Short Boiler Firing Cycle

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  #1  
Old 10-17-05, 08:47 PM
dsandstrom
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Short Boiler Firing Cycle

Just had to do some work on my mother-in-law's central, closed-loop, hot water heating system using traditional standing radiators. The system is up and running with a new expansion tank and no leaks (system re-filled and bled, no signs of air).

Both before and after the fix the boiler (natural gas) had a very short firing cycle. The pilot lights the main flame, the flame builds, lasts 2-3 seconds, then goes out. The system then relights the flame almost instantly. Total cycle time is 5-7 seconds. I figure this means that the boiler temperature sensor is telling the boiler that it's at temperature. The question is, is this kind of firing cycle normal (the main gas valve is on/off, so there is no low flame level)? If not, where to look? There is a "circulator cartridge," which I gather is a small pump similar to the water pump in my truck. Could this be fouled/broken (when I had the piping open did not see any significant scale buildup) and not moving water through the heat exchanger fast enough to take the heat out? Anything else to look at? All suggestions appreciated.
 
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Old 10-17-05, 08:58 PM
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Several Possibilities

The circulator could be the culprit as could some kind of restriction in the system. Another distict possibility is the boiler is grossly oversized for the load. When the flame shuts down, is the supply pipe hot? What is the boiler temp gauge reading?
 
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Old 10-18-05, 08:56 AM
dsandstrom
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Guess it's time to look at the circulator

Grady,

Thanks for the help. Guess I need to look at the circulator or for another restriction. Didn't see anything yesterday, and the system drained pretty freely, so I'll start with the circulator.

Boiler is 120,000 BTU/hr input, 104,000 gross output, 9?,000 net output (can't remember the ? value). Boiler heats a 3-unit house (2,500 sq. ft. +-) with a coldest day temperature of about freezing. There are a total of 5-radiators in the building. Don't know if that all sounds right or not.

When I fired it up yesterday the output temperature reached about 110F after half an hour. The piping was warm but not too hot to touch, so assume the gauge is close to accurate. This seems to be low to me (from what I've read here and elsewhere). Don't know what the setpoint is, so will investigate. The temperature climbed slowly throughout the half hour, so don't think the boiler was getting there too quickly and shutting down.

Thanks again for the help.
 

Last edited by dsandstrom; 10-26-05 at 04:40 PM.
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Old 10-18-05, 06:49 PM
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Short cycle

Did the boiler short cycle during this 1/2 hr run? If it did, I would look at the aquastat settings & the possibility of a defective flame roll out switch presuming there is no roll out. Other possibilities include loss of flame sensing or a gas valve breaking down.
 
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Old 10-26-05, 04:38 PM
dsandstrom
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Update on short cycle

Got it fixed, figured I'd post my experience in case anyone else encounters this.

Went through all the limit switches with a multimeter and couldn't find any problems, the limit line into the burner controller stayed energized throughout the time I was having trouble. So, opened up the burner box so I could watch the flame. When main gas opened, the main flame spread across the burners quickly, as it did this, it would blow the pilot flame away from the flame rod, causing the main gas valve to close. Pilot flame would then return to the flame rod, main gas would open, flame would spread and blow the pilot away from the rod (kind of surprised this didn't result in a lockout).....

Turns out the pilot has a shaped orifice that sends part of the pilot flame toward the flame rod and part towards the burner to light it. Had some junk in the orifice that needed to be cleaned out, once that was done the system worked fine.

Thanks for all the help and for providing this forum
 
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Old 10-26-05, 07:39 PM
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Thumbs up Fixed

Good job & good detective work. Actually seeing something like this happen is super good for a layman. It was loosing flame sensing because it was loosing flame to the sensor. I've seen many experienced techs get thrown for a loop on something like this. All it takes is a little fleck of rust or soot. Keep up the good work.
 
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