RV furnace won't ignite: fan runs but no heat, clicking, or lockout

How an RV furnace lights

Most RV furnaces are forced-air, direct-spark-ignition (DSI) propane units. When the thermostat calls for heat:

1. The control board starts the blower motor.
2. When the fan spins fast enough, airflow closes the sail switch, which proves there's enough combustion air.
3. Power passes through the high-limit switch (an overheat safety).
4. The board opens the gas valve and sparks the igniter.
5. A sensor confirms flame. If none is detected, the board typically retries a few times, then locks out.

The furnace runs on 12V even when you're plugged in, through the converter. Propane provides the heat, but the battery and converter make everything else work.

Danger: If you smell propane (a rotten-egg odour), don't try to light anything. Turn off appliances, shut the propane at the tank if it's safe to do so, open doors, get everyone out and call for help from a safe distance. Never use a furnace with a damaged exhaust, and never use an oven or stove for space heating. Test your propane/CO alarm before every trip and replace it at the end of its rated life.

Symptom: nothing happens at all

• Is the thermostat set to Heat (not Cool or Fan) and above room temperature? Replace thermostat batteries if it uses them.
• Check the furnace's 12V fuse at the distribution panel.
• Check the battery disconnect switch and battery voltage.
• Some furnaces have their own power switch on the unit itself, behind the access panel. Make sure it's on.

Symptom: fan runs but no flame (blows cold air)

This is the most common complaint, and the causes are usually simple.

1. Low 12V voltage

If the battery is low, the blower spins slowly, the sail switch doesn't close, and the board never attempts ignition. Or ignition starts but fails. Measure battery voltage with a Digital multimeter (auto-ranging, with DC amps) while the fan runs. If it's well below about 12V, charge the battery or plug in and try again. Many "dead furnaces" in Alberta winters are really dead batteries. See RV battery not charging: how to troubleshoot the converter and 12V system.

2. No propane reaching the furnace

• Is the tank valve fully open? Is the tank empty? (Weigh it, or pour warm water down the side and feel for the cool line where the liquid level is.)
• Light a stove burner first. If the stove won't light or burns weakly, the issue is supply: tank, regulator or valves. See RV propane problems: smells, weak flames, regulator issues and leak testing.
• After a tank change or when the RV has been stored, there's air in the lines. Running the stove for a minute purges it. The furnace may need two or three attempts.
• Open tank valves slowly. Opening them too fast can trip the excess-flow safety in the tank valve, which restricts gas to a trickle.
• In deep cold (around -30 °C and below), propane vaporizes poorly and pressure drops, especially from a nearly empty tank. Keep tanks fuller in winter and keep snow off them and the regulator.

3. Blocked intake or exhaust

Look at the furnace vent on the outside wall. Insects (wasps and mud daubers especially), birds, leaves, snow drifts or ice can block it. Blocked combustion air or exhaust trips the sail switch or causes ignition failure. Clear it carefully and fit a screen designed for RV furnace vents if insects are a recurring problem.

4. Blocked return air or ducts

The furnace needs to breathe. Make sure the return-air grille isn't blocked by storage, clothes or a cushion, and that floor registers are open. Too little airflow can trip the high-limit switch, which shuts the burner off to prevent overheating. It may start and stop repeatedly. A crushed duct under the floor does the same thing.

5. Sail switch or limit switch problems

If voltage is good, propane flows and the vents are clear, but the furnace still won't try to spark, a sticking sail switch or failed limit switch is likely. Lint and pet hair can jam the sail. Testing these switches means removing the furnace's access cover and checking continuity with power off. Many owners can do this, but follow the furnace manual's wiring diagram, and if you're unsure, leave it to a pro.

Symptom: clicking or sparking but no flame, then lockout

• Purge air from the lines (light the stove first).
• Check propane pressure issues: other appliances weak? Yellow, lazy flames on the stove? Suspect the regulator.
• The igniter electrode may be dirty, cracked or out of position, or the burner may be blocked by debris or spider webs. These are inside the combustion chamber, and adjustments must follow the manufacturer's specifications.
• To reset a lockout, turn the thermostat off for a minute and back on (some models need the furnace power cycled).

Symptom: lights, then goes out after a few seconds

The flame isn't being "proven". This usually means a dirty flame sensor, a poor ground, low gas pressure, or an air/exhaust issue. It's a classic one for a technician with a manometer to measure gas pressure.

Symptom: furnace short-cycles or overheats

Check filters (if fitted), return air and ducts. Short-cycling with hot air coming out usually means the limit switch is opening due to restricted airflow.

Winter use tips for Alberta

• Keep the battery charged. The furnace fan is one of the biggest 12V loads in your RV. Overnight at -20 °C, a single small battery may not make it to morning without shore power or a generator.
• Keep vents clear of snow, including after plowing at a campground.
• Run the furnace periodically during long storage checks to keep insects from nesting, but only when you're present.
• Have the furnace serviced every year or two by a certified RV technician, especially before a winter camping season.
• Carry a backup battery CO alarm in addition to the built-in alarm.

When to call a pro

Any work on the gas valve, burner, gas line or combustion chamber, any gas leak, a cracked heat exchanger (soot or exhaust smell inside), or repeated lockouts after you've confirmed power, airflow and propane supply. In Alberta, gas work should be done by a qualified, certified technician.