Furnace tries to ignite but can't


  #1  
Old 12-25-02, 11:03 PM
foaxaca
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Question Furnace tries to ignite but can't

My furnace is approximately 13 years old, gas fired, forced air type. After not having used it in several months, it is exhibiting the following symptoms:

I switch on the heater at the thermostat and get the following activity:

A small fan motor begins turning (looks like a computer cooling fan)

The electric ignition begins sparking/clicking
Another click in a squarish metal cube-shaped part with several wires attached and a control knob on top.

A small flame lights at the point where the igniter is sparking.

The small flame remains lighted for anywhere from 1 - 5 seconds and then goes out.

As soon as it goes out, the igniter begins sparking and the clicking sound from the part with the control knob on top begins again.

This goes on until I turn off the heater at the thermostat.

The burners never light nor does the main blower fan in the unit begin turning.

The air conditioning function in the unit works fine.

I replace the old analog dial type thermostat with a new digital Honeywell thermostat a couple of years ago and had to replace the transformer when I reversed a couple of the wires. The unit worked fine until now.

Any suggestions before I call a pro? If nothing else, it would be helpful to be informed about the possible solutions before I trust a repair person.

Thanks
 
  #2  
Old 12-26-02, 06:09 AM
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is this

a carrier/bryant/payne? if so it sounds like the pilot burner. determine if it is 2 or 3 wire.
 
  #3  
Old 12-26-02, 11:08 AM
bigjohn
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The startup sequence is ok until the board or module needs to recognize the pilot flame. The Carrier/Bryant furnaces of that vintage use a switch that is heated by the pilot flame but it takes more than a few seconds. Most other furnaces with Direct Spark Ignition and a pilot use the spark to lite the pilot and the pilot flame in turn lites the main burners. These systems use the principle of flame rectification to verify the pilot flame before allowing the gas valve to open and provide gas flow to the main burners. The circuit is from the flame rod thru the flame to GROUND. As the current passes thru the flame it is rectified from AC to a pulsating DC currrent which is what the module/control board looks for in order to verify the pilot flame. So, the problem could be that the ground connections are dirty or corroded. The pilot burner is part of the circuit and where it attaches to the furnace could be corroded. The pilot burner may be dirty and not supplying a large enough flame to cover the end of the flame rod. The flame rod may have an oxidized coating that is restricting the current in the circuit. You might have a bad module/control board. Measuring the strength of the pilot proving circuit is necessary in order to prove a bad module.
 
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Old 12-26-02, 12:20 PM
bigjohn
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The startup sequence is ok until the board or module needs to recognize the pilot flame. The Carrier/Bryant furnaces of that vintage use a switch that is heated by the pilot flame but it takes more than a few seconds. Most other furnaces with Direct Spark Ignition and a pilot use the spark to lite the pilot and the pilot flame in turn lites the main burners. These systems use the principle of flame rectification to verify the pilot flame before allowing the gas valve to open and provide gas flow to the main burners. The circuit is from the flame rod thru the flame to GROUND. As the current passes thru the flame it is rectified from AC to a pulsating DC currrent which is what the module/control board looks for in order to verify the pilot flame. So, the problem could be that the ground connections are dirty or corroded. The pilot burner is part of the circuit and where it attaches to the furnace could be corroded. The pilot burner may be dirty and not supplying a large enough flame to cover the end of the flame rod. The flame rod may have an oxidized coating that is restricting the current in the circuit. You might have a bad module/control board. Measuring the strength of the pilot proving circuit is necessary in order to prove a bad module.
 
  #5  
Old 12-26-02, 01:32 PM
H
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Carrier

Sounds like a bad pilot switch on a Carrier, Bryant or BDP, they are all basically the same manufacturer. Just determine if it's 2 or 3 wires to it , and change it out.
 
  #6  
Old 12-27-02, 01:00 AM
foaxaca
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The unit is a BDP and the pilot burner part has 3 wires coming out of it. Thanks for any additional information. Is it worth taking this item off and cleaning/dusting it off, looking for any obstructions on the pilot opening, etc?

Thanks for all the great information.
 
  #7  
Old 12-27-02, 01:03 AM
foaxaca
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By the way, is changing out this part a DIY project? Any advice if it is? I'm not shy about breaking out the tool kit and doing just about anything.

Thanks
 
  #8  
Old 12-27-02, 06:56 PM
ToolguyinDE
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Furnace tries to light but can't

I had the exact same problem with my 12yo Armstrong heater. There are 5 long burners in mine. All I did was remove them, and blow all the dirt off with compressed air. I also shopvaced out everything.
It's working great now (and it was a free fixit.

Good Luck,

John M. Claymont, DE
 
  #9  
Old 12-28-02, 05:28 AM
bigjohn
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It's usually the switch. The switch is a warp switch that is heated by the pilot flame which proves the pilot flame is lit. When it gets to temperature, the contacts in the switch transfer and send power to the gas valve. I usually replace the switch and pilot burner assembly together which is not beyond a handy DIY. When you take the tubing nut out of the new pilot burner, be aware that there is an orifice in there that could fall out. Be careful to not let it fall out when you put the burner in.
 
  #10  
Old 12-28-02, 08:34 PM
H
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DIY

If you are to the point of pulling it out to clean it, don't bother....just replace it. As far as the pilot is concerned, it's just a fixed orifice. It sits under the pilot tube (a small brass looking peice) The other tech is right though..if you don't look for it , it will fall out. If you already have it out, clean it off with alcohol and blow it off. Hold it up to the light to ensure it has a clear hole in it. Don't be tempted to poke something in there though. Reassemble and your cooking with gas! Easy job.
 
  #11  
Old 12-28-02, 11:02 PM
foaxaca
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Thanks for all of the info. Turns out it was a bad spark ignitor module. Got it changed out and the unit works just like it used to.
 
  #12  
Old 01-01-03, 10:01 AM
H
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Ignition

I'm a bit confussed...You said the spark ignition module was bad...Didn't you state, that the spark DID occur, and lit the pilot???
It sounds like its trying to light main gas, and the 24V power never makes it to the gas valve ( should be through the switch NOT the board). You've already proven power to the pilot (its lit), the bimetal switch pops over in an attempt to light the main gas, and when the main gas valve fails to get the 24V to release it's 3.5" of gas to keep the bimetal hot, the bimetal cools off fast enough to switch back over to re-light the pilot. This is a built in safety. I think you got a new pilot switch and didn't know it! I could be wrong!
 
 

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