How RV Furnace Ignition Cycles Work

When your thermostat calls for heat, it sends a 12V signal to the furnace control board. This board is the brain of the operation, orchestrating the entire sequence. First, the board energizes the blower motor to start air circulation. This pre-purge cycle clears any residual gas from the combustion chamber - a critical safety step that prevents explosions. After a few seconds of air circulation, the board opens the gas valve, allowing propane to flow into the burner assembly. Simultaneously, the igniter begins sparking - you'll hear rapid clicking if you're near the furnace. Modern furnaces use direct spark ignition (DSI), where electrodes create sparks to light the gas. Older models used glow plugs, but DSI is more reliable. When propane meets the spark, combustion begins. A flame sensor (a thin metal rod near the burner) detects the flame through electrical conductivity - flames conduct electricity, completing a circuit that tells the control board combustion succeeded. If the sensor doesn't detect flame within a few seconds, the board shuts down the gas valve and may retry the sequence. This is a crucial safety feature preventing raw propane from filling your RV.

Once combustion is established, the burner produces hot exhaust gases in a sealed chamber. Unlike your home furnace, the combustion chamber is completely isolated from the air you breathe - combustion gases exhaust outside through a tube. Heat from the combustion chamber warms metal fins or a heat exchanger, similar to a radiator. The blower motor forces air across these hot surfaces, absorbing heat. This warmed air then flows through insulated ducting to vents throughout your RV. The thermostat continuously monitors temperature. When your RV reaches the set temperature, the thermostat stops sending the 'call for heat' signal. The control board shuts off the gas valve, extinguishing the flame. However, the blower continues running for another 30-60 seconds to extract remaining heat from the exchanger and prevent overheating - this is called the post-purge cycle. Only after this cooling period does the entire system shut down. This cycle repeats as needed to maintain temperature, and it's completely automatic - your only job is setting the desired temperature.

The most common ignition failure is low battery voltage. The control board, blower motor, gas valve, and igniter all run on 12V power. If battery voltage drops below about 11.5 volts, the board may start the sequence but fail to complete it - there's not enough power to spark the igniter or open the gas valve fully. This is why furnaces often fail on cold mornings after running all night - batteries are depleted. Charging batteries or connecting to shore power instantly solves this issue. Dirty or misaligned flame sensors cause another common problem. The flame sensor rod accumulates carbon deposits over time, insulating it from the flame's electrical conductivity. The sensor 'sees' no flame even though combustion occurred, so the board shuts down for safety. This creates a frustrating pattern where the furnace lights, runs a few seconds, then shuts off. Combustion air intake and exhaust tubes can also become blocked by debris, bugs, or birds' nests. Without proper airflow, the furnace can't combust properly or exhaust gases build up, triggering safety lockouts.

Modern RV furnaces include multiple safety devices that prevent operation when conditions aren't right. The sail switch is a small flap inside the blower housing that only closes when adequate airflow is detected. If the blower motor fails or ducting is severely blocked, the sail switch won't close, and the control board won't attempt ignition. This prevents overheating and potential fires. Thermal limit switches detect if the heat exchanger becomes dangerously hot - this can happen if ducting is severely restricted or the blower fails after ignition. If a limit switch opens, the furnace shuts down immediately. These safety devices sometimes cause mysterious failures. A sail switch can stick or accumulate dust that prevents it from closing properly, making a perfectly functional furnace refuse to start. Limit switches can occasionally fail 'open' even when temperatures are normal, creating nuisance shutdowns. Troubleshooting these requires understanding the furnace logic and testing continuity through safety circuits - work best left to professionals who have diagnostic experience and appropriate meters.

While checking propane levels and battery voltage are owner-level tasks, furnace diagnosis requires specialized knowledge. Furnace control boards are complex electronics that can fail in subtle ways - maybe they'll light the furnace but won't maintain flame, or they'll run the blower but never attempt ignition. Accurately testing these requires understanding 12V circuits, flame sensing current, and proper gas pressures. Improper diagnosis leads to replacing expensive parts unnecessarily. Gas-related work on RV furnaces must be performed by qualified technicians. Propane is heavier than air and can accumulate in low areas, creating explosion risks. Gas valve adjustments, regulator settings, and leak testing require professional training and specialized equipment. If your furnace won't stay lit, produces soot or unusual odors, makes popping sounds, or cycles on and off rapidly, shut it down and call a technician. These symptoms can indicate incomplete combustion, cracked heat exchangers, or gas pressure issues - all potentially dangerous conditions requiring immediate professional attention.