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Tuesday, October 20, 2009

Fixing Furnace Faults

I have a Suburban 17000 btu propane furnace in my pop up camper that will not light when it is cold ( 45 degrees or less) outside. I have checked the battery and have sufficient power. The fan comes on but the ignitor never tries to ignite. When the outside temp is above 50, it will start right up. Also, once it starts once, it will re-start without fail as long as the unit is not allowed to cool. Could this be the sail switch or control board? Obviously the thermocouple works as it lights when warm.
Reed, (Mt. Crested Butte, CO)


 Reed, based on your statement “the fan comes on but the ignitor never tries to ignite”, we can probably rule out an LP delivery problem due to the colder weather. But electronics can be very problematic in colder weather though the problem is usually related to faulty or dirty connections.

Try this: unplug the connector to the control board on the furnace and carefully inspect both the plug and the board traces for corrosion or oxidation. Corroded or oxidized connections and their ability to conduct electricity can be hugely impacted by temperature and humidity changes. If there is any corrosion or oxidation on the board, temporarily clean it off using a standard pencil eraser. Be sure to blow away any eraser debris. The plug is best cleaned using a contact cleaner and protectant; I favor the products of Caig Laboratories. You can also scrape it off, but be extremely careful not to bend the connectors in the plug. Once you have inspected and cleaned the contacts, firmly re-seat the plug onto the board and make sure it is tight.

Next check all other connections to make sure they are clean and tight, including the ground connection. It is possible that your control board has an open solder joint or trace that is opening and closing with temperature changes, but this would be unusual for the temperatures you mention. 

If you still have problems after performing this maintenance I would suggest the board be tested on a bench board tester. Weather could also cause the gas solenoid valve to stick, but since you have stated that the ignitor never fires chances are the gas valve is okay. It is possible the sail switch is heavy-laden with lint and needs cleaning, but typically this would result in the problem happening every time the unit tried to fire. It is, however, important to have the furnace cleaned and serviced annually which includes cleaning and testing the sail switch.

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