You may not be aware, but your furnace has a special temperature limit switch that determines the operating temperature range. The primary purpose of the limit switch is to make sure the furnace kicks on as soon as temperatures drop below a predetermined setting.
It also keeps track of operating temperatures to make sure the furnace doesn’t become too hot, as overheating is dangerous to both the furnace and the home’s occupants.
Unfortunately, the limit switch can malfunction. For instance, it can break down due to short-circuiting like any other electrical component. It will also stop working when it reaches its end of life.
So, often, homeowners wonder whether you can bypass the limit switch, perhaps temporarily, when troubleshooting the furnace or for longer, perhaps overnight, when the switch fails. Is it safe? Let’s begin with what the experts say.
Can You Bypass the Furnace Limit Switch?
Yes, you can temporarily bypass the temperature limit on your furnace. For example, you can bypass the limit switch when trying to find out why your furnace shuts down too frequently or runs but doesn’t blow warm air. However, it’s highly unsafe to bypass the furnace limit switch for an extended period, given the crucial role of the switch in averting possible fires due to overheating and flame rollout incidences.
What is the Limit Switch on a Furnace?
The furnace limit switch is a component of forced-air furnaces responsible for turning on the furnace blower once air inside the furnace reaches a pre-defined temperature. It works alongside the thermostat to ensure that you get the necessary heat as soon as it becomes a little too cold indoors. Of course, the thermostat kickstarts everything.
When temperatures drop below the thermostat setting, typically 68°F to 72°F, the thermostat sends a heat request to the furnace’s control board. Then the furnace initiates ignition, and the burner flames come alive.
As soon as temperatures inside the heat exchanger reach a certain level, the limit switch engages the blower motor to begin drawing cold air into the furnace through the return air plenum and past the heat exchanger where the cold air absorbs heat.
Then it blows the now-warm air out of the furnace via the supply air plenum, through the ducts in your home, and disperses it throughout your home for even heating.
What Does the Limit Switch Do?
The limit switch has two primary responsibilities, i.e., regulating the blower fan and averting overheating incidents.
- Regulating the blower fan
Blower fans down come automatically when the furnace kicks on. Instead, it waits until it’s hot enough inside the furnace. Also, it doesn’t automatically go off when heating stops. Instead, it waits until temperatures inside the furnace drop below a specified point.
The limit switch is responsible for both actions. It determines the temperature at which the fan starts to run and tells the blower when to stop running.
- Averting overheating dangers
Unfortunately, furnaces can overheat. For example, a furnace will overheat if the heat exchanger is worn or damaged. Furnaces can also overheat if the blower fan is damaged or when there’s a flame rollout. Flame rollouts mainly result from the concentration of combustion gases in the combustion chamber.
The limit switch is designed to quickly sense overheating and shut down the furnace to prevent potential fires.
How Does it Work?
Standard limit switches comprise a bimetallic spring inserted in the plenum located above or next to the heat exchanger. This string expands when the air in the plenum warms up, moving a gear that turns the limit switch dial. The rotating control dial is connected to contacts inside the switch that turns the fan on or off depending on the furnace temperature.
Most limit switches have three operating settings as follows;
- LOW limit (100°F): This is the temperature at which the limit switch turns off the blower motor, typically following a heating cycle. The reason is that the air inside the furnace is no longer hot enough to warm the room at this stage.
- ON setting (130°F): This is the temperature at which the switch completes the circuitry that kickstarts the motor blower.
- HIGH limit (200°F): This is the highest temperature the furnace can safely run. Any temperature rise beyond this point constitutes overheating and triggers the limit switch to discontinue the heating process.
Troubleshooting the Furnace Limit Switch
A failed or dead limit switch can spell doom to your heating wishes, given the component’s critical role in keeping the furnace going. Here’s what can go wrong and what to do.
How Do You Know if the Limit Switch is Bad?
- The blower runs nonstop
- The furnace won’t start
- You can’t hear the blower humming
- Furnace blows cold air
Why Do Limit Switches Go Bad?
Fan limit switches can malfunction or stop working for many reasons ranging from natural wear and tear to electric short-circuiting. However, the four most common reasons are;
- The switch is stuck: Furnace switches can become stuck open (or closed) for many reasons, including electrical malfunction and dirt buildup.
- A defective/dirty sensor: If the bimetallic sensor on the limit switch is dirty or corroded, the switch loses sensitivity and can no longer work accurately.
- Damaged/loose wiring: The switch depends on electricity to power its operation. Thus, loose or damaged electrical wires can render the switch useless.
- End of life: Finally, the switch will also naturally fail once it reaches its end of life. When this happens, you must replace the switch.
Can You Bypass a Limit Switch?
Yes, you can bypass a limit switch temporarily, such as when troubleshooting your furnace. Here’s what you need to know about bypassing the limit switch.
How to Bypass the Furnace Limit Switch
The following is a step-by-step guide to locate and bypass the furnace limit switch. You need a screwdriver and voltage tester for the process.
- Turn off the furnace: Switch off the circuit breaker at the main panel. It may also help turn off the gas supply at the main valve. However, this isn’t absolutely necessary.
- Remove the access panel: Most furnace access panels can be removed without tools. However, others may require a screwdriver. Refer to your owner’s manual for further guidance.
- Locate the furnace limit switch: The limit switch is mounted supply air plenum where heated air exits the furnace. It’s easy to recognize as it connects to two pairs of wires. One pair leads to the limit switch relay while the other pair go to the fan switch.
- Ensure there’s no power in the terminals: Touch one probe of the voltage tester to the L1 terminal and the other to the L2 terminal and check whether there’s voltage flow. Only proceed if there’s no voltage.
- Remove the switch relay wires: You need to plug out the first pair of wires (that lead to the limit switch relay). Insert a flathead screwdriver into the horizontal slot on the L1 terminal and press the terminal to release the first wire. Then do the same for the L2 wire.
- Jump the L1 and L2 wires: We recommend using a jumper to connect the two wires. However, you can also twist the open ends of the wires together manually.
- Test the furnace: Turn on the gas valve and switch on the circuit breaker. Then turn on the furnace to see if it works. If it works, the fan limit switch is bad and needs resetting or replacement.
- You can now shut down the furnace and call your HVAC technician to fix or replace the fan limit switch.
Dangers of Bypassing the Furnace Limit Switch
Don’t forget that you’re taking a significant risk as you bypass the limit switch. For instance, bypassing the limit switch means you’re entirely exposed in case of overheating or a flame rollout.
Therefore, you should only consider bypassing the limit switch as an option of last resort. More importantly, don’t even think about bypassing the limit switch for the night, perhaps hoping to replace it the next day. It’s too risky.
Why Does My Furnace Limit Switch Keep Tripping? The fan limit switch is likely tripping because of overheating in your furnace. You may want to shut down the furnace and call an HVAC professional.
How Do You Reset a Limit Switch on a Furnace? Open the access door, locate the limit switch, and set the temperature on the left component to 100°F, the central component to 130°F, and the right component to 200°F.
How Much Does It Cost to Replace a Limit Switch on a Furnace? The average cost to replace a furnace limit switch is $4 to $100+ depending on the type of furnace, limit switch model, and whether you DIY-install it or hire a professional.
Are limit switches usually open? Yes, limit fans are usually open when the furnace is off. They only close when the furnace is ON and heating. You have a problem if the limit switch is open even when the furnace is ON.
Yes, you can bypass the fan limit switch on your furnace. However, no, it’s not entirely safe to do so, especially for long periods. Fortunately, now you know how to bypass the switch, the inherent dangers, and the safety precautions.