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CampWrench
ClimateModerateAbout 45 min

RV furnace blows cold air

The blower runs but the burner never lights, so only cold air comes out.

Applies to: Forced-air propane furnaces in towable and motorised RVs, including Suburban NT and SF series and Dometic/Atwood AF and HydroFlame series.

Quick answer

If the blower spins but no heat follows, the furnace is failing before ignition. The three causes that account for most cases are low propane supply (including a regulator that locked out after a tank change), house battery voltage under roughly 10.5 volts, and a sail switch that never closes because airflow is restricted. Listen to the start sequence: blower spins up, purges for 15 to 30 seconds, the gas valve clicks, then the burner lights. Whichever stage it stops at tells you which circuit to test first.

What usually causes it

  • No propane reaching the burner

    Most common

    An empty tank, a closed valve, or an excess-flow lockout in the tank's OPD valve caused by opening the valve too quickly after a refill. The furnace draws more gas than a stove burner, so it is often the first appliance to fail on marginal supply.

  • House battery voltage too low

    Most common

    The furnace is a 12V DC appliance even when you are on shore power. Below roughly 10.5V the control board will spin the blower but will not hold the gas valve open. A battery that reads fine at rest can sag badly under blower load.

  • Sail switch not closing

    Common

    The sail switch proves airflow before gas is allowed. Closed floor registers, a crushed or disconnected duct, a blocked return-air grille or a dirty blower wheel all reduce airflow enough to keep it open.

  • Limit switch tripped on overheat

    Common

    Restricted airflow overheats the plenum and trips the limit switch. It resets itself once cool, which is why the furnace often works for one cycle then blows cold. Repeated trips are an airflow fault, not a switch fault.

  • Obstructed burner tube or orifice

    Common

    Mud dauber and spider nests inside the burner tube are a classic cause after storage. The furnace will attempt ignition and fail, or light briefly and go out.

  • Failed igniter, electrode or control board

    Less common

    If the gas valve clicks and you can smell a brief gas cycle but never get flame, the electrode gap, igniter or board is the suspect. This is the point where most owners hand it to a technician.

Step-by-step diagnosis

Work in this order. Each step is cheaper or safer than the one after it, so stop as soon as the symptom clears.

  1. Confirm propane actually reaches the appliances

    Check the tank gauge, then open the tank valve slowly. Opening it fast trips the excess-flow device inside the OPD valve and starves every appliance. Light a stove burner: a strong steady flame means supply is good, a weak or yellow flame points at the regulator or a lockout. If the stove will not light either, stop here and solve the supply problem first.

  2. Measure house battery voltage under load

    With a multimeter at the battery terminals, look for 12.4V or higher at rest. Then call for heat and watch the reading while the blower runs. If it drops below about 11V, charge or replace the battery before diagnosing anything else. A furnace that only fails at night on battery power is almost always this.

  3. Run the start sequence and note where it stops

    Set the thermostat at least 5°F above room temperature. You should hear the blower spin up, run a 15 to 30 second purge, then a distinct click as the gas valve opens, then ignition. No blower at all points at the thermostat or board. Blower with no click points at the sail switch, limit switch or a lockout. Click with no flame points at gas delivery or ignition.

  4. Restore full airflow

    Open every floor register, clear the return-air grille of rugs and bedding, and check under dinettes and beds for a duct that has been crushed or pulled off its collar. Vacuum the return grille. This single step resolves a large share of sail switch and limit switch complaints.

  5. Inspect the burner tube and exterior vent

    With the propane off at the tank, remove the exterior access panel and look into the burner tube for spider webs and mud dauber nests. Blow them out with compressed air. Check that the exterior intake and exhaust vent is clear of nests and debris.

    Shut the propane off at the tank before opening any part of the burner assembly.

  6. Let a tripped limit switch cool and retry

    If the furnace ran briefly then went cold, wait 15 minutes with the power on so the blower can cool the plenum, then call for heat again. Heat that returns and then fails again on the same cycle confirms an airflow restriction rather than a bad switch.

  7. Test the sail switch for continuity

    With the blower running, check continuity across the sail switch terminals. An open circuit while the blower is at full speed means either the switch has failed or airflow is still too weak. Compare against your model's service literature for the correct terminal pair before condemning the part.

    Never jumper the sail switch to make the furnace run. It exists to stop the burner firing without airflow, and bypassing it creates a genuine fire risk.

  8. Escalate ignition faults

    If gas is confirmed, voltage is good, airflow is good and the valve clicks without producing flame, the remaining suspects are the electrode gap, igniter and control board. Replacing a gas valve or control board on a sealed-combustion appliance is the point to book a certified RV technician.

Safety notes

  • If you smell propane at any point, shut the tank valve, ventilate the RV, avoid switches and flames, and leave until it clears.
  • Never bypass a sail switch or limit switch. Both are burner safety interlocks.
  • Confirm your carbon monoxide alarm works before running any propane heating appliance overnight.

When to call a professional

Call a certified RV technician for gas valve replacement, any suspected propane leak, a cracked heat exchanger, or if you find soot around the exterior vent. Soot indicates incomplete combustion and is a carbon monoxide concern.

Frequently asked questions

Why does my RV furnace run for a few minutes then blow cold air?

That pattern almost always means the limit switch is tripping on overheat. The burner lights, the plenum overheats because airflow is restricted, the limit switch cuts the gas, and the blower keeps running to cool it down. Open all floor registers, clear the return-air grille, and check for a crushed duct.

Can I run my RV furnace on shore power alone without a battery?

No. The furnace blower, control board and gas valve all run on 12V DC. On shore power the converter supplies that DC, but many RVs will not deliver stable voltage to the furnace without a battery in the circuit to buffer it. A weak or disconnected battery is one of the most common causes of a furnace that only spins the fan.

Why did my furnace stop working right after I filled the propane tank?

Opening the tank valve quickly after a refill trips the excess-flow protection inside the OPD valve, which restricts gas to a trickle. Close the valve, wait about a minute, then reopen it slowly over several seconds with all appliances off.

Is it normal for the furnace to smell for the first minute?

A faint burning-dust smell on the first run of the season is normal and clears in a few minutes. A sharp propane smell, or a smell that persists on every cycle, is not normal and should be investigated before further use.

Also described as

  • rv furnace runs but no heat
  • camper furnace blowing cold air
  • rv heater fan works but no heat
  • suburban furnace not igniting

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