high limit on page 6.
https://www.manualslib.com/products/Honeywell-Aquastat-L7224u-9008343.html

Screen Shot 2021-12-07 at 6.16.32 PM.jpg
 

Kudzoo83

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Here is a manual if you don't have one. Page 5 has a anti short cycle setting enable it run it out max time. Without knowing all your setting need to know all of them. When boiler was service was the nozzle size the same or did they go larger? What is model of t stats for the zones. Outdoor reset would save you money varying boiler temp by outdoor air temp.
https://www.google.com/url?q=https:...EQFnoECAgQAg&usg=AOvVaw2iGuyY2eJHlOKVI6BSinvP

Unfortunately, I'm not sure if the nozzle size has been changed recently. Thermostats are Honeywell RTH6360D1002/E (or something similar), which I'm just now reading are prone to frequent cycling. I'll definitely look into the outdoor reset. Thank you so much!
 

Fitter30

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Honeywell stat has two places to select boiler. Would have to call honeywell to see if there is more settings. What are your heat emitters? Have any idea what nozzle u have. Most if they change it just on a yearly tune up leave the old nozzle for a emergency.
 

Kudzoo83

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Honeywell stat has two places to select boiler. Would have to call honeywell to see if there is more settings. What are your heat emitters? Have any idea what nozzle u have. Most if they change it just on a yearly tune up leave the old nozzle for a emergency.

We have hot water baseboards with fins. The nozzle is 60 80A, and I can confirm that size hasn’t been changed from at least the last couple service reports.
 

Fitter30

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Don't make more that one adjustment at a time. Write down where u started and the change. Baby steps make a few changes don't know what helped or not.
 
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Kudzoo83

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Don't make more that one adjustment at a time. Write down where u started and the change. Baby steps make a few changes don't know what helped or not. Room stat change if set wrong could short cycle boiler

Will do. Thank you again for your input!
 
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