Furnace ignition issue


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Old 01-10-10, 11:32 AM
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Furnace ignition issue

I have an 80% gas furnace that uses intermittent pilot ignition. Normally, once the flame sensor senses the pilot flame, the ignition control will stop sparking and open the main burner valve, and the furnace runs until there is no longer a call for heat from the stat. Lately, sometimes while the furnace is running, the main burner flames go out, while the pilot stays lit. But the control does not start sparking again. After several seconds, the main burners will ignite again. This is strange, since presumably if the main burners go out, it means that the control is not sensing the pilot and is turning off the main burner. But if it isn't sensing the pilot, then it should start sparking again. But it doesn't. The flame sensor is clean, there is always a good pilot flame impinging on the sensor, and the ground and wiring connections are good. I have confirmed that the problem is not the gas valve, as the 24 volts to the valve is dropping out when the burners go out. So it means that the control for some reason stops sending voltage to the main valve terminal on the gas valve. Any ideas on what could cause this? Ignition control going bad? The control is a Robertshaw SP-715A.
 
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Old 01-10-10, 12:24 PM
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Measure the AC voltage being applied to the flame sensor wire by the ignition module and see if that stays on when the main burners should be on and stay on.

Also, measure the DC microamps flowing along the wire to the flame sensor when the burners are lit. The measurement here might be from 1/2-5 microamps DC current, with 1,000,000 microamps= 1 amp.
 
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Old 01-10-10, 01:32 PM
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Thanks for the quick reply. When I connect the leads of my (analog) multimeter (set on the 50 AC volts scale) to ground on one lead and to the sensor terminal on the other lead, while the furnace is running, the meter pegs and the main burners go off and the sparking begins; when I disconnect them the furnace will re-ignite. When checking current, it reads about 4 microamps on the sensor lead while running; of course, the burners haven't dropped out since connecting it, so I have not yet been able to determine if the current drops out when the burners go off. Love those intermittent problems! In case it matters, my setup utilizes one rod for the sensor and a separate rod for the spark.
 

Last edited by eclipse; 01-10-10 at 04:08 PM.
 

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