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Gas furnace goes through several steps of firing sequence but will not light

Gas furnace goes through several steps of firing sequence but will not light

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  #1  
Old 04-03-15, 02:43 PM
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Gas furnace goes through several steps of firing sequence but will not light

Having trouble with my LP Gas furnace located in my attic. The thermostat calls for heat and I hear a motor turn on, the ignitor lights up and there is a flame present (looks like a pilot light where the ignitor/flame sensor is) but then the main burner fails to fire up. After 10 or 15 seconds the flame at the ignitor then goes out and the motor I heard running continues. It's not the blower motor which sends heat through the duct work.

The troubling thing is it happens every now and then and there is nothing I can do to get it to fire up. At times it will run flawlessly for 1 month at a time and then it starts to act up.

I recently replaced the ignitor and flame sensor (all 1 unit) thinking this would fix it but it did not. I'm ready to call in the experts unless someone here can give me some ideas as to what is causing this. And as a point of reference I have a furnace in my basement and it does not experience this problem.

Thanks in advance for the help.
 
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  #2  
Old 04-03-15, 03:08 PM
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Welcome to the forums.

What is the make and model of your furnace ?
The ID plate can be found on the side wall next to the burner.

It sounds like you may have a problem with the pilot. Possibly a partially plugged orifice.
How does the pilot look..... it should be a nice big blue flame ? If it is not burning correctly the sensor will not open the gas valve to the burner.
 
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Old 04-03-15, 03:49 PM
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Thx

Thanks, Pete. I can't seem to find out the model # but it's an Aire-Flo and it uses a honeywell gas control valve. Not sure how to tell if the pilot light is strong enough but it does light up about 1 inch, making full contact with the flame sensor rod, turning it bright red.
 
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Old 04-03-15, 05:08 PM
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A Honeywell gas valve may mean you have a SmartValve system which are trouble prone.
How about the part number off the gas valve ?
 
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Old 04-03-15, 05:45 PM
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Have you tried tapping the valve with a screwdriver handle when there is a call for heat, the valve could be sticking.
Geo
 
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Old 04-03-15, 07:15 PM
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It's a SV9500 which is a smart valve. I'll try tapping it with a screw driver but it may in fact be shot which is what I suspect. I did try to adjust the pilot by turning the screw on the valve and it did not increase or decrease. Maybe another sign the valve itself is shot?

Thanks again for all the help. If you have other suggestions outside of the valve please let me know.
 
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