Short Cycling Gas furnace

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  #1  
Old 01-11-09, 07:55 PM
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Short Cycling Gas furnace

Furnace burner lights and shuts down several times( for about two seconds each time ) before staying lit and furnace fan coming on.

Any help would be appreciated.
 
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Old 01-11-09, 08:15 PM
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What is the make and model of the furnace? You can get that information off the rating plate for the furnace in the burner compartment.
 
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Old 01-11-09, 08:22 PM
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I would assume you have a flame sensor to prove ignition. You need to locate this sensor and polish it with steel wool or sand cloth. If you have a hot surface ignitor (glow bar) the sensor us usually located on the opposite side of the burner rack. It will have a ceramic base with a metal probe that is in the flame when the burners light. If you have a pilot it should be in the pilot flame. Could also use the model and serial #s.
 
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Old 01-11-09, 08:58 PM
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Hello Skip--

I was interpreting tdbn's post as saying that the burners were turning on and off rapidly several times before staying lit. That's not likely to be a hot surface ignition system if my interpretation is correct.

My guess is that it might be a Carrier BDP three wire pilot system, which allows that kind of symptom to occur due to a dirty pilot.

But my interpretation may be wrong, or it could be something else. Not much information to go on so far.
 
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Old 01-11-09, 09:58 PM
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The furnace is an Intercity HGD-125, here is a picture of the furnace.
If you need any further information about it, i will try to provide it
DSC01536.jpg - Image - Photobucket - Video and Image Hosting
 
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Old 01-11-09, 10:46 PM
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Ahhh. A furnace I've never seen before.

Looks like an intermittent pilot ignition system --- there's pilot tubing and some wires going to the pilot burner. One must is presumably to light the pilot. I can't really say whether it's a flame rectification system or some other means of detecting that the pilot burner is lit before turning on the main burner.


Can you describe the ignition process from when the thermostat first gets a call for heat? I'm supposing you would see a spark light the pilot --- how long after the pilot lights before the main burner turns on?

If it's a flame rectification system, the main burner would typically turn on within a second or two of the main burner lighting. Some pilot burners have a bimetal heat sensor that has to be heated by the pilot burner to verify that the pilot is lit. That usually takes a minute or so before the main burner lights.

If that's the system you have, a dirty pilot burner would explain the on/off cycling of the burner, since the pilot burner isn't adequate to keep that bimetal sensor hot enough for reliable operation. So when the main burner switches on, it colls off enough to turn the main burners off a couple of times.

That's my guess anyway, based on not enough information.
 
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Old 01-11-09, 11:02 PM
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This is an old system. the pilot is always lit. I assume that when the thermostat requests heat,it tells the gas valve to release gas to allow the main burner to light. When the temp in the heat exchanger is reached, the blower motor is switched on. When temp in house is reached gas is turned off, burner outs, motor runs till heat exchanger temp is lowered, then turns off. It then waits for the next cycle.
This is how I think it works. I really don't Know.
Someone please set me straight.
 
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Old 01-11-09, 11:33 PM
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Pretty much any furnace has a way to verify that the main burner is going to light.


Since you have a standing pilot with a couple of wires going down to it, I'd say you have the kind of bi metal system I described in the previous post. That means that there is a switch on the pilot burner assembly which is heated by the pilot flame. The switch has to be hot to close, which verifies that the pilot burner is lit and allows the gas valve to turn on.

That's what I'm guessing, anyway. Since the pilot is lit all the time, that switch should be closed all the time.

If you measure the voltage on both sides of the pilot switch, you are probably going to find that the pilot switch opens and shuts the burner off, causing the problem that concerns you.

That suggests that the problem is a pilot burner that needs to be disassembled and cleaned to get reliable operation.
 
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Old 01-11-09, 11:53 PM
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Since you have a standing pilot with a couple of wires going down to it,...
You must be looking at a different picture because I see no wires going to the pilot burner.

It looks to me to be a standing pilot with a thermocouple safety combination gas valve. There is the pilot gas tubing, a vent tube from the combination gas valve and the thermocouple line.

I suspect a dirty pilot, dirty burners, dirty burner orifices, misplaced pilot burner or a malfunctioning pressure regulator in the combination gas valve.
 
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Old 01-12-09, 12:17 AM
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I bow to your observations, furd. The thermocouple tubing looked like a couple of wires until you prompted me to look closer.

Never mind.
 
  #11  
Old 01-12-09, 05:25 AM
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If it helps any gas to the furnace was turned off for about a year until recently.
 
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