Furnace Rollout Switch Keeps Tripping: What It Means

By Ray Bennett · Updated 2 October 2026 · 5 min read · Checked against the sources at the end

Furnace Rollout Switch Keeps Tripping: What It Means

Short answer

The rollout switch trips when flame comes out of the front of the burners instead of going into the furnace. That is not a fault in the switch, it is the switch doing its one job. Reset it once to get heat, then find out why the flame came out, because a blocked flue and a cracked heat exchanger both start this way.

Of all the safety switches on a gas furnace, this is the one that deserves the most respect, and it is the one people most often reset and forget.

A rollout switch trips when flame spills out of the front of the burner box instead of being drawn into the furnace. Flame is supposed to go one way. When it goes the other, the switch cuts the gas, and it is usually telling you about something you cannot see.

Before you start

What to carry: a torch and a screwdriver. Fifteen minutes, and most of it is looking.

Gas or electric? Only a gas or oil furnace has one. An electric furnace has no flame, so no rollout switch, and none of this applies.

Know which switch you are looking at. A rollout switch is a small disc with two wires, mounted right by the burners, often with a little red or black button in the middle. The limit switch looks similar but sits higher up on the cabinet, and it means something different: what the limit switch is telling you.

The rule for this switch is different from the others. Reset it once, and only once. If the furnace runs normally afterwards and it never trips again, it may have been a gust of wind down the flue. If it trips a second time, stop resetting it. Lennox goes further for its ML180UH and says to call a technician the first time, so if your furnace’s label says the same, follow the label.

Never bypass it. Not with a wire across the terminals, not with the button taped down. Flame rolling out of a furnace sets fire to the wiring and anything stored nearby, and it can mean exhaust coming back into the house.

Fit a carbon monoxide alarm now if you do not have one. A rollout often means the exhaust is not leaving the way it should. If an alarm sounds, everybody goes outside and you call from there.

What it actually does

The burners fire into the heat exchanger and the exhaust is pulled out through the flue. If that path is blocked, flame looks for another way out, and the only other way is towards you.

Check 1

Understand why flame comes out the front

Flame goes where the air goes. On a working furnace the inducer pulls air in through the burners, through the heat exchanger and out of the flue, so the flame is drawn inwards.

If the exhaust path is blocked, that pull fails. The flame has nowhere to go but back out of the front of the burner box, where the rollout switch sits waiting for exactly that heat.

So the switch is a messenger. It is rarely the fault. Replacing a rollout switch that tripped for a reason puts a new part in front of an old problem.

The heat it reacts to is high. These switches are set to trip at temperatures the burner box never reaches when everything is working. Normal operation does not trip them.

The reset, done properly

Once to get heat, and then watch. A second trip is the end of the resetting and the start of finding out why.

Check 2Free, ten minutes

Reset it once and watch a full cycle

Power off at the furnace switch first. The switch that looks like a light switch on or beside the furnace.

Press the button on the rollout switch. It clicks when it resets. Some older ones are one time fuses with no button at all, and those have to be replaced rather than reset, which is itself a reason to have it looked at.

Power on, and stay there for a whole cycle. Turn the thermostat up five degrees and watch the burners through the sight glass for the full run.

What good looks like. Blue flames, steady, drawn into the heat exchanger, and no change when the blower starts.

What bad looks like. Flames lazy and yellow, flames licking out of the front, or flames that jump when the blower comes on. Switch it off and leave it off: why flames that move when the blower starts matter.

Put the thermostat back to where it was when you have watched the cycle.

What trips it, most likely first

Check 3Nothing to a new furnace

The causes, in the order they are found

A blocked flue. A bird’s nest, a fallen liner in an old chimney, leaves in a flue cap, or on a high efficiency furnace, snow or ice over the pipes outside. This is the common one, and some of it you can see from the garden.

A blocked or loose vent connection. The pipe from the furnace to the chimney can sag, separate or fill with debris, especially where it joins an old masonry chimney.

A cracked heat exchanger. The one nobody wants. A crack lets the blower push air into the burner area and push the flame out of the front. It is found with a camera and a combustion reading, not by guessing.

A dirty or misaligned burner. Rust flakes or debris in the burners can make the flame sit wrong and spill out. A cleaning fixes it.

Not enough combustion air. A furnace shut into a small cupboard, or a room sealed up tight with a new door, can be starved of the air it burns. Opening the door and seeing if it behaves is a useful free test.

Wind down the flue. A one off trip on a stormy night with no repeat is often just that.

Questions people ask

It works fine after I reset it. Can I just leave it?

If it tripped once and has not tripped since, yes, keep an eye on it. If it has tripped more than once, the reset is covering up a flue or combustion problem that will not fix itself.

Where exactly is the rollout switch?

On the burner box, near the front of the burners, behind the lower door. There may be one or two. They are small round discs with two wires each.

Is it the same as the reset button on my water heater?

No. That one is a high temperature cut out for the water inside the tank. Different part, different meaning: which reset button you are actually looking at.

Can a bad rollout switch trip for no reason?

It happens, but rarely, and it is the last thing to assume rather than the first. Check the flue and the flame before blaming the part.

My furnace code says rollout. Is that the same thing?

Yes. Many boards flash a specific code when a rollout switch opens. Read the code from the label inside your own furnace door: how to read the light the board is flashing.

Is it safe to run the furnace while I wait for somebody?

If it has tripped more than once, no. Use space heaters safely, keep the carbon monoxide alarm on, and leave the furnace off until the cause is found.

Sources: Goodman installation instructions IOG-2009E (*MSS9, *CSS9: rollout limits, flame appearance), Lennox installation instructions 507323-01 (ML180UH: flame rollout switches), Carrier 58SC0A/58SC1A installation, start-up, operating and service instructions (status code 33) and Trane "Cracked Heat Exchanger"