Furnace short cycles on and off for less than 30 seconds?

airplane401 · November 2016

My 33 year old 84,300 BTU Burnham Boiler and Beckett burner with tankless 3 zone system short cycles 20 to 30 seconds. When it starts for the 20 seconds the burner is ignited and the circulator runs then it shuts itself down. I have adjusted the heat anticipator in both upstairs thermostats and played with the high low limit settings in the control box. All three zone valves are working properly and thermostats are working properly. I set the high temperature at 180 and the low at 160. The differential is set at 25 recommended from online. Lastly, I set both heat anticipators in the round honeywell thermostats for .5. Still short cycles. Sometimes it does this once or twice and then will start up and run fine the third time. Checked the photo eye and there is no suit, perfectly clean.

If anyone has any ideas, much appreciated. Thank You.

Leon82 · November 2016

I had some random cycles where my boiler would start then stop before it even ignited.

I changed the thermostat to one which I could adjust the differential. I set it to 1.5 No more spazing out.

Is the thermostat over a radiator? I also stuffed insulation in the wall cavity to stop any drafts coming from the nail plates.

STEVEusaPA · November 2016

There's a lot to check.
I would change the diff back to 10. I'm assuming a triple aquastat.
It's possible your high and low dials are inaccurate. IOW, if the high is shutting it off at say 170, and the low is turning it on at 169, you'll get the short cycling.
Usually when checking this, I would turn the low down to 120, and check the temperatures with an accurate thermometer. See if when it shuts off on high limit, the dial matches the temperature. Then see if it turns back on after it drops 10 degrees. You can also hold the reset button (so burner doesn't fire) and see when the circ shuts off-does it match the low limit?Then shut off the t--stat, and run the hot water and watch the thermometer and see if the burner turns back on when the temp drops to the low limit setting.
I would also check all wiring connections. Power off check ohms across the high limit, etc

GW · November 2016

You could jump TT at boiler control and also the burner to see what changes. Process of elimination

billtwocase · November 2016

What zone valves do you have? Thermostat baseplate level? Sounds like thermostat, try to pinpoint which one

Furnace short cycles on and off for less than 30 seconds?

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