In my last two columns I have been discussing the RV water heater, and this week I will extend the appliance discussion to the furnace. Because they are so inherently reliable, RV appliances are often taken for granted until they fail. Furnaces are perhaps the strongest example of this, as they rarely fail, and even when they do, it may go unnoticed until the cold weather sets in. This can make for a long, cold day or night. There is not much to worry about in an RV furnace, but in this two-part article I will present the basic furnace theory of operation and some repair and maintenance issues. Note that this article will deal exclusively with direct spark ignition (DSI) ducted forced-air furnaces, as these are the most common models in most recreational vehicles.
RV furnaces work on the principle of heat transfer by blowing ambient air over a hot sealed combustion chamber, heating the RV. Heat from the combustion chamber is not blown directly into the RV because of the risk of carbon monoxide (CO) entering the coach. A limit switch is used to protect the furnace from overheating. The switch opens if the furnace housing gets too hot, cutting off the voltage to the burner. There is also a sail switch to ensure adequate airflow over the combustion chamber. The sail switch opens once there is a high enough flow of air from the blower. A time delay relay (TDR) or control circuit allows current to flow to the blower motor when the furnace is turned on, but delays current to the gas valve until the combustion chamber is clear of residual gasses. The TDR or timing circuit also allows the combustion chamber to cool down after the temperature set point is reached.
The DSI furnace heating cycle is as follows:
- The TDR or timing circuit in the DSI board allows the blower to purge the chamber for 20 – 30 seconds.
- The DSI board supplies current to the gas valve and causes it to open.
- As the valve opens, a high-current spark is sent to the electrode at the burner. The board will lock-out the furnace after three ignition attempts if the flame sensor does not detect a flame within about 7 seconds. Some older furnaces only make a single attempt at ignition.
- If the furnace fails to ignite and the thermostat remains closed, the blower will remain on until the thermostat is manually reset.
When the temperature reaches the desired set point, the thermostat will open the contacts and power will be removed from the TDR or DSI board timing circuit. This will close the gas valve and the burner will go out. The TDR or timing circuit will keep the blower motor running until the combustion chamber cools.
Although the RV furnace may seem like a complex appliance, they are inherently reliable, as mentioned above. Although furnace failures are rare, they do happen. Next week I will present some possible failure modes and troubleshooting steps to help keep your RV warm and cozy.
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.