Ask Dave: Why does RV’s rear furnace work but front one blows cold air?

By Dave Solberg
Dear Dave,
My RV has two propane furnaces. The rear furnace works fine, but the front one blows cold air only. Any suggestions? Thanks. —Ray, 2001 Triple E Signature

Dear Ray,
Are you running both furnaces at the same time? If yes, you might not be getting enough 12-volt power to run the fan fast enough on the front one. You are obviously getting 12-volt power to the furnaces, as the fan is blowing. However, the fan must be going fast enough to blow out the back of the furnace cage and lift the sail switch. This tells the module board there is enough airflow and to open the gas valve and spark.

If the front one works fine when it is the only one running, then I would look at the condition of your batteries. Does it do this when plugged into shoreline or just when boondocking? If it does it with the shoreline cord plugged in, then it is not the battery.

Check vents for furnace

Check to make sure all your vents are open and unobstructed for that front furnace. If they are closed, have a rug over the vent, or the hose leading to the vent is bent, it will restrict airflow and cause a backflow, limiting airflow and not raise the sail switch.

It could also be a defective sail switch, which would require you to remove the furnace and test it with a meter. You should have continuity between the wires when it is raised.

Listen to the furnace

Listen at the furnace to see if you hear any clicking noise or spark attempt. The fan should run for about 10 or more seconds. If the sail switch is good and it lifts, the module board will send power to the gas valve and you should hear a click followed by spark attempts from the piezo or spark ignitor. Typically this will only attempt to light a few times. If it does not, it will shut down. If you do not hear the click and spark attempt, it is the sail switch or could even be the module board. You you can take that out and have it checked by an authorized service center. However, this usually costs more than just replacing the module board.

If you do hear the click and spark attempt, it means the fan is running fast enough and the sail switch is working. Then it could be that the spark ignitor is not sending the spark to the burner assembly. That could be a spark probe alignment issue or a crack in the ceramic which causes the spark to travel along the crack to a ground.

If it actually lights but only runs for a few seconds, then it could be an issue with a low flame due to a weak LP regulator or dirty burner assembly. It could also be a defective thermocouple, which is the upper probe the flame should reach to tell the module board there is a flame, otherwise it shuts the system down.

If you hear any of the attempt to light situations, typically the system will try a few times and shut down and the fan motor will not continue to run and blow cold air. I included this as some people get impatient and turn the thermostat off and on repeatedly without knowing the sequence.

Walk through these steps and you should be able to identify where the problem is.

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RV furnace has low airflow and isn’t hot. Why?

Dear Dave,
The RV furnace has always worked well, but this last week it seems it isn’t heating the interior as quickly as it did a few months ago. I checked the vents and nothing is blocking them. The air blowing is warm but used to be much warmer. And while the heat is on, I can’t light the stove burners; I have to turn the furnace off, and then the stove will light. That has never happened before, so I think the two may be related. —Cheryl, 2016 Coachmen Leprechaun 220QB

Read Dave’s answer.

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

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