RV Furnace Blows Cold Air Then Shuts Off

The thermostat calls, the blower starts, cold air comes out of the registers, and then the furnace quits. Sometimes it never lights and shuts down after a minute or two. Sometimes it lights, you get heat for a few minutes, the flame drops out, and the blower keeps pushing cold air until it stops. Then it starts over.

If the furnace never attempts to light at all, the won’t-ignite guide covers that sequence. This guide is for the furnace that behaves like it’s trying and then gives up. In order of likelihood: a sail switch that closes at startup and then chatters open, a limit switch tripping because the furnace is starved for return air or duct area, flame sensing dropping out, a weak gas supply, a partly blocked vent, a short-cycling thermostat, and last of all the board.

You need a multimeter, screwdrivers, a flashlight, and a timer. Budget an hour.

What’s normal and what isn’t

Every Suburban NT/SF, Atwood and Dometic Hydro Flame 8500 series, and the rebadged Dometic furnaces run the same timeline when the thermostat calls.

1. The board waits a few seconds, then starts the blower.
2. The blower runs 15 to 30 seconds with no flame. This is the pre-purge. It clears the combustion chamber and lets the sail switch prove airflow. The air at the registers is cold, and that’s correct.
3. Sail switch closed, limit switch closed, the board opens the gas valve and sparks for about 7 seconds.
4. Flame is sensed and the furnace heats.
5. The thermostat is satisfied, the valve closes, and the blower runs another 60 to 120 seconds to cool the heat exchanger. The air goes from warm to cool during this post-purge. Also normal.

If a trial for ignition fails, the board closes the valve, purges, and tries again. Most Suburban and Atwood DSI boards try three times, then lock out. In lockout the blower finishes its cool-down and stops until the thermostat is turned down and back up.

So the problem isn’t cold air. The problem is cold air that never turns warm, or warm air that turns cold while the thermostat is still calling.

Step 1Time the cycle and match the pattern

Turn the thermostat up and stand at a register with a timer. Note when the blower starts, whether you hear the valve click and the igniter tick, when heat arrives, and when it leaves. Two or three cycles is enough. The pattern points to the cause before you pick up the meter.

Pattern	What it tells you	Go to
Blower runs 1-3 min, no click or spark, stops; repeats 3 times then quits	Board never got a closed sail switch, or it opened again during purge	Steps 2 and 3
Click, spark, flame for 2-10 s, drops out; repeats 3 times	Flame not sensed, or gas too weak to hold	Steps 5 and 6
Lights, heats 3-15 min, flame drops, blower runs on for minutes, relights later	Limit switch opening from overheating	Step 4
Lights, heats 1-3 min, whole furnace shuts down including blower, restarts soon	Thermostat satisfied too early (short cycling)	Step 8

Step 2Read voltage at the furnace under load, then watch it

The blower on a Suburban NT-30/SF-30 draws 7 to 11 A, and an Atwood 8535 around 8 A. On a battery that’s down to 12.2 V at rest, that load pulls the furnace terminals into the 11 V range, and it sags further every minute the blower runs.

The sail switch is a microswitch pushed closed by a paddle in the blower stream. At design speed the paddle is held firmly. At a slow blower it’s barely held, and a little more sag, or the drag of a dirty wheel, lets it flutter open. The board sees the switch open and drops the flame. This is why a furnace that runs fine on shore power quits after 10 minutes on battery.

Pull the access panel, find the 12 V feed to the board, set the meter to DC volts, and read with the blower running. Keep watching for a couple of minutes.

• 12.0 V or higher and steady: power is fine. Go to step 3.
• 10.5 to 12.0 V, or dropping while you watch: marginal. Plug in to shore power and rerun the cycle. If the dropouts stop, the battery, the converter, or the wiring to the furnace is the fix.
• Below 10.5 V: most boards drop out between 9.5 and 10.5 V, and the blower is far too slow before that. Charge or replace the battery, and check the battery terminals and the furnace ground screw.

If the battery reads 12.6 V at the terminals and 11.2 V at the furnace, the drop is in the wiring. Look for a corroded ground, a loose spade connector, or a connection that’s warm to the touch.

Step 3Clean the blower wheel and prove the sail switch

With voltage good, the next reason a sail switch chatters is airflow. Dust, pet hair, and insulation fibers pack between the blower vanes and reduce the air it moves without changing how it sounds.

Disconnect 12 V to the furnace (pull the fuse). Get a brush into the blower wheel and clean every vane, then clean the sail switch paddle. Test the switch: disconnect its two wires, set the meter to continuity, and push the paddle closed by hand. You want a beep or near 0 Ω closed and an open circuit released. A switch that reads intermittently as you wiggle the paddle is worn out; replace it. Suburban commonly uses 230575 or 232258 depending on model, Atwood 33181 or 37150. Verify against your service manual.

Do not wire the sail switch closed to test. It is the only thing proving airflow before the valve opens.

Step 4Find out why the limit switch is tripping

If the furnace heats for several minutes and then the flame drops while the blower keeps running, the limit switch has opened. It’s a thermal disc on the casing near the heat exchanger, normally closed, and it opens when the casing passes its rating, typically 190°F or 200°F on Suburban units (stamped on the switch). When the casing cools it closes and the furnace relights. Warm, cold, warm is the signature.

The limit switch is almost never the problem. It’s reporting one. Too little air is moving through the heat exchanger, and that comes from a short list.

• Closed registers. The installation manual lists a minimum number of open 4 in ducts per model (a 30,000 BTU/h Suburban typically needs at least three or four). Closing bedroom registers to push heat up front is the most common way owners trip the limit.
• Crushed or kinked flex duct. Follow each run from the plenum to its register. Ducts get crushed under drawers, kinked behind the water heater, or flattened where a pipe was routed over them. A register with much weaker flow than the others has a restricted duct.
• Blocked return air. Towels, shoes, or a dog bed against the return grille will trip the limit in minutes. Suburban publishes a minimum return air area per model; a factory cutout covered by trim or a mat is a common find.
• Duct disconnected at the plenum. Heat dumps into the cabinet instead of the ducts, and the casing overheats.

Check the switch itself only after the airflow is fixed: disconnect the two wires and confirm continuity at room temperature. Open at room temperature means it’s failed; replace with the same rating. If a new switch keeps opening with every register open and the return clear, stop. A cracked or sooted heat exchanger can do this and that’s a tech’s call.

Step 5Flame sensing dropout

If the furnace lights and the flame drops within 2 to 10 seconds, every time, the board isn’t seeing the flame. The spark electrode doubles as the flame sensor by flame rectification, which needs a clean tip and a solid electrical path from the burner to the chassis.

Pull the burner assembly (two or three screws). Clean the electrode with fine abrasive, check the ceramic for cracks, and set the gap to 1/8 in (Suburban and Atwood manuals specify 1/8 in; some list a range to 3/16 in). Then check the burner mounting screws and any ground strap. Rust or a loose screw there raises resistance in the sensing circuit and the board gives up. Clean to bare metal and tighten.

Step 6Gas supply that weakens under load

The furnace is the biggest propane load in the trailer, 20,000 to 40,000 BTU/h, and it fails on a weak supply long before the stove does. If the furnace lights and drops out, or runs with a lazy yellow flame, check the supply.

• Stove test. Light two burners on high. Strong blue flames that don’t shrink when the furnace valve opens means supply is probably fine. Flames that shrink or go yellow point to the regulator or the tank.
• Near-empty tank in the cold. Propane has to boil inside the cylinder to supply vapor, and it takes heat from the surrounding air to do it. A 20 lb cylinder with a few pounds left at 20°F can’t keep up with a 30,000 BTU/h furnace, and pressure sags until the flame drops. Swap to a fuller tank and retest. The propane usage guide has the numbers.
• Excess flow valve. If the tank valve was opened fast after a swap, the excess flow device in the pigtail may be passing only a trickle. Close the tank valve, wait a minute, and open it slowly.
• Regulator. Two-stage RV regulators deliver 11 in water column (about 0.4 psi) at the appliance. A regulator that’s frozen, waterlogged, or past about 15 years old can hold pressure at idle and collapse under load. With a manometer, 11 in WC with the furnace and two burners running is the spec; without one, the stove test is your proxy.

Step 7Intake and exhaust blockage

Shine a light into both tubes at the exterior vent. Mud daubers build in the warm exhaust tube; the intake collects webs and leaves. A partial blockage changes the air-fuel mix enough to give a flame that lifts off the burner, or one that soots the electrode every cycle. Clear with a brush and compressed air.

Step 8Thermostat short cycling

If the furnace lights, heats a minute or two, and the whole thing shuts down including the blower (after the post-purge), then restarts a few minutes later, the thermostat is being satisfied too fast. The furnace is fine. The room never warms because each call lasts 90 seconds.

• Mechanical thermostats (basic Suburban, Coleman, and Duo-Therm wall units) have a heat anticipator, a small adjustable resistor under the cover with a scale. Set too low, it ends the call early. Move it toward “longer” in small increments and watch the cycle length.
• Digital thermostats have a swing or differential setting, often 1°F to 3°F. Set it to 2°F or more. Replace the thermostat batteries while you’re there.
• Location. A thermostat in direct register airflow or afternoon sun is being warmed by something other than the room.

Step 9The board, last

If voltage holds at 12 V or better under load, the blower wheel is clean, the sail switch is firm, the limit switch has continuity with every register open, the electrode is clean and gapped with a tight burner ground, the stove burns strong while the furnace fires, the vent is clear, and the thermostat holds a call, then the board is a fair suspect.

Read the diagnostic LED first. Suburban and Atwood boards blink a code for limit open, sail switch open, and ignition lockout. If the code says sail switch, the board is telling you to go back to step 2.

Replacement options are OEM or a Dinosaur Electronics board (UIB 64 for most Suburban and Atwood DSI furnaces, FAN 50 Plus where the fan relay is on the board). Dinosaur boards tolerate low voltage better, but a new board on a furnace with a packed blower wheel fails the same way the old one did.

Symptom → likely cause

What you see	Most likely	Check
Cold air 15-30 s, then heat	Normal pre-purge	Nothing
Cold air 1-3 min, never lights, three times, quits	Sail switch not held closed: low voltage or dirty wheel	Voltage at furnace under load; blower wheel; sail switch
Lights, out in 2-10 s, repeats	Flame sensing or weak gas	Electrode, burner ground, stove test
Heats 3-15 min, flame drops, blower runs on, relights later	Limit switch tripping	Open all registers, ducts, return air
Flame gets lazy or yellow as it runs, then drops	Gas supply weakening	Tank level and temperature, regulator, excess flow
Heats 1-3 min, everything shuts off, restarts soon	Thermostat short cycling	Anticipator or swing setting, location
Board LED blinking	Whatever the code chart says	Service manual

Parts and tools mentioned

• Digital multimeter with DC volts and continuity
• Sail switch (Suburban 230575 / 232258, Atwood 33181 / 37150; verify against your model)
• Limit switch, matched temperature rating
• Soft brush, compressed air, fine abrasive, 1/8 in drill bit shank for the gap
• 4 in flexible duct and clamps if a run is crushed
• Dinosaur Electronics UIB 64 or FAN 50 Plus, or OEM board
• Manometer (optional) for the 11 in WC check

Questions people ask next

Why does the furnace blow cold air for a while before it lights? That’s the pre-purge. The board runs the blower 15 to 30 seconds to clear the chamber and prove airflow through the sail switch before it opens the gas valve. The cool air at the end of a cycle is the post-purge. Neither is a fault.

Is the limit switch bad if it keeps tripping? Almost never. It’s opening because the casing is reaching 190°F to 200°F, which means not enough air is moving through the heat exchanger. Fix the registers, ducts, and return air. Replace the switch only if it reads open at room temperature.

The furnace works fine plugged in but quits overnight on battery. Battery or furnace? Battery, or the wiring to the furnace. The blower pulls 7 to 11 A, and a battery that reads fine at rest sags under that load as the night goes on. Read voltage at the furnace terminals with the blower running.

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