What Is a Roll-Out Switch?

A roll-out switch is a safety feature found on a gas furnace. A furnace may have only one roll-out switch, or it may have several. The roll-out switch stops the furnace from igniting if a part of the furnace that is supposed to stay cool gets hot.

It's essentially an alarm system that lets you know the flames in your furnace are going places they should not go. A tripped roll-out switch is a symptom letting you know there is a bigger furnace problem you need to address.

When a gas furnace operates properly, the igniter lights the burners, and the flames begin. As they burn, they are pulled toward the back of the furnace where they warm the heat exchanger and then vent their exhaust to the outside. Sometimes, however, things can go wrong during this process. When they do, the flames and the heat they generate can both get pushed back into the furnace and the room it occupies. HVAC techs call this a "flame roll out."

This is dangerous for two reasons. One is that burning gas generates a colorless, odorless, and potentially deadly gas called carbon monoxide. If it backs up in your home, your family could get sick or even die from carbon monoxide poisoning. Flame roll outs can also overheat parts of the furnace that aren't meant to get hot, potentially starting a fire.

Roll-out switches prevent this problem by monitoring the heat buildup in the furnace. If something gets hot that shouldn't, the roll-out switch trips and turns off the gas. This prevents the possibility of a flame roll out until you can check the furnace.

When a roll-out switch trips, your furnace blower motor may run, but you won't get any hot air out of the unit. There are several ways you can determine if a tripped switch is the problem. One is to peek through the window on your furnace cover and see if the control board is blinking. Count the blinks and then check the sticker inside the furnace cover panel. This error code sticker will tell you if the problem is a tripped roll-out switch or if you have a different error code to address.

Sometimes, you can actually feel heat coming off the front of the furnace or see discoloration inside the furnace cover. Plastic parts inside the furnace may melt before the roll-out switch trips, making it clear that things inside the furnace were getting pretty hot.

You may also have the option of checking the switch yourself. Roll-out switches are typically located on either side of your burner box. You could have one switch, but you may have three, so look carefully around the burner box to make sure you check them all. Many roll-out switches have a manual reset button that pops out when they're tripped. If the button is popped, your switch is tripped.

In some cases, you may even witness the flame roll out firsthand. If you reset your flame roll-out switch and then run the furnace with the cover off, you may have time to observe flame roll out before the furnace switches off again. The flames coming from your furnace burner should be blue and conical in shape, moving away from you. If they appear rounded or reach back toward you, your furnace has a flame roll-out problem, and the roll-out switch should trip.

Some roll-out switches are once and done. Once they trip, they cannot be reset, so you'll need to have a furnace repair company replace them. Other switches have a manual reset, which is just a button that pops out. Push it back in and the switch is reset.

It's fine to reset a roll-out switch once to get your furnace back up and running. If it trips a second time, however, you need to call a furnace pro for some help. Do not under any circumstances bypass the roll-out switch. Its job is to tell you that something is wrong with your furnace, and your job is to listen to that warning.

You could experience flame roll outs and a tripping roll-out switch because of a problem with your vent or chimney. The problem could be as simple as a bird's nest blocking your chimney, or you could have a more serious problem, such as blockage or collapse of the chimney. Tripped roll-out switches can also indicate a cracked heat exchanger or low gas pressure. All of these issues can make operating your furnace unsafe, so shut down your furnace and call in an HVAC professional if your roll-out switch trips and you don't know why.