As long as gas is burned in a furnace, it has a potential for overheating, which is why there is a high-limit switch on each furnace to prevent catastrophes. This switch cuts the gas off when a preset temperature is reached inside the plenum, just above the heat exchanger.
A fan limit switch is also found on most furnaces, preventing the blower from blowing cold air into the house. It usually is open during regular operation and closes when the temperature reaches the preset level.
On the other hand, high-limit switches are normally closed and open when the temperature exceeds their preset threshold. A combination switch performs both functions on most furnaces, while some feature separate switches for each.
An overheating furnace limit switch will shut off the furnace once it reaches a certain temperature. The limit switch is a safety device that monitors the heat level of the furnace. Bad furnace limit switches exhibit the following symptoms:
- There is a problem with the pilot light
- When the thermostat is turned up, the blower does not come on
- There is no heat from the furnace even though the blower is on
You need to understand how furnace limit switches work before you can tell if they are malfunctioning.
How It Works
Undoubtedly, the built-in safety devices in modern furnaces make them one of the safest appliances on the market. A furnace’s internal temperature is kept under control by this switch. Here are some tips on how to identify a bad limit switch and how to work it.
It usually takes a little while for your furnace burners to reach the preset temperature. The limit switch signals that the warm air can be distributed once the furnace reaches your set temperature.
Furthermore, the limit switch monitors the temperature to ensure it stays comfortable. The switch shuts off the burners to cool down the furnace if the temperature becomes too high.
Identifying A Bad Limit Switch
There is a possibility that a limit switch will develop problems over time, unfortunately. There are several reasons for these problems:
- The limit switch is defective
- The temperature sensor and filter are dirty
- Problems with the furnace’s airflow
If the furnace does not receive adequate airflow, it may repeatedly overheat, causing excessive wear and tear on the limit switch and eventually malfunctioning. You can also be at risk of overheating and injury if you have an overheating furnace due to a malfunctioning limit switch.
On the other hand, a defective limit switch may turn off the burner gas even in the absence of an overheating danger, preventing the furnace from producing any heat. Your furnace will run more efficiently if you get timely maintenance from Everett heating contractors.
There are several signs that may indicate that your furnace limit switch is malfunctioning, so you may need to schedule a gas furnace repair to fix the problem. You should also look for the following signs.
- There is a constant cycle of shutting off and turning on the furnace
- It distributes cool air through the furnace
- You can’t turn off your furnace’s blower
What Does a Limit Switch Look Like?
There are two types of limit switches. First, an old electro-mechanical switch consisted of a 6-inch probe extending into the plenum. The air is distributed just below the attachment point on the ductwork by a box attached to the ductwork.
Nowadays, solid-state switches are disk-shaped and extend only about an inch into the plenum. When you remove the furnace cover, you will see two types of switches mounted on metal plates that screw onto the burner housing.
Dials are found on electro-mechanical ones. By adjusting this setting, you can set the temperature at which the fan limit switch starts and shuts off the fan. This is the point at which the high-limit switch shuts the gas off.
Unlike solid-state switches, solid-state switches don’t have this adjustment dial. There may be a reset button on this type of device. There has to be a match between the replacement and the preset value.
There are usually two or four wire terminals on the plate of a limit switch. For example, to connect a fan limit switch to a furnace blower, two 120-volt terminals are required. Likewise, the gas valve is controlled by two 24-volt terminals on a high-limit switch.
Troubleshooting Limit Switch Problems
Due to this fact, the combination furnace limit switch is both a fan limit switch and a high limit switch. The blower will switch on and off by monitoring the plenum temperature and shutting off the gas if it becomes too hot.
The furnace can blow cold air in two different ways as a result of this. First, this causes the blower motor to not start or stop properly if it fails in its first function.
If this second function fails, the furnace will stop producing heat, so the gas will be shut off when there is no danger of overheating. A tripped high-limit switch will place the furnace in hard lockdown mode, which prevents it from starting until a professional has it serviced.
In contrast, the furnace may be overheating and shutting off the gas properly because of the high limit switch. In this case, either the blower motor is malfunctioning, or the filters are extremely dirty, resulting in poor airflow.
In addition to their return air filters, the outflow filters should be cleaned or replaced before assuming the limit switch is faulty. Dirty filters can also cause a malfunctioning blower, so it’s important to clean them regularly.
You should clean the furnace filters and the temperature sensor on your furnace’s limit switch when necessary. You can also ensure smooth airflow within your furnace by replacing your furnace filter when necessary.
A lockdown mode can be triggered if your limit switches trips repeatedly. As a result, it completely shuts down. Your furnace limit switch may need to be reset. Then turn your furnace back on after 30 seconds if you have a newer model.
A heating expert should be called to reverse or reset the limit switch if it does not work or if the furnace shuts down repeatedly.
When In Doubt, Replace
Despite giving you helpful information, some tests are impractical. When the furnace suddenly cuts out, conducting a continuity test on the high-limit switch may reveal that the furnace is opening at an incorrect temperature.
It’s easier and safer to replace the switch than to work inside a hot furnace since it costs as little as $4 for some models. The blower fan or filters might cause the furnace to overheat and shut down after installing the new switch.
Make sure to use a replacement part that is identical to the original when replacing a furnace limit switch. Detailed information about each switch’s cut-in and cut-out temperatures can be found on its front panel.
On the new switch, the temperatures must be displayed exactly the same. So, for example, a label reading L210 indicates a limit that cuts off at 210 degrees.
The furnace will not work properly and may even be damaged if you use a switch with different cut-in and cut-out temperatures. An overheated furnace could crack, releasing dangerous fumes like deadly carbon monoxide into the furnace room.
Can I Replace A Furnace Limit Switch?
There’s no problem with that, and yes, it’s possible. First, follow the directions above to locate the limit switch. Then, check the continuity of the limit switch with a multimeter.
Having an infinite resistance on the multimeter indicates a bad limit switch. Removing the malfunctioning limit switch and installing a replacement is as simple as unscrewing the switch plate.
Depending on the manufacturer, Furnace limit switches cost between $3.98 and $25. All parts are recommended to be OEM, though some furnaces may work with universal switches.
You should remember that if the high limit switch weren’t working, the furnace wouldn’t burn. To clear the heat, the blower will continue to run when the high-limit switch trips. So, either the system has been reset, or the limit switch has been closed to stop the blower from running.