Furnace
A guided diagnosis for RV techs. Each step is a check, a reading or a question.
- What is the furnace doing? Turn the thermostat up at least 5 degrees above room temperature and stand at the furnace for two minutes. The sequence tells you where it stops: call, blower, sail switch, purge, spark and gas, flame proved, heat. Expect: Blower within a few seconds of the call, click and whoosh 15 to 30 seconds later, warm air shortly after
- 12V at the furnace on the heat call wire? With the thermostat calling, meter the thermostat wire at the furnace (usually the single small wire into the board or the relay) to furnace ground. Expect: Battery voltage, 12 V or more, on the call wire while the thermostat calls
- Is the thermostat alive and calling? Check the thermostat's own 12V feed and ground. Then jumper the thermostat's heat terminals (or short the two wires at the furnace end) to simulate a call. Expect: A jumpered call starts the blower immediately on most furnaces
- Is the furnace fuse good and is 12V reaching the board? Pull the furnace fuse at the distribution panel and meter it. Then meter the main 12V feed at the furnace (the heavy wires at the board or the motor relay) to ground. Expect: Fuse under 1 Ω, battery voltage at the furnace feed
- Does the blower motor get power during the call? Meter directly at the two motor leads while the thermostat calls. Then clamp the motor lead for amps. Expect: Battery voltage at the motor with 5 to 9 A flowing and the wheel turning
- Do you hear a spark? Stay at the furnace. After the blower has run 15 to 30 seconds you should hear the igniter snap for about 7 seconds. Some boards try three times with a pause between. Expect: A rapid ticking at the burner once the purge is done
- Is there gas at the furnace? Light a stove burner to prove supply. Then put a manometer on the furnace valve test port and watch it during a trial for ignition. Meter the gas valve coil for 12V during the trial. Expect: 11 in WC during the trial, 12V at the valve coil for the 7 second trial
- Look at the burner, orifice and electrode Pull the burner access cover. Check the electrode gap and its ceramic, look for rust and debris in the burner tube, and check the orifice for a spider web or dirt. Expect: Electrode about 1/8 in from the burner, clean tube, open orifice
- Did the sail switch close? The board won't spark until the sail switch proves airflow. Meter across the sail switch terminals with the blower running: closed reads near 0 V across it. Expect: Near 0 V across the sail switch with the blower at speed
- Why isn't the sail switch closing? Meter voltage at the furnace under load with the blower running. Listen to the blower speed. Look at the return air and the blower wheel. Expect: 12.0 V or more at the furnace with the blower running
- Is the high limit switch closed? The limit is normally closed and in series before the valve. Meter across it: near 0 V across a closed switch. Let the furnace cool first if it just ran. Expect: Near 0 V across a closed limit
- Spark at the electrode, or none? With sail and limit closed, the board should spark. Watch through the burner view (if there is one) or listen. Check the high tension lead and the board's ground. Expect: A strong snap at the electrode tip for about 7 seconds
- What does the flame look like before it drops out? Watch through the burner view if there is one. A healthy flame is blue with sharp cones, steady, and wraps the electrode tip. Expect: Steady blue flame covering the electrode
- Is it the limit tripping, or the thermostat cycling? Put your hand at a register. If the air goes hot then the burner drops while the blower keeps running, the limit opened. If the whole furnace stops and starts, the thermostat is cycling. Expect: Steady heat until the room reaches setpoint
- Blowing cold: is it in lockout or just slow to light? Watch the board LED if it has one, and listen for sparks. Count the tries. Expect: Light-off 15 to 30 seconds after the blower starts
- Blower only, or burner too? Turn the thermostat all the way down. The burner should drop immediately and the blower should stop within about 90 seconds to 3 minutes. Expect: Burner off at once, blower off a couple of minutes later
Causes and repairs
Thermostat or thermostat wire
The furnace runs when you bypass the thermostat, so the furnace is fine. The thermostat isn't closing the heat circuit, or its wire is open between the wall and the furnace.
Tools: multimeter
Check the wire for a staple through it or a broken splice. Replace the thermostat if the wire is good. If it is a multi-zone or networked thermostat, see the Thermostat guide.
Furnace fuse blown
A fuse that blows the instant you reinstall it is a short, usually a chafed wire or a seized motor. A fuse that blows after the blower runs a while is a dragging motor or a wheel rubbing the housing.
Tools: multimeter, clamp meter
Unplug the motor and try a new fuse. Holds: the motor is drawing too much, check the wheel for rub and the motor amps (typically 5 to 9 A, compare to the label). Blows: find the short in the harness.
Open 12V feed to the furnace
The panel sends power and the furnace never gets it. Look at the connector behind the furnace, the splice at the panel, and any disconnect or relay in between.
Tools: multimeter
Walk the wire with the meter until the voltage disappears. Repair the open with a proper crimp, not a twist and tape.
Open blower motor
Voltage at the motor with no current and no movement is an open winding or brushes. Spin the wheel by hand with the power off: if it turns freely, the motor is simply dead.
Tools: multimeter, clamp meter
Replace the motor. Check the wheel for rub and set the wheel position per the manual so the sail switch gets full airflow. Verify amps on the first run.
Blower seized or wheel jammed
A motor that hums and pulls high amps is stalled. Look for a wheel rubbing the housing, a mud dauber nest, a bad bearing, or debris in the combustion wheel.
Tools: hand tools, clamp meter
Power off. Free the wheel, clean the housings, replace the motor if the bearings are rough. Recheck amps against the motor label.
Control board or time delay relay not energizing the blower
The call is present and the board has 12V, but it never sends power to the motor. On Dometic (Atwood) boards a solid red LED is a board fault. Check the board's ground wire and its connector pins first; a corroded ground does this.
Tools: multimeter
Clean and tighten the board ground. If the relay never clicks with a good call and a good ground, replace the board (or the time delay relay on older units).
LP pressure drops under load
The regulator can't hold 11 in WC with the furnace drawing. A frozen or failing regulator, a nearly empty cylinder on a cold day, or a kinked or undersized line.
Tools: manometer
Test with the stove burners also lit. Replace a regulator that sags or that is over 10 years old. Check for a crushed line between the regulator and the furnace. See the LP guide for the leak and lock-up test.
Electrode out of gap or cracked
A spark that jumps to the wrong place won't light gas. A cracked ceramic lets the spark leak to the bracket.
Tools: feeler gauge
Set the gap to about 1/8 in (check your model). Replace a cracked electrode. Confirm the high tension lead is tight at the board and the electrode.
Burner or orifice fouled
Rust from the heat exchanger and insect debris choke the burner. Mud daubers love the exhaust.
Tools: hand tools, leak detector
Remove the burner, clean the tube and ports, check the exhaust and intake for nests. Replace the orifice if it is suspect. Reassemble and leak check the gas connection.
Gas valve not opening
12V at the coil, good pressure in, clean burner, and still no gas: the valve is stuck or the coil is open. Ohm the coil and compare to spec; an open coil is obvious.
Tools: multimeter, leak detector
Replace the gas valve. Leak check the connections after. Some valves are two-stage; make sure both coils get power during the trial.
Board not powering the gas valve
The board sparks but never opens the valve, or the valve wire is open. Check the valve connector and wiring back to the board.
Tools: multimeter
Meter at the board's valve terminals during the trial. 12V there and not at the valve: wiring. Nothing at the board: replace the board.
Low voltage, blower too slow to close the sail switch
Under about 11 V the blower can't move enough air to close the sail switch, so the board never sparks. The battery is weak, the converter is off, or there is drop in the wiring.
Tools: multimeter
Charge the battery or plug in. Then measure the drop from the battery to the furnace with the blower running; over 0.5 V points at a bad connection or undersized wire. See the 12V guide.
Restricted return air or ducts
Closed registers, a blocked return grille, an aftermarket filter, crushed flex duct, or a rug over the floor vent. The blower can't move air so the sail never closes.
Tools: flashlight
Open at least two registers, clear the return, remove any filter, straighten crushed duct. Then retest the sail switch.
Sail switch failed
Good voltage, good airflow, and the switch still reads open. The arm is bent, stuck, or the contacts are gone.
Tools: hand tools, multimeter
Replace the sail switch. Do not bend the arm to make it close; it is a safety.
High limit switch open
If the furnace is cold and the limit reads open, the switch has failed. If it just ran hot, find out why: restricted ducts or a dirty blower wheel overheats the exchanger and the limit does its job.
Tools: multimeter
Cold furnace: replace the limit. Hot furnace: fix airflow first, then check the limit again.
Control board not sparking
Airflow and limit proved, lead and electrode good, and no spark. The board's ignition section has failed. On Dometic (Atwood) boards you may see a 3 flash lockout after three tries.
Tools: multimeter
Replace the board with the correct part for the furnace. Confirm the board ground and all connectors.
Flame not proved: sense electrode or ground
The board opens the valve and watches for a small current through the flame to the grounded burner. An oxidized electrode, a wide gap, a cracked ceramic, or a rusty burner ground gives a weak signal and the board shuts the gas off in seconds.
Tools: emery cloth, feeler gauge, multimeter
Clean the electrode with emery cloth, set the gap to about 1/8 in, check the ceramic. Ohm the burner to the board ground: under 1 Ω. Clean the board ground. Then run a full cycle.
Poor flame: pressure, orifice or combustion air
A yellow or lifting flame won't hold the sense signal. Low LP pressure, a partially blocked orifice, or a blocked exhaust/intake starving combustion air.
Tools: manometer, hand tools
Manometer at the valve under load: 11 in WC. Check the orifice and burner, then the exhaust and intake for nests. Clean, correct, retest.
High limit tripping: overheating
Closed registers, blocked return, a filter someone added, crushed duct, or a dirty blower wheel. The exchanger overheats and the limit opens to protect it.
Tools: flashlight, hand tools
Open registers, clear the return, remove filters, inspect and clean the blower wheel, straighten ducts. If airflow is right and it still trips, replace the limit.
Thermostat location or anticipator
A thermostat in a warm spot (near a register, in sun, over a lamp) satisfies early. Analog thermostats have an anticipator; set it per the furnace spec. Digital thermostats have a cycle rate setting.
Tools: hand tools
Move the thermostat or redirect the register. Set the anticipator or cycle rate. Replace a thermostat that chatters.
Slow light-off: air in the line or weak spark
Air after a tank change clears itself in a few tries. A persistent slow light is a weak spark, a marginal gap, or low pressure that takes time to build.
Tools: feeler gauge, manometer
Set the gap, check the lead, manometer under load. If it lights clean on the first try three times in a row, it is fixed.
Blower relay or time delay stuck
The board's blower relay (or the separate time delay relay) is welded closed, or the thermostat wire is shorted to 12V somewhere. Disconnect the thermostat wire at the furnace: if the blower keeps running, it is the board or relay.
Tools: multimeter
Replace the board or time delay relay. If pulling the thermostat wire stops it, find the short in the thermostat wiring.
Gas valve not closing
A burner that stays lit after the call ends means the valve is not closing or the board is holding it open. This is a safety problem. Shut off the LP at the cylinder.
Tools: multimeter, leak detector
With LP off and the furnace cold, disconnect the thermostat wire. Meter the valve coil: 12V with no call means the board. No voltage and it still flowed gas means the valve. Replace the failed part and leak check.
Stop: combustion problem
Soot, a yellow flame at the exhaust, a strong smell, or a CO alarm means combustion products may be getting into the living space. A cracked heat exchanger, blocked exhaust, or a burner out of position.
Tools: CO meter, inspection mirror
Shut the furnace down and the LP off. Ventilate. Inspect the exhaust and intake for blockage, the burner alignment, and the exchanger for cracks. Replace the furnace if the exchanger is cracked. Do not return it to service on a guess.
Keep going on furnaces
Learn it
Prove it
- Furnace blows cold
- Furnace sparks but never lights
- Furnace keeps shutting off, fan keeps blowing
- Thermostat clicks, furnace does nothing
- Comeback: furnace quits again