RV furnace ignites on second attempt with flames shooting out exhaust

Dear Dave,
I am having problems with my Suburban furnace. Just before we closed our travel trailer up for winter storage, it started igniting late. In other words, it would start the fan, click to ignite, but not ignite. It would then try again and ignite with a boom and flame shooting out the exhaust outside of the trailer.

I have not tried anything yet and do not want to try starting it again until I determine the cause. I am an autobody technician and have some aptitude with tools that may be needed.

Any light you can shed on common issues or causes and solutions would be greatly appreciated. —Tom, 2007 Springdale 252RL with a Suburban furnace

Dear Tom,
If you have flames shooting out the exhaust, I recommend you take it to a certified technician immediately.

However, if this occurs while you are dry camping, try plugging the unit into a 120-volt source and try it again. When you are plugged in, the converter/charger will provide a constant 13.2 volts. If it works, your house batteries are getting weak.

How the furnace works

When the thermostat is set to a desired temperature, it creates a closed circuit sending 12-volt power to the module board when it calls for heat. The module board then starts the fan to move air from the outside vent through the combustion chamber, purging it out the exhaust port. This is designed to last for 15 seconds.

It also draws in ambient interior air through the cold air return passing over the burner assembly and past the sail switch, which rises with the appropriate air flow. This sends 12-volt power to the module board, indicating proper air flow. If the sail switch does not raise due to low airflow after 30 seconds, the control module will go into shutdown mode and turn the blower off.

If the sail switch raises and creates a closed circuit, the gas valve opens and the electronic ignition sparks at the ignitor, or piezo, attempting to light for 7 seconds. The module board will verify flame through the spark wire and electrode, so it is essential it is positioned properly. If a flame is not sensed within 7 seconds, a 15-second purge cycle will begin followed by a second try for flame. After three attempts, the module will go into shutdown mode and the blower will run for another 3 minutes.

Possible causes of non-ignition

So, if it does not light on the first or second attempt, it could be obstruction in the combustion air intake or restriction in the burner assembly. It could also be the electrode probe is greater than 1/8” from the top of the burner. That would mean the spark is not getting to the burner at full strength. In that case, a buildup of LP could happen, causing the burst you are experiencing. All these require removing the burner chamber, access door and manifold—which should be done by a qualified technician.

 You might also enjoy these posts from Dave 

• Why does RV furnace blow warm air then turn cold?
• Why won’t RV’s furnace work when camping but works for service tech?
• Why does RV furnace light on second attempt but not first?
• Furnace airflow issue follow-up with photos

Dave Solberg is a leading expert in the RV industry and the author of the “RV Handbook.”

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