Furnace Rollout Switch Vs. High Limit Switch: What’s The Difference?

It's not surprising that people often relate the furnace rollout and high limit switches as one device. Given how similar they are but that's just not the case. We researched this topic to help you differentiate these two safety switches. 

The flame rollout or rollout switch detects an overheating issue in the furnace combustion chamber. In this case, it instantly shuts its system to stop the gas supply and let the flame go off. Typically, you'll see this device nearby the burners or burner compartment.

In contrast, furnace high limit switches have a probe sensor to detect high thermal levels inside the heat exchanger. Additionally, the high limit switch is responsible for turning furnace blower fans on or off. You can access this device inside the heat exchanger itself.

You can only reset the rollout switch manually when the furnace unit is tripped. On the other hand, a limit switch can automatically reset the furnace system to get it working properly again. Stay with us and keep reading to know the other significant answers relating to this topic. 

What Are Rollout And High Limit Switches On Furnaces?

Technician is holding wrench in front of combi gas boiler

Your furnace uses several components to provide heat in your home and operate its system properly. It includes the rollout and the high limit switches. Though sometimes, people often confuse recognizing both switches' distinct functionalities. Safeguarding and monitoring your furnace system are these switches' common tasks.

The rollout switch is a flame sensor that shuts the system gas supply when the combustion chamber becomes too hot. This safety device is vital to prevent any potential fire risk due to overheating furnace components.

Meanwhile, the high limit switch is also a vital part of furnace safety. It can sense the temperature in the air and close the burner as the temperature gets to a limit. The blower will continue to operate until the furnace's interior temperature drops to the lower limit.

What Causes Rollout Switch To Trip The Furnace?

The flame rollout switch on your furnace can certainly trip. This problem is due to a lack of regular service to its component. If this switch keeps on tripping, it indicates that there might be serious issues that need to be addressed.

Here are the common causes why rollout switches trip the furnace unit:

Dirty Burners

Severely dirty burners can completely reduce gas pressure in your furnace. As a result, it increases the likelihood that the flames could spread out rather than enter the heat exchanger.

Gas heating boiler flames

You must clean the furnace burners along their holes to resolve this problem. We advise cleaning them with steel wool and vacuuming them to securely remove all dirt.

Check this dependable vacuum cleaner on Amazon.

Blocked Exhaust Vent

A blocked exhaust vent can hinder the airflow of your heat exchanger. As the heat exchanger continues to impede the air in its flue pipe. This will result in the flame going out of the heat exchanger and possibly exiting anywhere.

You can blame the pesky rodents or even spiders building their nest inside the exhaust vent. Cleaning your exhaust pipes at least each year or two will prevent rollout tripping from occurring.

Jammed Heat Exchanger

If your heat exchanger becomes clogged, it will reduce the airflow necessary for the furnace to function smoothly. This will cause the same effect as blocking your exhaust vent; thus, it trips the furnace occasionally.

The soot build-up is what causes your heat exchanger to clog. An eco-friendly soot cleaner is all you need to clean your heat exchanger.

Buy this soot cleaner on Amazon.

Damage Heat Exchanger

Over time, it's impossible to avoid your heat exchanger manifest breakdown. Although, heat exchangers have a long life expectancy of around 15- 20 years. Even so, an overly old heat exchanger is more prone to crack or damage.

This is due to excessive heat exposure in all its service life within the furnace system. Therefore, a tripping issue in your rollout switch can normally occur in this condition. Replacing the heat exchanger can help solve this issue.

What Causes The High Limit Switch Tripping Issues?

Don't worry if the high limit switch on your furnace trips. The system's primary safety feature is to warn you early on in case of abnormal activities arising in its system.

Here are the few causes you can look for to help avoid tripping issues:

Technician repairing Gas Furnace

Short Cycling

The term "short cycle" refers to the furnace briefly turning on before shutting off. It's not necessarily that you suspect your furnace that it is the root cause of why short cycles happen.

Sometimes, improper thermostat settings can falsely command your furnace to operate wrongly. Ideally, a 68°F setup in your thermostat can effectively warm your home in the winter season aside from saving power.

Flame Sensor Rod Is Dirty

If you keep ignoring the maintenance of your furnace system for some time, the development of dust or soot in the rod is likely unavoidable. This is exactly what causes your high limit switch to trip more often.

Limited Airflow Issues

Since a dirty air filter can restrict airflow and cause your furnace's high limit device to trip and open irregularly, it is best to clean or change your furnace air filter every 30 days to maintain proper airflow and save energy.

Get this Air Filter on Amazon.

High Limit Switch Is Defective

If somehow the tripping problem didn't go upon thorough inspection of the previous suspect causes. This might simply be that your high limit device is failing or completely becomes defective. Replace it as necessary as possible to attain a long-term solution and get rid of this specific dilemma for good.

Have a quick look at this high limit switch on Amazon.

How Can I Tell That My Furnace High Limit Switch Is Defective?

Technician repairing combi Gas Boiler

A defective high limit switch is the result of a wear and tear process throughout its service cycle. This will lead to overheating your furnace more often. After all, you can observe this particular problem with so many system failure manifestations. As such, the blower might also fail to switch on or off at the time when you essentially need it.

Consequently, the furnace overheats without shutting off, putting you at risk or your entire family. Once you've noticed or encountered these possible signs, you can still attempt to reset your furnace unit.

How To Test Furnace Rollout Switch?

The rollout switch in your furnace system could inevitably malfunction. In such cases, when the rollout switch might be at fault if your flames are not going out. The most accurate way to test this switch is to use a multimeter and test it.

See this Multimeter on Amazon.

Follow our instructions to test your furnace rollout switch properly:

  1. Start the procedure by shutting off your furnace unit.
  2. Gently remove the furnace unit panel door to access the rollout switch.
  3. Go to your furnace burner chamber to access the rollout switch; it is a small device with two wires in it.
  4. Securely detach the wires from the terminals.
  5. Properly set your multimeter to ohms (Ω) setup and connect its two probes in the terminals of the rollout switch.
  6. The reading outcome should show a good circuit result around 0Ω, 1Ω, or 2Ω.
    • Otherwise, it will show an open line result of two digits or even thousands if it is defective.
    • Upon verifying that it is indeed defective, remove it and ensure to get the exact model before replacing it.
  7. Ensure to reattach its wires correctly into the rollout switch.
  8. Gently close its panel and switch the power supply on again.

How To Reset Your Furnace Unit?

New gas boiler, Heating system with copper pipes, valves and other equipment in a boiler room gas, Furnace Rollout Switch Vs. High Limit Switch: What's The Difference?

The resetting process must come next after thoroughly troubleshooting your rollout and high limit switch. First of all, we want to point out that regardless of the brand of your furnace unit, the resetting procedure should come all in the same steps.

The following are the steps on how you correctly reset your furnace unit:

  1. First, shut off your thermostat to avoid turning the furnace while you're performing these steps.
  2. Turn off your furnace unit power source or just detach the furnace unit's power cord from the socket.
  3. Allow the furnace system to reboot or refresh its settings for at least 30 seconds.
  4. Switch the breaker to "on" again and securely reinsert the furnace power cord into its socket.
  5. Finally, you can now power on your thermostat again.

In Conclusion

New gas boiler, Heating system with copper pipes, valves and other equipment in a boiler room gas

Though rollout switches and high limit switches are very similar in terms of safeguarding your furnace or home. However, they offer a distinctly unique set of features and functionality. Knowing both switches' major differences or how they work help you maintain or care for these devices.

Made it this far? Read also our other article below!

Furnace Limit Switch Troubleshooting [In 10 Easy Steps]

Can You Bypass Furnace Limit Switch? [And Should You?]

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