Jarco Floor Heat Boiler cycles on/off constantly

[acbruno]

I have an older Jarco Boiler 700,000 BTU built in 1981. It runs floor heat for 7 bays. Recently it has started to short-cycle on/off every 5 seconds. That's 12 times a minute, 720 times a hour. I hope I don't break anything allowing this to continue. I have replaced the low temp and high temp thermostats and overridden the flow sensor but still the burner turns itself off after only 1 or 2 seconds of igniting each time. There is no flame sensor on the unit that I can find. Gas flow appears to be good. I have also swapped out the igniter controller.

When the system first turns on from a cold start, the boiler will run for about a minute until it goes into this short cycling issue.

Thanks in advance for any ideas you might have.

 

[Bubbles Galore]

Watching.....

 

[Jim Caudill]

Study your main gas valve. I would think there has to be a "flame sensor" of some sort that tells the main gas valve to keep letting the gas flow. I am assuming that you have a "spark type" igniter that jumps a gap to light off the burner. I think the cycling is actually the momentary gas flow that comes from the ignition "light-off" circuit. It never senses a successful light, and therefore re-attempts ignition. I have a similar system in my home furnace built around the same time period. It definitely has a thermocouple to signal the main gas valve to stay open.

 

[Randy]

Sounds like you might want to call in a professional HVAC guy to look at it.

 

[acbruno]

I have a HVAC guy coming out this afternoon. At $85/hour labor rate, this was my last option.

 

[Randy]

Keep us posted. Let us know what the problems is.

 

[MEP001]

Get down and look at the flame sensor as mentioned. When the burner nozzles get dirty, the air/fuel mixture is off and the air gets sucked up into the boiler suddenly when it ignites causing it to suck out the pilot, and then the boiler shuts down and reignites. The burner assembly comes out pretty easily (other than lack of maneuvering room), and you can clean them with a torch tip cleaning file as long as you use one smaller than the opening. I've always taken the whole burner array to a bay and cleaned the tips, then sprayed each one thoroughly with water. You'll see a lot of crud come out the inlet.

 

[acbruno]

I had a HVAC guy come out yesterday. It took him an hour and a half to fix this issue. Apparently there is a flame sensor built into the igniter unit. He just used a wire brush to clean the igniter since he could not seem to remove the burner for a more thorough cleaning. It is ok now. I'm going to take the burner apart next summer and go through it like MEP001 recommended.

 

[MEP001]

It should take care of it - I've pulled out and cleaned a couple dozen burners for that same reason, some Weben Jarcos, some Raypacks, some Teledyne-Laars and I think one Lochinvar. The reason cleaning the sensor worked for now is because the pilot not getting good contact with is had caused a build-up of soot.