Did your furnace’s flame rollout switch trip? Then there definitely IS a cause for alarm.
A tripped flame rollout switch is not a normal thing- it means there’s a problem with your furnace. And it can be a lethal one if not addressed.
In this article, I’ll go over the causes of a tripped flame rollout switch, and what you should do if your flame rollout switch is tripping.
I’ll also explain how to test a flame rollout switch if you think yours is bad. Let’s get started!
What is a flame rollout switch?
The flame rollout switch is used to detect if there is a fire or hot exhaust gases inside of your furnace’s burner compartment.
The rollout switch is located on the side or top of the burner compartment. Most flame rollout switches look like a little metal circle with two wires coming out of the back.
During normal furnace operation, the flames from the burners will be sucked into the heat exchanger.
If there is low suction into the heat exchanger, the flames will start “rolling out” from the heat exchanger.
When flame rollout occurs, the flames from the burners move backward and start burning all the parts inside your furnace. The flame rollout switch is used to detect this.
Oftentimes, there will be two or even three flame rollout switches in the furnace. For example, some furnaces have a rollout switch on each side of the burner compartment.
What to do when your flame rollout switch keeps tripping
If the flame rollout switch is tripping, then you need to find what’s causing it to trip.
Flame rollout switches only trip when there is an underlying issue with the furnace.
You SHOULD NOT keep resetting the rollout switch and running your furnace. You need to figure out what’s causing the switch to trip, and address that issue.
What causes a flame rollout switch to trip?
Flame rollout switches trip when the flames from the burners escape from the heat exchanger into the rest of the furnace.
Here are a few possible causes of tripped flame rollout switch:
1. Clogged exhaust vent
A clogged exhaust vent will restrict airflow through your heat exchanger.
If airflow is restricted through the heat exchanger, then flame rollout can occur.
Be sure to check your exhaust vent for any possible obstructions.
Here are a few possible causes of a clogged furnace exhaust vent:
- Bird’s nests
- Tree branches & leaves
- Soot buildup
For more information on blocked exhaust vents, check out my article here:
2. Clogged heat exchanger
A clogged heat exchanger has the same effect as a clogged exhaust vent– airflow restriction.
When the heat exchanger is clogged, flame rollout can occur.
So what causes a clogged heat exchanger? Usually soot buildup or corrosion.
Soot is the product of gas molecules that don’t completely combust. It’s sort of like ashes from a campfire.
Over time, unburned gas molecules will form and you’ll get soot inside your furnace’s heat exchanger.
A little bit of soot is a normal thing. But if you find an excessive amount of soot buildup, there is another issue with your furnace.
A clogged heat exchanger will reduce the performance of your furnace.
It may even prevent it from functioning at all.
If you have a high-efficiency furnace, its heat exchanger might be able to be removed and cleaned.
I recommend that a qualified HVAC professional cleans your heat exchanger if it needs cleaning.
3. Cracked heat exchanger
A cracked heat exchanger will reduce suction inside your heat exchanger. This leads to less airflow and flame rollout.
Besides flame rollout, another sign that your heat exchanger is cracked is if your carbon monoxide alarm is going off.
A crack in your heat exchanger will allow combustion gases to infiltrate the air in your home. This is dangerous because carbon monoxide from the furnace’s combustion gases is extremely toxic.
So a cracked heat exchanger is not only bad for your furnace but bad for your health!
Unfortunately, if your furnace’s heat exchanger is cracked, then it will need to be replaced.
Replacing the heat exchanger is very labor-intensive. The entire furnace will need to be taken apart. The heat exchanger is an expensive part, too.
If your heat exchanger is cracked, chances are that your furnace is pretty old. So it’s almost always worth it to replace the entire furnace instead of just the heat exchanger.
4. Low gas pressure
Low gas pressure is another cause for flame rollout.
If the gas pressure in your furnace is low, then the flames have a good chance of rolling out instead of going into the heat exchanger.
Low gas pressure is usually caused by dirty burners.
When the burners’ orifices get clogged, the gas pressure will drop.
To fix this, you need to clean the burners and the orifices.
I recommend using compressed air and a wire brush to clean them out.
Check out this video from Word of Advice TV. Jay does an excellent job showing you how to clean the burners in a furnace.
How to test a flame rollout switch
Sometimes, your furnace’s flame rollout switch might go bad.
For instance, if your flames are not rolling out, but the rollout switch is tripping, then the rollout switch might be bad.
Normally, a flame rollout switch will close when it’s not tripped. A bad flame rollout switch will have an open or high-resistance circuit when not tripped.
To test your furnace’s flame rollout switch, follow these steps:
- Turn off power to the furnace. Find the disconnect to the furnace to turn off power. You don’t want the furnace turning on while you’re working on it.
- Open up the furnace and locate the flame rollout switch. The flame rollout switch is usually located in the burner compartment. It is a small, button-shaped device that has two wires coming out of it. Most furnaces have at least two rollout switches- one on either side of the burner compartment.
- Remove wires from flame rollout switch. Disconnect the wires from the flame rollout switch terminals. Also, be sure that you reset the rollout switch by pressing the button on it. The button is usually located between the terminals.
- Use a multimeter to test the rollout switch. Get out your multimeter and set it to the continuity or ohms (Ω) setting. Use the two multimeter probes to touch the terminals on the rollout switch- one probe on each terminal.
If the flame rollout switch is good, the multimeter will show a short circuit or a very very low resistance reading (0, 1, or 2Ω).
If the flame rollout switch is bad, the multimeter will show an open line (OL) or a higher resistance reading. For example, somewhere in the hundreds or thousands of Ω.
- Reconnect wires to rollout switch. After you test the rollout switch, it’s time to start putting everything back together. Of course, if your rollout switch is bad, then you’ll want to remove it and replace it with a new one.
Reconnect the wires to your flame rollout switch. The rollout switch is not polarity-sensitive, so it doesn’t matter which wire goes to which terminal.
- Close furnace & turn power back on. After you reconnect the rollout switch, close up your furnace and turn its power back on. Now you’re ready to test it out! Set your thermostat to “heat” and see if your furnace works now.
Flame rollout switch versus high limit switch: What’s the difference?
Furnace high limit switches and flame rollout switches are often confused with one another.
It’s not surprising that they are mistaken for each other because they are very similar.
Here’s the difference:
- The flame rollout switch detects if there is a fire or hot exhaust gases in your furnace’s burner compartment. The flame rollout switch needs to be reset manually by pressing a button on the switch.
- The high limit switch detects if there is a high temperature in your furnace’s heat exchanger. The high limit switch will reset automatically when the temperature inside of the heat exchanger falls below a certain level.
Both flame rollout and high limit switches will close the gas valve when they’re tripped. This causes the burners to shut off in both scenarios.
In fact, sometimes it’s tough to tell if your furnace is tripped on high limit or flame rollout since they both shut off the burners!
One way to tell is to take a look at the indicator light on the furnace’s control board.
The indicator light will flash in a certain manner depending on what kind of error it has.
You’ll need to get your furnace’s instruction manual out to decode the flashing light and see whether your high limit or rollout switch is tripped.