Table of Contents
A furnace limit switch is a component of a forced-air furnace that is responsible for turning on the furnace blower once the desired heat has been met inside the furnace. Without a functioning furnace limit switch, a furnace would not be able to regulate the temperature in conjunction with a thermostat.
How do you know if a limit switch is bad?
The most common sign of a malfunctioning limit switch is the continuous operation of your furnace’s blower. Just as the limit switch won’t allow the fan to turn on until the warm air has reached the right temperature, the limit switch also shuts down when the air cools to a certain temperature.
What trips a limit switch on furnace?
If the air inside your furnace gets too hot, the limit switch trips to prevent overheating. If the furnace high limit switch trips due to overheating, there is likely an airflow issue that is preventing proper air movement through the system. A dirty filter is the most common cause of airflow issues.
Is my furnace limit switch bad?
One common symptom of a malfunctioning limit switch is a blower fan that doesn’t shut off. A bad switch also can prevent a furnace from operating at all. If the switch has completely failed and is stuck in the open circuit (off) position, the furnace won’t run.
What is the purpose of a high limit switch on a furnace?
The high temperature limit switch (also known as a furnace plenum thermostat) is a safety device on a furnace that is designed to cut the gas supply when the burner temperature exceeds the safe operating range. When the temperature drops back within the safe range, it will allow the gas valve to open again.
Can you run a furnace without a limit switch?
Without a functioning furnace limit switch, a furnace would not be able to regulate the temperature in conjunction with a thermostat. Typically, a furnace limit switch consists of an exterior plate and a temperature probe that lives inside the furnace housing.
What conditions can cause the limit switch to open during furnace operation?
The limit switch shuts off the burner when the temperature exceeds a preset temperature. When the temperature reduces to normal, the burner will come back on. The most common reason for limits to open is a plugged filter or a dirty blower wheel. On the left are some high temperature limits used in furnaces.
What does a tripped limit switch look like?
The most common way to tell if a limit switch is tripped is to observe the behavior of your furnace: Your furnace’s heating cycles end right after they begin. If the furnace won’t ignite, but the blower fan stays running, then the high limit switch might be tripped.
How do you reset the limit switch on a Trane furnace?
To access it, you’ll need to remove the front panel of the furnace. You may find that many models have limit switches where the reset button is located. The button is often located on top of the switch and will need to be pressed to initiate the reset.
Should a limit switch be open or closed?
The limit switch is a normally closed switch in series with the sail switch and the thermostat. Being temperature actuated, it is designed to “snap open” when an over temperature condition occurs. Thus the switch is normally opening under full load conditions and that causes arcing of the contacts.
Why does my furnace keep tripping the reset button?
If you have a gas or oil furnace, the reset button may trip if you are out of fuel. If a furnace isn’t receiving enough airflow, it may overheat and trigger the reset button to trip. Make sure to clean or replace the air filters and keep the air vents open enough for proper airflow.
Is limit switch a sensor?
Limit switches are a type of sensor that detect presence and absence. Specifically, mechanical limit switches are switches that are mechanically activated, meaning that they have some sort of arm, lever, knob, plunger, etc., which is physically—or mechanically—activated by making contact with another object.
What activates a limit switch?
In most cases, a limit switch begins operating when a moving machine or a moving component of a machine makes contact with an actuator or operating lever that activates the switch. The limit switch then regulates the electrical circuit that controls the machine and its moving parts.
What is an example of a limit switch?
A good example of this is an automatic garage door. Without a limit switch that stops the door when it reaches the bottom rail, the door would keep moving downwards and eventually crash into the ground, causing serious damage. Limit switches can also be used as part of a larger control system.
Where is the furnace high limit switch located?
The high limit switch is usually attached to a metal plate, which is bolted to the outside of the furnace plenum (the large metal box where air is heated.) You will have to remove the furnace cover to find it.
When should a furnace be reset?
If your furnace doesn’t have any air coming out at all and the pilot light is out, it’s worth it to try resetting the furnace. If this doesn’t work, there may be a problem with the thermostat setting, the power, or the gas line.
Why is limit switch stuck open?
When the temperature exceeds a preset temperature, the limit switch shuts off the burner. The burner will come back on when the temperature is normal. A plugged filter or a dirty blower wheel are the most common reasons for limits to open.
