Although it’s not a very common occurrence, sometimes you might ignite the furnace only to be greeted by four LED flashes.
Don’s panic. It’s just a sign that one of the furnace’s safety mechanisms has failed. Specifically, it means that your high limit switch has malfunctioned.
The bad news is that a failed high limit switch makes the home heating process very dangerous. It can lead to overheating, which is unhealthy for you and your family. Overheating can also lead to a house fire. However, the good news is that the modern furnace is designed to go OFF right away to prevent any of the above issues.
So, as long as you take the flashing lights seriously and fix the problem, you should be completely safe. Moreover, you can often fix limit switch malfunctions yourself without the input of an HVAC professional.
Read on to learn more about high limit switch issues, what causes them, and what to do when you experience one.
A Recap: What Does it Mean When the Furnaces Flashes Four Times?
If your furnace light blinks four times, it’s telling you that your high limit switch is open. Typically, the furnace will go off right before the flashing begins. Or, the fan may run while the burner is off. Thus, you will only get cold or room temperature air from your ducts.
What’s the High Limit Switch and What Does it Do?
The high limit is one of two switches found on the furnace limit switch. The other is the low limit switch.
The low limit switch, found on the left of the limit switch, determines the temperature below which the furnace is no longer necessary. When furnace temperature drops below this level, the air is too cold to blow into the home. Thus, the limit switch will keep the blower OFF.
In other words, the furnace must be at a temperature higher than the lower limit for the blower fan to engage.
Meanwhile, the upper limit switch is the maximum heating temperature, beyond which the home would be too hot for the occupants. If the temperature inside the home rises beyond the high limit, the furnace will react by cycling off to prevent heat damage and heat-related illnesses.
Overheating can result from various issues, including wrong thermostat setting, a faulty thermostat, a compromised heat exchanger, and flame rollout.
The high limit switch is part of the furnace responsible for shutting the furnace whenever there’s overheating.
How the High Limit Switch Works
When the thermostat calls for heat, the furnace burner ignites, and heating commences in the heat exchanger.
However, the air above the heat exchanger at the supply plenum isn’t warm enough to heat the house. So, at this stage, the limit switch keeps the blower off.
However, slowly, the temperature at the supply plenum begins to rise. As soon as it passes the lower temperature limit, the switch auto-activates the blower fan and begins to circulate warm air via the heat exchanger into your home.
Heating continues for as long as necessary, with the thermostat solely responsible for regulating the degree of “warmth” inside the home. Whenever the temperature reaches a pre-defined level, the thermostat will request the furnace to cycle off or “step down” the heating process by operating at a fraction of total capacity.
Heating resumes again when temperatures in the supply plenum drop below the lower temperature limit.
However, the heat exchanger can become too hot during the heating process, resulting in overheating in your rooms. Overheating is bad as it can damage the heat exchanger. It also poses a health risk for the home’s occupants and a fire risk to the home.
This is where the high limit switch comes in handy. The switch opens when temperatures within the exchanger rise too high, effectively shutting down the furnace.
Why is the High Limit Switch Open? Causes and Solutions
The high limit switch can be open for many reasons. If your furnace keeps blinking four times even after you restart it, it could be one of the following issues;
The first thing that comes to mind when the high limit switch is open is overheating. This switch guards explicitly against overheating and is purposely designed to “open” when there’s overheating. So, if your furnace is blinking four times, you may want to check the furnace and home temperature.
Solution: You need to address the underlying issues. Why is your furnace overheating? Are the filters blocked? Are airflow paths obstructed? Are the air vents blocked? Regular maintenance can fix most of the issues.
A closely related but slightly different issue is flame rollout. A flame is said to have “rolled out” if it exists outside the pre-defined areas. If this happens, the limit switch will sense overheating and may shut down the furnace.
Solution: Flame rollouts typically result from gas leaks, a damaged heat exchanger, and poor/compromised airflow within the furnace. It’s best to hire an HVAC specialist to diagnose and fix the underlying problems.
Defective Limit Switch
The limit switch itself can be damaged. For example, the sensors can wear off, thus lose the capacity to detect heating levels. Or some components may break down altogether. When this happens, the switch may begin to malfunction and may even refuse to close when necessary.
Solution: If your limit switch is damaged or has malfunctioned, you can attempt to repair it. However, here too, you need the help of an experienced HVAC technician. If it’s damaged beyond repair, replacement is the only other solution.
Signs of Malfunctioned Limit Switch
Furnace issues can be a little complex to diagnose, given that most problems share symptoms. A furnace that refuses to start, for instance, could signal so many things. Nevertheless, two or more of the following symptoms usually point to an open high limit switch.
- The furnace blinks four times: If your furnace blinks four times, stops, then blinks four times again, you certainly have an open high limit switch.
- The blower runs continuously: Just as the limit switch keeps the blower fan off until the supply air is warm enough, it can also keep the fan running if the fan is broken.
- Burner goes off and refuses to light: If the limit switch detects overheating, it immediately instructs the furnace to cycle off and not allow resumption of services until the underlying issue is fixed.
How to Replace the Furnace Limit Switch
The first step here is to purchase a matching replacement. Make sure to confirm with the manufacturer before you head out shopping. Ideally, you should get an Original Equipment Manufacturer (OEM) replacement.
Once you have the replacement switch, proceed as follows;
- Deactivate the furnace: Turn off the power at the circuit breaker and turn off the gas valve.
- Note down the old switch readings: Take a picture of the readings on the switch using your phone. You’ll need them to set the new switch.
- Remove the old limit switch: The limit switch is found behind the furnace cabinet door. Then gently remove it. You may need to remove a few screws.
- Install the new limit switch: Make sure it fits snugly. Also, ensure to screw it into place as required.
- Configure the new limit switch: Refer to the readings you captured from the old switch to configure the new one. Don’t change the settings.
- Test the new switch: Replace the cabinet door, restore the gas supply, and switch the power. Then set the thermostat and switch ON the furnace. If all else is fine, the furnace should resume operation without further ado.
The furnace limit switch is a critical component of the gas furnace, without which the heater cannot run. If you see the LED on your furnace flashing four times between breaks, this switch is bad and needs troubleshooting.