An RV furnace that won’t light on a cold night is a real problem. The good news is that most furnace failures trace back to a small list of common causes, and several of them can be diagnosed without any special tools. Work through this list before calling for service.
How an RV Furnace Works (Quick Overview)
RV furnaces run on propane and are controlled by your thermostat. When the thermostat calls for heat, the furnace: (1) runs the blower motor to purge the combustion chamber, (2) opens the propane valve, (3) fires the igniter, and (4) the burner lights. A sensor called the sail switch confirms the blower is moving air, and a limit switch prevents the unit from overheating. If anything in this sequence fails, the furnace won’t light.
1. Low or Empty Propane Tank
Check it first. A tank that reads 20% can still struggle to deliver adequate pressure in cold weather because propane stays liquid at low temperatures and pressure drops. If it’s below 20% and it’s cold outside, try the furnace with a fuller tank or after warming the tank slightly.
2. Propane Valve Not Open
Confirm the propane shutoff valve at the tank is fully open, and that any manual shut-offs along the line to the furnace are also open. This sounds obvious but gets missed regularly, especially after winterization or after someone turned it off for storage.
3. Dirty or Clogged Burner
Spiderwebs and insect nests inside the burner tube are extremely common, especially in RVs that have been stored. Even a small obstruction can prevent the burner from lighting. If you can access the furnace combustion chamber, visually inspect the burner tube for any blockage. A can of compressed air often clears it.
True story: Mud dauber wasps love RV furnace exhaust vents. One or two nests can fully block the burner tube or exhaust. This is the #1 cause of furnace failure in stored RVs.
4. Tripped High-Limit Switch
The high-limit switch is a safety device that shuts the furnace off if it overheats. Once tripped, it requires a manual reset or replacement. Overheating is usually caused by restricted airflow — blocked return air vents, closed registers, or a dirty filter. Clear the airflow issue first, then find and reset the limit switch (usually a small red button on the furnace body).
5. Sail Switch Failure
The sail switch is a thin flap inside the furnace that proves the blower is moving air before allowing ignition. If the sail switch is stuck, bent, or broken, the furnace will run the blower but never attempt to light. Sail switch replacement is an inexpensive repair but requires accessing the furnace internals.
6. Igniter or Igniter Board Failure
The igniter sparks to light the propane. You should hear a rapid clicking sound when the furnace tries to light. If you hear the blower run but no clicking, the igniter circuit or control board has likely failed. If you hear clicking but no ignition, it may be the igniter itself or a propane delivery issue.
7. Low Battery Voltage
RV furnaces run on 12V power for the blower, igniter, and control board. If your battery bank is low (below 11.8V), the furnace may not operate correctly. The blower may spin slowly, or the igniter may not fire reliably. Charge your batteries and try again.
Before any repair: Make sure the thermostat is set to HEAT mode (not just fan), and is set higher than the current temperature. Also confirm there’s no error code on the thermostat display.
Still Won’t Light?
If you’ve worked through these seven steps and the furnace still won’t fire, Thomas can diagnose it at your location. He carries common furnace parts and can often fix it in one visit.
Request Heating Service