Just because cold weather is right around the corner, or has already frozen the corner in your neck of the woods, doesn’t mean you’re done camping for the year! More RVers are using their rigs for winter activities such as skiing, ice skating, and even ice fishing trips rather than trying to find a hotel miles away from the activities. If you plan to do any cold weather camping, it’s important to make sure your RV furnace is operating properly and some tips to make it run more efficiently.
Let’s first cover how the liquid propane (LP) furnace operates;
The thermostat is set to the desired temperature, once the inside temperature drops below that setting, the thermostat calls for heat from the furnace. Most thermostats have a variance that can be as much as 4 degrees below the setting. Your RV thermostat is powered by the 12-volt house battery system, therefore you can not replace it with a residential model such as the NEST or other devices as they operate on 24-volt power. Older models were designed with a bi-metal interior coil that would shrink and expand with temperature changes and we all remember the old “mercury in a tube” type models! These should be changed with a newer RV specific model. Also, make sure you match your heating and cooling system to the proper thermostat as some control both the heater and air conditioner, others have a heat pump, and some models have several zones with temperature sensors in individual rooms.
Once the thermostat calls for heat, the control module starts the blower motor inside the furnace which blows out any older air that may be trapped inside the burner assembly which could contain trace amounts of LP. This is exhausted outside through the vent tube. The blower motor also draws cold air from inside the RV and routes it back over the burner tubes and through the interior vents. This forced air pushed up a “sail switch” located behind the burner tubes and once the switch reaches the desired height, creates a closed circuit that tells the module board there is sufficient airflow and to open the gas valve and light the burner assembly. One common problem with RV heaters is low house batteries that will not provide enough airflow to raise the switch! If your batteries are below 11-volts, the blower fan will run, but the unit will not light. This can also be deceiving when the furnace does light and runs for some time and then just the blower runs. This typically means the house batteries started off with enough voltage, but then drained fast due to sulfation.
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 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. Some models have two vents on the outside of the rig, with the upper vent designed for fresh air, and the lower for heated air coming out of the heat exchanger.
Tips to make your heater run more efficiently
As mentioned before, do not cover vents with carpeting, rugs, or close off to many vents inside the rig. Not only does it limit airflow at the sail switch, it will also build up excess heat in the heat chamber and trigger the high limit switch.
(High Limit Switch)
Verify your exterior exhaust and intake vents are unobstructed and your house batteries are in good working order. Lead-acid batteries will become sulfated and lose storage capacity if not properly charged and maintained. Verify your LP pressure is sufficient which should be at 11” of water column and should be checked by a certified technician. Make sure your LP regulator is not exposed to the elements and does not get snow and ice buried around it.
If the furnace does not light, verify there is 12-volts going to the thermostat and then at the control board on the furnace.
Check to see if the blower motor is working but the valve is not opening and trying to spark. If the blower motor is not working, check to make sure there is no obstruction such as a mouse nest. This requires removal of the shroud or often time the entire unit to access. Removal will also allow you to test the sail switch and high-temperature switch which can be done with an ohm resistance test.
Blower runs, no heat
Test for 12-volt at the control module, listen for the gas valve to open with a click, listen for spark attempt. If the gas valve does not open, it could be a faulty control board. If it attempts to spark, it could be a cracked ceramic insulator on the lighter probe causing the spark to follow the crack to ground known as carbon tracking which means the spark does not get to the burner. It could also mean a dirty or contaminated spark ignitor or improper gap. And finally, it could mean low LP pressure due to a weak or faulty regulator.
Furnace runs for a short time, shuts off too soon.
This is typically due to weak house batteries that are sulfated and do not hold a charge very long as mentioned earlier. Once the furnace shuts off, immediately check voltage at the control board, anything below 11-volts will create this situation. Even if the rig is plugged into shoreline power, the converter or battery charger will not engage until the batteries hit 10.5-volts so there may be a power gap between what the furnace needs to light and the converter senses for charging.
It can also mean the high limit switch is getting too hot due to obstruction in the vents or is getting weak.
Give your furnace a helping hand!
It’s important to understand your furnace systems’ capacity and sometimes provide a little help with supplemental heat and additional insulation. A catalytic heater is a safe supplement in the bedroom at night or living area in the daytime.
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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