Last week I published an overview article on how direct spark ignition forced-air RV furnaces work. This week I will discuss some possible failure modes and troubleshooting steps to help keep the appliance functioning properly.
Unlike other RV appliances, furnaces utilize a purge cycle and are also prone to short-cycling. These characteristics are relevant because they may cause the user to believe the furnace is not working properly. During the purge cycle, the furnace blower will run for up to thirty seconds before the burner lights and after the burner shuts off when the requested temperature is met. This is to ensure that the combustion chamber has adequate air available and does not have any residual gas present. At the end of the heating cycle, the purge acts to cool the combustion chamber. This may leave some owners feeling that their furnaces are malfunctioning, since the appliance will blow cold air for a short time. You should hear your furnace burner ignite after the blower starts, but there could be a delay of up to thirty seconds. Also note that there may be a slight delay between turning on the furnace or setting the temperature and the blower motor starting.
With short-cycling, the furnace will shut down before the set temperature is reached. This is a common problem in very cold weather and results from the furnace overheating before the RV heats up. This is a normal occurrence meant to protect the furnace and does not indicate a faulty appliance. When this occurs, the furnace will relight when it cools down. Short-cycling may occur more than once in a heating cycle, but eventually the RV will reach the set temperature and the short-cycling will stop. If your furnace short-cycles, be patient, as it may take slightly longer for the RV to heat up. You may choose to slightly reduce the set temperature.
The most common failure mode of an RV furnace is low voltage. Because it uses a blower motor, the furnace is the RV appliance most susceptible to low-voltage failure. A furnace will not run if the house battery voltage falls below 10.5 volts. Voltages higher than 10.5, but lower than the nominal 12V may allow the blower to start but prevent the burner from lighting. This is because the sail switch will fail to close under low airflow conditions. This switch is designed to close and allow current to flow to the ignitor when adequate airflow is achieved. Since low voltage will prevent the blower from spinning fast enough, the sail switch will not close and the furnace will not light.
If your furnace will not start or if the burner does not light after thirty seconds, measure the battery voltage, as well as the voltage reaching the furnace. This can be measured at the fuse panel or at the red and white or black and white wires at the furnace control board. If the voltage is lower than about 11V, you should identify and rectify the cause of the low voltage then test the furnace for proper operation.
If the furnace blower starts and the burner tries to light but fails, I would first check the propane system. Make sure your other propane appliances are functioning properly. It is a good idea to have your propane system inspected annually by a qualified RV service center.
If your furnace is not functioning properly, your battery voltage is higher than 11V, and your propane system is functioning properly, you should have your RV furnace professionally repaired. The problem could lie with the control board, limit switch, sail switch, gas valve, blower, wiring, or elsewhere. Fortunately, most furnace problems are voltage-related.
About the Author:
Steve Froese, an avid RV owner, traveler, and Coach-Net member since 2013, is the principal of “A Word to the Wise Technical Communications”, a published RV author, certified RV technician, and licensed Professional Engineer. He frequently collaborates with the “RV Doctor”, Gary Bunzer, and has worked with the RVIA/RVDA as a technical and training writer and consultant. Professionally, he works as a quality engineer and musician. Watch for more of Steve’s work in upcoming Coach-Net publications.