Today we are seeing more RV enthusiasts using their RV for winter activities such as skiing, ice skating, and even ice fishing. Even if you are not one of the die-hard cold weather enthusiasts, your RV adventure may take you to a location that will need the onboard heater to keep things warm. An understanding of how your RV LP furnace operates is important to help make it run more efficiently and to prevent a malfunction.
Just like in your home, the thermostat senses the ambient temperature at the point of installation and can be set to the desired temperature. Once the temperature falls below the setting, it creates a closed circuit which sends power to the furnace control board. If you have an older model with a bi-metal thermostat it’s best to replace it with a newer digital RV model. Make sure it matches the mode of operation your current thermostat has.
Some thermostats also control the air conditioner and either a heat strip or heat pump option. A heat strip is simply a heat element or coil that is placed in the air conditioner and when operated heats the coils similar to an electric stove top or portable heater. The air conditioner uses the fan only to send the heat into the coach. This is a good option if you have free electricity as it does have a huge amp draw for the heat it produces.
Here is the confusing part…some thermostats have a mode that is labeled “Elec Heat” which RV owners think is an electric operation of the onboard furnace. This is actually a heat pump option incorporated in the air conditioner. A heat pump extracts BTU’s from the outside air to bring into the rig and remove cold air from inside the rig. Heat pumps are only efficient with temperatures above approximately 35-40°! So when the temperatures get below that, it blows cold air. One more thing to know about thermostats is zones! Keep in mind the thermostat can only sense the temperature in the location it has been placed, unless it has an option for zones with a temperature tensor in the bedroom or other location.
Once the thermostat senses the temperature has fallen below the temperature that was set, it creates a closed 12-volt circuit which is similar to touching two wires together that allows the voltage to flow to the furnace. Keep in mind this could be 2-4 degrees lower before it creates the closed circuit.
The 12-volt circuit alerts the control module on the furnace which starts the blower as well as the combustion air wheel, which draws fresh air in from the vent on the outside of the coach. A switch called the “sail switch” is located inside the blower assembly and the airflow pushes the switch up to make contact with the module and create another closed circuit. This provides a feed back to the control module verifying everything is open and there is good air flow. If your house batteries are below 10.5 volts, the blower will run, but will not have enough power or airflow to raise the sail switch enough to create the closed circuit. Therefore the blower will continue to run but the furnace will not light. Also, if you have a few heat vents closed, a rug over the floor vents, or a pinched or kinked supply hose in the rig, it will restrict the airflow and the sail switch will not rise, therefore the gas valve will not open and the spark igniter will not light the flame.
Once the sail switch has verified sufficient airflow and created the closed circuit, the circuit board will open the gas valve which you should hear a distinctive click, followed by the spark ignitor clicking and lighting the LP mixture in the burner assembly. Then the heat exchanger will heat to approximately 200 degrees and the air wheel pushes fresh air over the heat exchanger to provide warm air to the inside of the rig. The air going through the heat exchanger is exhausted to the outside of the RV through an exhaust vent.
A high limit switch is located on the end of the heat chamber which measures the temperature and will shut the unit off if it gets higher than normal. This can happen when interior registers get covered with a rug, vents are shut off, or supply hoses get pinched.
To troubleshoot a nonworking furnace, first check the fuse at the distribution center, then the voltage supply to the furnace. This can be done without removing the furnace. If the blower wheel starts, but there is no heat or ignition, the sail switch typically is not raising high enough to close. If the 12-volt supply is less than 10.5 volts, the fan will not turn fast enough to raise the sail switch, therefore it will not close and light.
If the fan does not turn on and there is 12-volt supply, check the thermostat by removing and touching the two wires together which will bypass the thermostat and send a signal to the module board. If all this is good, then you will need to remove the furnace and verify the module board and the high limit switch.
If the unit starts, ignites, and runs for a short period of time, the high limit switch is sensing high temperature or is faulty. Verify you have all vents open and unobstructed and all supply lines to the registers under couches and inside cabinets are not pinched or kinked.
About the author:
Dave Solberg: Managing Editor, RV Repair Club
For the last 25 years, Dave has conducted RV maintenance and safety seminars, developed dealer and owner training programs, written RV safety and handyman articles, authored an RV handbook reference guide and logged over 100,000 miles on the road in an RV.
RV Repair Club is your go-to online resource for enthusiasts who want quality RV maintenance, repair, and upgrade information – a community where passionate RVers can come together to gather knowledge and share their experiences.
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