As much as modern heating furnaces are a blessing to homeowners and renters, allowing us to “set it and forget it”, they also come with their own set of potential problems. Because they are situated inside the home, there is always the possibility of dangerous flue gases getting into our living space if certain issues should arise.
Fortunately, modern technology has kept pace with the development of furnaces and has provided built-in safety devices to alert us to a situation that could threaten the health of the inhabitants of our home.
One of these devices is the Flame Rollout Switch, also called a flame roll out switch. Either works.
What is a Flame Rollout Switch? How Does it Work?
When the furnace begins a heat cycle, a draft inducer fan starts pulling air through the furnace and out the flue. That’s the humming you hear coming from your furnace when the thermostat first calls for heat and before the furnace fires up.
After 15 to 45 seconds, it varies by furnace model, the gas burners ignite with a “whoosh,” and the flames are drawn by this draft into the heat exchanger. After another minute or so, the main blower turns on and begins drawing air from your rooms through cold air returns. The air passes over the very hot heat exchanger, and heat from the combustion gases is transferred into the air. Heated air is then pushed back through the supply registers (vents) in each room or zone.
Flame rollout is when some of the burner flames come backwards, or “roll out” of the heat exchanger of your furnace. In this situation, rather than just heating the heat exchanger in the heart of your furnace, stray flames are now heating the area where important control components are located behind the burners.
If this situation were to continue very long, internal furnace components such as wiring and the control board could be destroyed. If unchecked, a fire hazard could develop.
Here’s where the flame rollout switch enters the picture.
How does it work? The flame rollout switch is a safety device that will detect the presence of flame rollout and shut off the gas valve. This will, of course, cause the furnace to stop firing. The draft inducer fan might continue to run, and the main blower should run until the built-up heat inside the furnace is disseminated. But there will be no more flames causing potential damage and danger. The furnace will cool down and not start up again as long as the flame rollout switch is tripped.
Can I bypass a flame rollout switch? You shouldn’t because it performs a safety function to maintain your equipment in good condition and to protect your home and property.
Is it safe to just reset the flame rollout switch? Probably once or twice, and if the problem exists, the switch should do its job.
We give brief answers to those questions because complete answers to each are offered on separate pages of Pick HVAC. See Related Topics on this page to locate the articles with the answers.
What Can Cause a Flame Roll Out Switch to Trip?
There are several things that can cause flame rollout and the flame rollout switch to trip. They all have something to do with the interruption of a smooth draft passing over the flames, through the heat exchanger and out the flue. Here are some of them:
- Buildup of soot. This could be in the heat exchanger itself or in the flue. Soot buildup is usually caused by poor combustion and sooty flames instead of nice clean, blue flames. Excessive soot buildup can eventually block enough of the gases from traveling through the system to cause them to back up into the exchanger and combustion box where the burners are located. When this happens, flame rollout occurs.
- Blocked Flue. Whether your furnace sends its exhaust gases through a brick chimney, a metal pipe, or a plastic pipe, they can become blocked – especially during the summer months when the furnace is idle and there is no heat going through it. Common blockages include such things as:
An insect, bird or animal nest, a dead animal, or even a buildup of acorns or other nuts that some animal has stashed there.
Another way the flue can become blocked is if the flue cap is resting directly down on top of the pipe or chimney instead of above it. Rather than preventing rain and/or snow from coming down the flue, the cap may be preventing flue gases from going up the flue.
And there’s another possible reason. In wintry climates, snow and ice can surround and cover a flue vent, though the heat in the exhaust usually melts it fairly quickly.
- Blocked tube in the heat exchanger. This can occur if a large amount of nest material or other debris comes down the flue and actually gets inside the heat exchanger. It might also be caused by a buildup of rust flakes, as sometimes metal flue pipes and heat exchangers rust when they get older. Some heat exchangers have polypropylene liners. In defective units, the liner separates from the metal, and this problem can block the heat exchanger too. These issues might only affect one or two of the burners in your furnace (most have from 2 to 5 burners), but it could be enough to cause flame rollout in the ones that are blocked.
- Cracked or defective heat exchanger. Normally, when the blower turns on that sends warm air through your registers, it blows air past – as in over and around - the heat exchanger. The exchanger tubes or chambers are sealed so that the burning gases only pass through them to the flue. If there is a crack, a hole, or a defect that allows some of the blower air to get inside the heat exchanger, it can push backwards on the gas flames and cause rollout. This condition can also result in the dangerous situation of flue gases getting into your indoor air supply.
- A bad flame sensor switch: If genuine flame rollout has been ruled out but the switch keeps tripping, it could be bad. It’s not a common problem. However, if it has been determined that the switch is bad, you can DIY replace it for under $50. A quick search of parts suppliers shows generic switches for under $10 while OEM Rheem and Lennox switches cost $35 to $45. Disconnecting the bad switch, installing and securing the new switch are pretty easy tasks.
Where is the Flame Roll Out Switch on a Furnace?
The flame rollout switch (some furnaces have more than one) is located inside your furnace, behind the cover panel – usually the upper panel on an upright furnace. It will be mounted slightly above and behind the burners, so that if flame rollout occurs, it will sense the excess heat, operate (trip), and shut off the gas. The flame rollout switch in most furnaces may be manually reset, but if the reason it tripped is not addressed, it will trip again. See our page on Is it Safe to Reset the Rollout Switch?
What Are the Signs of Flame Rollout?
How do I know if my flame rollout switch is bad?
The average homeowner will usually not get into trying to figure out what the problem is if the furnace stops putting out heat. For most, it’s time to call the local HVAC company to send out a technician.
However – if you are somewhat handy and like to see if you can fix it yourself, there are a few things that could indicate that flame rollout is the reason you have no heat. Here are the most common:
- Button on the switch is popped out. Some, but not all, flame rollout switches are “one-time” switches. Once they trip, you have to replace them. Many other rollout switches have a button in the center that pops out when it’s tripped. To reset it, just push it back in. Caution: If the flame rollout switch trips a second time, it’s time to call in a trained technician to diagnose and repair whatever is causing it to trip. Do not, under any circumstances, bypass this switch!
- Blower and/or inducer fan running, but nothing else happening. This was probably caused by either an overheating or flame rollout situation, which caused the gas valve to be shut off.
- Evidence of burns or discoloration. This would be behind the cover panel, but outside of the furnace combustion chamber. In a high-efficiency furnace, there are sometimes plastic components because the exhaust temperature is low. These especially may have been burned or melted before the flame rollout switch tripped.
- Control board blinking an error code. In the removable panel, there is usually a small, round window through which you can see the control board. It has a light on it that blinks a specified number of times for each condition that shuts a furnace down. There should be a sticker or card inside the cover panel or on the control board itself to tell what the codes are indicating.
- Visually seeing the flames rollout. Flame rollout is an observable event with the front panel removed. Normally, the flames from the burners are nicely blue, indicating complete combustion, and are steadily drawn into the heat exchanger when burning. Flame rollout will look like the flames are pulsating, bouncing, or being blown back toward you as you observe them burning. They might also have a lot of yellow mixed in with the blue.
The flame rollout switch is one of a few safety devices that are designed to signal a problem with your furnace and to prevent serious damage to a fairly expensive piece of equipment in your home. While it’s always good to know what these devices are and how they operate, repairing them is usually not an option, since they are electronic.
The best way to avoid having any of them trip is to have a routine checkup of your furnace yearly – before the heating season after the furnace has sat idle for several months is a good time. This way, you can be fairly confident that you will have a warm and worry-free winter without having to make that middle of the night call to your HVAC technician.