Dunkirk Q90-100 Short Cycling

Joe_Petruzzo

I have a Q90-100 that is shutting off after reaching 120 degrees. I disconnected the air intake going outside, and now it shuts off after reaching about 140 degrees. The Boiler Control (IBC) is brand new. Igniter is about a year old. When it shuts down the power light on the IBC stays on, so it can't be the casting temperature safety switch. I've set the High limit Aquastat to 200 degrees instead of 180, but this did not work either. I've ruled out simple problems like the room thermostat. Only potential problem I see is 2 small hairline cracks in the plastic housing for the draft inducer.

Thanks in advance for any help.  

Joe

Steve Whitbeck

High limit

You need to check if the limit switch in the first picture is opening. Put a volt meter accrost it and if it is you will see 24 volts when the burner shut off.

Sounds to me like the heat exchanger is dirty and the flue gasses are getting through to the vent limit switch. Is the vent pipe hot just before the burner cuts off?

Steve Whitbeck

High limit

removing the intake pipe increased the airflow - that cooled the exhaust gasses and allowed a higher boiler temp.

GTI

Q90-100

I think I know your problem. On the boiler there is an access cover/panel on the narrow side of the boiler opposite the side where the intake exhaust piping is located. Remove the sheet metal screws holding this panel. It's painted blue just like the boiler casing.

Once that's off you will see the boiler cleanout plate. It's silver stainless steel. This plate is fastened with hex head bolts. On my boiler ALL were finger loose. I discovered this of course in winter and had short cycles and little heat. Realizing the gasket below had been compromised due to it being directly exposed to the heat from burning gas I temporarily fixed the boiler with high temp copper silicone by permatex. I ran a bead of silicone around the entire perimeter and tightened the hex bolts. I then ordered a new gasket and removed the temp silicone gasket. Upon installing the new gasket I used new hex bolts. Once tightened I marked each bolt with a black sharpie at the 12 o'clock position. I monitored the bolts and observed that they were once again loosening up on their own. The final solution was to one by one thread lock each bolt. To this day I have the blue cover plate off to monitor the bolts. It's been 3 heating seasons since and the unit is working flawlessly.

My short cycling problem went on for the first 3 years after installation. The gas company changed the electronic control board 4 times, 3 igniters and hours of baffled labor. In the end I the homeowner figured it out. If you need photos let me know.

It seems the air leaking in where the gasket was damaged was tripping the pressure safety switch.

Good luck.