If you’ve never run into a “limit switch open” issue with your furnace, you’re lucky. Although it doesn’t happen too often, it’s one of the most annoying things.
Why would the switch open in the first place? And, even if it opens, shouldn’t it at least self-resolve when you restart the furnace?
Unfortunately, that’s never the case. If your furnace limit switch is open, you must manually fix the issue. Until then, the furnace will stay off.
Below we discuss what causes open limit switches, signs that you have an open limit switch, and what to do when your limit switch opens. First though, a definition.
What is a Limit Switch, and What Does it Do?
The furnace limit switch is a safety mechanism built into modern furnaces to prevent overheating. It’s a small plate with arms that keeps furnace (and indoor) temperature limits, beyond which the switch can call for heating or shut off the furnace.
If temperatures dip below a certain point, the limit switch, working alongside the thermostat, requests heating to keep you warm. During heating, the limit switch also stays on the watch to keep the temperatures within a safe level. If it’s becoming too hot inside the house, the limit switch will request the furnace to cycle off to prevent overheating.
Where is the Limit Switch Located on the Furnace?
The limit switch is located in the furnace cabinet on a special panel beneath the heating ducts. To access it, reach below the ducts and open the panel. You’ll find the limit switch on top of the panel.
It’s a small plate the two metallic arms sticking out. The two arms regulate indoor temperature. The first one on the left keeps temperatures from dropping below a pre-defined level. Meanwhile, the one on the right is known as the high limit switch. It prevents furnace overheating.
The reading between the two arms is the temperature limit for the blower system.
How Do You Know if a Limit Switch is Bad?
It’s not very easy to conclusively pinpoint limit switch issues. Indeed, homeowners often replace other parts of the furnace when all they need to do is reset the limit switch. You may even think that the burner system is bad or that it’s an ignition problem.
Nevertheless, there are a few signs that, when observed together, almost always point to a bad limit switch;
- Overheating: Hopefully, you don’t get to this point. But, yes, overheating is almost a confirmation of a malfunction. If the limit switch is up and running, it will instantly sense overheating and initiate an off-cycle to protect you and your furnace.
- Extremely cold conditions: Most people immediately head to the thermostat when indoor conditions become too cold, even with the furnace up and running. Unfortunately, it’s not always the thermostat. The limit switch also has a part to play. So, if you adjust temperature settings on the thermostat but the problem doesn’t go away, you may want to check the pressure switch.
- Unexplained temperature fluctuations: Besides overly hot conditions and extremely cold conditions, a faulty limit switch can also cause extreme temperature fluctuations inside the house. One moment the conditions can become very hot, and the next minute it’s cold.
What Does it Mean When a Limit Switch is “Open?”
It’s a sign of overheating. In other words, your furnace is generating too much heat relative to what your home needs.
Overheating is bad for your health and the health of your furnace. Too much heat in your home can cause heatstroke, prickly heat, heat cramps, edema, and heat syncope, among other heat-related health issues.
Remember that the effects can extend to your pets and plants. For example, too much heat can cause plants to wither and may also spur some diseases.
Meanwhile, excessive heating within the furnace can cause stress cracks in the heat exchanger. Cracks near the bends and welds are prevalent when the exchanger is exposed to too much heat.
What Causes the Limit Switch on a Furnace to Open?
Another way to ask this question is – what causes overheating in furnaces? There are several possible causes;
1. Poor Air Circulation
Poor air circulation is considered the primary cause of furnace overheating. If there isn’t a good supply of fresh air and unimpeded ventilation of exhaust gases, you’re almost sure to have heat buildup within the furnace. The leading cause of poor/restricted airflow in furnaces is dirty filters.
2. Dirty Internal Components
Dirt and debris can also find a way into the furnace and cover critical components in a thin layer. When this happens, there’s a significant risk of furnace overheating. Specifically, if the dust entering the furnace covers the coils in your furnace, overheating is almost guaranteed. The blower motor is also very sensitive to dust buildup.
3. Mechanical Failure
The furnace comprises several parts that work together harmoniously. Unfortunately, nearly all these parts experience extreme stress during furnace operation.
For instance, the electrical wiring and blower systems are under stress the entire time the furnace is ON. If these parts succumb to stress and fail, the normal functioning of the furnace may be compromised, resulting in overheating.
4. Flame Rollout
Finally, the furnace can also overheat if there’s a flame rollout. Under regular operation, the furnace flame is fully contained within the combustion chamber. A flame rollout means the flame is burning outside the chamber. Besides being a significant fire risk, a flame rollout can cause overheating and even result in a furnace shutdown.
How Do You Reset an Open Limit Switch on a Furnace?
If you find that your limit switch is open but are confident that it’s not broken, you can reset/restore the furnace to regular operation.
- Shut off the furnace: Switch off the electrical power at the breaker and turn off the gas supply at the gas valve.
- Locate the furnace’s heating duct: You’ll typically find the ducts in the home’s basement utility or utility closet. You need to reach beneath the ducts to locate the panel on which the limit switch is found.
- Locate the limit switch: The limit switch is found at the top of the panel you just removed. You’ll know you’ve found it when you come across a plate with two metallic arms sticking out. Remove the cover (if one is present) to reveal the body of the switch.
- Set the fan limits: You’ll see a temperature setting if you check between the two arms. This value controls the furnace’s fan. It’s the temperature at which the fan switch comes ON when the furnace is running. Set it to 105°F-115°F.
- Set the lower temperature limit: The lower temperature is the lowest temperature at which the furnace can run. Below that, it must go off as it would be inefficient and largely unhelpful. It’s set on the left arm of the limit switch and must not be lower than 90°F.
- Set the high-temperature limit: The high-temperature limit is the highest temperature at which the furnace can operate. The furnace will automatically go off to protect the house’s occupants and the furnace’s components if it goes beyond this setting. It’s set on the right arm of the limit switch and must not be higher than 200°F.
- Set the switch to automatic: When set to “auto” mode, the furnace automatically implements the three settings above. There’s no need for further effort on your part.
- Restart the furnace: You’ve done the first part. Now, to complete the reset, you need to restart the furnace. Close the panel, turn on the gas supply and switch back electricity. Then turn on the furnace.
Can You Bypass the Furnace Limit Switch?
Yes, you can bypass the limit switch on your furnace. It’s not recommended as it exposes you to significant risk given the function of the limit switch. However, you can temporarily bypass the switch to test the rest of the furnace or to confirm that the switch has malfunctioned.
Just don’t make it a habit, keeping in mind that every time you bypass the limit switch, your furnace operates without temperature limits. Lack of heating limits makes overheating is a genuine concern.
How Do you Bypass the Pressure Switch on a Furnace?
To bypass the limit switch, you need to disconnect the two wires attaching to the switch and touch the wires directly. Alternatively, use a short wire (jump wire) to connect the two wires. One with alligators at both ends would be perfect.
The moment you touch the two wires directly, you take the limit switch out of the equation. So, the furnace will run even if you have a broken limit switch.
There you have it – a brief guide on open furnace limit switches. But, again, we must stress the importance of the limit switch and why you should keep it in the best working condition at all times. Otherwise, you’d be exposing yourself to a significant danger.
Remember that you can also replace the limit switch. If resetting doesn’t do the trick, contact your preferred HVAC professional to replace the compromised unit. The entire job costs $60 to $200.