Ask Dave: Furnace issues: I’ve replaced the sail switch but now it runs intermittently – Why?

Dave Solberg is a leading expert in the RV industry and author of the “RV Handbook” as well as the Managing Editor of the RV Repair Club. Today he discusses a furnace that runs intermittently.

Dear Dave,
I had a sail switch installed in my Atwood furnace. Sometimes it runs great for three or four days, other times it won’t ignite but for a couple of seconds. What’s happening? —Ty

Dear Ty,
There are several situations that can cause a furnace to not work properly, starting with house battery power to airflow. I call them “Gremlins,” as they seem to hide for long periods of time and then jump out and create mayhem only to go back into hiding and everything works for some reason. Especially when you take it to the service center!

How the furnace is designed

Let’s look at how the furnace is designed to work and what might be causing your intermittent issue. The thermostat is set to the desired temperature and when the inside ambient temperature drops to that point, it creates a closed circuit and sends a 12-volt “signal” to the module board calling for heat.

The module board starts the fan which blows through the burner assembly, cleaning it out and exhausting it outside through the vent. It also pulls inside air in through the cold air vent blowing it over the burner assembly to the inside vents. This airflow must be strong enough to lift the sail switch – which you had replaced. When the sail switch is up, it also creates a closed circuit which tells the module board there is enough airflow and then opens the gas valve and starts the ignitor.

Even if you have a new sail switch, if the airflow is low it will not raise it enough to create a closed circuit. If the unit starts up for three or four seconds and shuts down it could be the batteries are sulfated and had good power to start the unit, but then nosedived and the airflow is not sufficient to keep the switch up. The “Gremlin” comes into play here as you might have other things inside the rig that are running and drawing the batteries down at the same time. When this happens, I typically install a portable battery booster to verify if it is or is not the battery.

Back to the furnace operation

As the warm air blows over the burner tubes, it passes by a temperature gauge known as the high limit switch. If the temperature is too high, it will create an open circuit and shut down the furnace. This could be a faulty switch. However, it could also be a blockage in the vents or tubing, which creates enough airflow to lift the sail switch, but creates backpressure and increased heat in the chamber.

Check all your vents and tubing if you have the silver corrugated type running under cabinets. You might have a rug or runner over the vents that was not there the last time. Another “Gremlin”!

It could also be a weak LP regulator and low pressure which could be intermittent due to other things running or not running at the time. When troubleshooting, it is important to document every scenario you come across such as the temperature, e.g., does it only work when warmed up or under a certain temperature? What other appliances are running and drawing LP or 12-volt power? Is it only when hooked to shoreline power or when dry camping? All these things can help narrow down the exact situation and find the “Gremlin.”

Testing for low LP pressure

To test for low LP pressure or a weak regulator, typically you would have a technician pull a line and use a water column tester or manometer looking for 11” of water column. However, you can do a rudimentary test by turning on a stovetop burner and verifying that it has a steady blue flame. Then turn on a second, and third, watching to make sure the flames don’t go low or jump around with orange coloring. Then turn on something else that takes LP such as the refrigerator or furnace and see what the flame does. You can also shut all other appliances off to make sure nothing else is using LP and see if that helps.

And finally, the burner assembly could be plugged, rusted, or even have liquid from bad LP. In the photo below, the LP supply tube is in the upper left-hand corner going into the burner assembly. This tube could be blocked or rusted. Inside this cavity are the ignitor and the burner assembly. If the unit starts for a short period of time and stops, do you hear a “poof” sound?

This burner assembly must be clean and intact, have no open rust holes that can cause a flare-up or flame-out condition, and not be contaminated with liquid from bad propane.

So why would this not happen all the time? Temperature differentials. Solder connections can expand and contract with temperature changes and make it a challenge to troubleshoot. Also, different altitudes can be a factor, as higher altitudes will have less oxygen and make LP appliances hard to start.

Keep in mind, getting to this stage generally requires removing the furnace and bench testing. It would also require more advanced technical expertise.

Read more from Dave here. 

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