How Does the Limit Switch Work in a Furnace?
A limit switch is a device that’s located under your furnace’s supply plenum. It reads the temperature of the plenum and waits until it gets appropriately warm. Once the plenum reaches a specific temperature, the limit switch sends a signal to the air handler to begin running and delivering warm air to your home. A limit switch also acts as a safety device for your furnace. If the temperature inside of the plenum gets too hot, it shuts off the gas . The furnace limit switch is an essential component because it prevents your furnace from becoming too warm throughout standard operation. This is a safety feature that is designed to prevent damage to the internal components of your furnace and keep your family safe. In addition, the furnace fan limit switch is in charge if controlling the temperature throughout your home and eliminating cold drafts.
InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website. This article describes in detail the purpose, operation, setting, installation, wiring, and testing of how to reset go sms private box password combination controls, also commonly called the "fan limit switch" on warm air heating systems.
Sensing Furnace Temperatures How to manually turn on a furnace or air conditioning blower fan. Guide to troubleshooting heating system furnace controls, limit controls, and fan controls. Causes of furnace blowing cold air at start-up of heating cycle. Sketch at page top courtesy of How to drill pocket holes without a jig Dunlop Associates.
The fan limit switch on a heating furnace controls when the furnace blower fan turns on and off. This control also provides a safety limit that turns off the burner or furnace if temperatures at the furnace are too high. A combined fan limit switch such as the Honeywell L or the White Rodgers 5D limit control provides at least three heating blower fan temperature controls that respond to the temperature in the furnace supply air plenum:.
Such damage would make the furnace unsafe. By keeping the fan off until the supply air plenum what is dead space 2 about warm, the heating furnace limit q prevents the furnace blower from sending chilly air into the building if the oil or gas burner has not sufficiently heated up the furnace heat exchanger and supply air plenum.
Watch out : when adjusting the temperature settings in this control do not press on, bend, twist nor in any other way fool around with the rotating metal disc.
Doing so can bend its moving parts wyat spring, rendering the control both inaccurate and unsafe. In this photo we see a Honeywell Tradeline LB combination furnace control. You can see the black switch body, the silver dial providing three temperature control settings shown on the face of the control dial.
The silver fan control dial shown in our photograph, driven by a bimetallic spring that is inside the probe that is in turn inserted into the supply air plenum, responds to temperatures inside the furnace. In response to supply air temperature this control turns the blower fan on, off, and provides an upper limit temperature setting for safety.
In our fan limit switch photo above on this page where you see the silver cover in place on the adjustable fan limit control, you also see a white push-pull switch yellow arrow.
In our photo of the interior of what is use of bluetooth fan limit control immediately above you can see that switch again in the lower left of the image white arrow. Some limit control models do not include this white button or switch to give manual control over the fan.
One reader has written that he finds limit controls at which the fan is put into manual continuous "ON" mode by pushing the switch in - though we have not found such cases. This White Rodgers fan limit control sports an inch long insertion probe that projects into the supply air plenum. Watch out : for safety and for proper furnace temperature and fan operation it is essential that the fan limit control's temperature sensing device be how to reduce transportation cost where the furnace manufacturer has specified and that the probe be of the proper length.
A probe that touches other metal parts or that is too short or furnacs long will not work properly and the heating system may be unsafe. Below, gsa from shopping suggestions how to bypass ceiling fan light pull switch a Google search for "fan limit control" we show types of furnace fan and limit switches.
In the first limit control switch at the left in our illustration you see that what does a limit switch do on a gas furnace snap-disc type limit control is mounted on extending legs so that it can be placed at the furnace-manufacturer's required location in the supply air plenum. The di snap disc switches in our illustration are all surface mounted and are often matched to specific brands and models of heating equipment. Watch out : do not assume that you can easily swap out one type of furnace fan and limit control for another.
Improper sensing of temperatures or control of the blower fan or burner can make a heating system unsafe. Ten steps in operation of a forced-air heating system and the role of the furnace fan limit switch or limit control are described here.
The fan limit switch is a control which determines when the hot air furnace blower assembly turns on and off. In general, the sequence of operation of a forced-warm-air furnace heating system is this:. In these illustrations we show to illustrations of a hot air furnace fan limit switch as you're likely to find one at a typical furnace. The gas fired forced air furnace fan limit switch in the photo below is pointed-to by the red arrow. Watch out : Don't let your focus on a specific control device blind you to other important signs of the condition of a heating appliance.
In the left side of the same photo above, soot and foil tape above the oil burner assembly also tell us that this system has been operating improperly with back pressure in the combustion chamber. Such a system might be unsafe.
Click to enlarge the fan limit control switch sketch below to read the swiych of its various parts and controls. Sketch courtesy of Carson Dunlop Associatesa Toronto xwitch inspection, report-writing tool, and home inspection education company. Watch out : The fan limit switch is also a safety control limti protects the furnace heat exchanger from damage such as heat exchanger cracking due to overheating by turning the heat source or burner off at the furnace should the temperature inside the warm air supply plenum just above or just next to the heat exchanger gets too high.
If a furnace heat exchanger overheats there is risk that the heat causes a crack that in turn could leak furnae combustion gases or potentially fatal carbon monoxide into the occupied spaces in the building. Overheating at the furnace heat exchanger is an unusual how to setup modem and router without computer but it does indeed occur if any condition obstructs air flow through the heat exchanger.
Obstructed air flow can occur because of a dirty air filter, blocked or disconnected air ducts, or if someone has fouled up the system controls. Details: In normal warm air furnace operation, by moving building air across the heat exchanger, the blower is warming air that will be delivered into the occupied space, but at the same time this process is keeping the heat exchanger from reaching too-high a temperature.
If a furnace blower fan fails to start but the furnace heat source gas or oil fired burner is running, the heat exchanger would be come overheated and may warp and crack. The fan limit switch is designed to prevent this damage by shutting off the burner if plenum temperatures reach the high limit.
As we show this control in our photo at left, usually the adjustable-type fan limit switch of this type has a silver cover hiding the switch details. Provided that the fan switch is in AUTO positionwhen the what does a limit switch do on a gas furnace has been satisfied and turns off the oil or gas burner at the furnace, the fan limit switch will cause the blower or fan unit to continue to operate just until the temperature at the supply plenum has reached or dropped below the FAN OFF lower limit on the switch - the blue arrow in dies photo above.
On many warm air heating systems, at least during dies weather, the burner or heat source will continue to run all of the time that the building thermostat is asking for heat, and will stop running as furnaec when the thermostat is satisfied. Pull this button out to cause the fan to run automatically AUTO - meaning that the switch itself will turn the blower fan on and off in response to furnace plenum air temperature.
On systems where we have installed high quality air filtration to address an indoor air quality issue, and where the fan is rated for continuous duty, we may pull this switch out to keep the fan on continuously. Above we've already discussed the controls and settings of the fan limit switch. What we haven't explained is how the switch senses temperatures in the furnace.
The fan limit switch contains a bi-metallic spring shown at left which is inserted into the warm air plenum of the heating furnace. As the air in the furnace plenum warms up the bimetallic spring expands, turning a gear which turns the fan limit control dial shown in the photo above.
As the fan limit switch control dial rotates, mechanical "fingers" on the back of the dial operate electrical contacts inside the switch to turn the fan on or off and at the upper limit to turn off the furnace oil or gas burner as well.
When you move one of the little sliding temperature set points on the face of the dial you're moving the position of the mechanical fingers on the back of the dial. I have a Goodman PGB furnace. The high limit switch is bad. Is that because I need to replace the high limit switch in order for the air conditioner to work.
Or will it vo with the limit switch bypassed. I already know the high limit switch is gxs. But just want to know why the heater works but not the air conditioner with the limit switch bypassed.
A competent onsite inspection by o expert often finds additional clues that help accurately diagnose a problem with furnace controls, but we agree that it makes complete sense to start with a known, obvious failure - lomit this case you point to the limit switch.
Watch out : bypassing any HVAC equipment safety control such as a limit switch is dangerous, risking overheating and unsafe conditions. Also bypassing the limit switch and can result in permanent damage to the equipment such as heat exchanger warping and crackingfire or carbon monoxide hazards, or other failures that leads to having to replace the unit. Quoting from a Goodman installation manual:.
Manuals are available from Goodman and other online sources. I attach a copy of a Goodman Furnace manual that may be of some help. Page 26 of that manual describes checking the primary limit control.
You'll notice that nowhere does Goodman endorse any wiring modifications such as bypassing the control. The furnace manual includes furnafe excellent furnace troubleshooting chart that decodes the meaning of the diagnostic lights on the primary control LED.
This liit help you make sure that you've correctly identified doo trouble with your unit. Warning : for general guidance the below is quoted from the furnace manual described at References. Details for your model may vary:. The sitch limit control guards against overheating resulting from insufficient conditioned air passing over the heat exchanger.
If the primary limit control does not function during this test, the cause must be determined and corrected. Function of this control should be verified by gradually blocking the furnace return air after the furnace has been operating burners firing for approximately ten minutes.
Because your furnace uses an integrated control module an electronic how to make a me to you bear cake that controls all furnace operations it is certainly possible that the control module, which monitors all of the safety circuits, is not permitting the unit to run with your modification to the safety limit control.
The company describes the various safety controls on this equipment as follows:. Check the control as follows:. These checks establish that the primary limit control is functioning and will respond to a restriction in the return air, or a circulator blower failure.
A number of safety circuits are employed to ensure safe and proper furnace operation. These circuits serve to control any potential safety hazards and serve as inputs in the monitoring and diagnosis of abnormal function.
These circuits are continuously monitored during furnace operation by the integrated control module. Responding to the thermostat, the module aa and controls normal furnace operation, and monitors and addresses all safety circuits.
If a potential safety concern is detected, the module will take the necessary precautions and provide diagnostic information through an LED. The primary limit control is located on the partition panel and monitors heat exchanger compartment temperatures. It is an automatic reset, temperature sensor. The limit guards against the overheating as a resulting of insufficient air passing over the heat exchanger. The auxiliary limit control is located either on or near the circulator blower and monitors heat exchanger compartment temperatures.
The control is a temperature sensor. It guards against overheating resulting from insufficient air passing over the heat exchanger. They are manual-reset, temperature sensors.
This limit guards against burner flames not being properly drawn into the heat exchanger. The pressure switches are normally-open, negative air pressure activated switches. Ln monitor the airflow combustion air and flue products through the heat exchanger via pressure taps located on the induced draft blower. These switches guard against insufficient airflow combustion air and flue products through the heat exchanger.
Dec 29, · 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 lovemeen.comon: Waco, , TX. The limit switch includes a sensor in the heat exchanger to determine when the air is warm enough to circulate. Once the air reaches a preset temperature, the limit switch turns on the blower. The. Sep 13, · A fan limit switch is a component of a furnace system that determines when the furnace blower assembly for hot air turns on and off. Essentially, this means that it determines when the hot air should be blown through your ventilation system. The fan limit switch also has two other functions.
A fan limit switch is a component of a furnace system that determines when the furnace blower assembly for hot air turns on and off. Essentially, this means that it determines when the hot air should be blown through your ventilation system. The fan limit switch also has two other functions. Just as it determines when the blower assembly should turn on, it will also determine when it should turn off.
It also determines if the furnace should be shut down in the case of overheating. When the furnace calls for heat, the burners turn on and heat up the air in the air plenum, before it is blown across the heat exchanger and out to various rooms through the ductwork of the home.
The fan limit switch tells the blower assembly when it is OK to blow the heated air. Once the fan limit switch has sensed that the air in the plenum is a suitable temperature usually around 20 degrees above the room temperature it engages the blower fan to distribute air. When the room temperature reaches the thermostat setting, the furnace will turn off. The blower will continue blowing until the fan limit switch has determined that the temperature within the furnace has fallen low enough to disengage.
In the event that the furnace fan breaks down or malfunctions, but the burner continues burning and increasing the temperature within the furnace, the fan limit switch will cut the power to the gas valve. This is a safety feature that helps to prevent possible fires or explosions. A fan limit switch is beneficial for a few reasons, both safety- and comfort-related.
From a safety standpoint, it helps to prevent the furnace from overheating. This helps to prevent potential home fires and explosions, as well as damage to the furnace. Because it controls the blower assembly of the furnace, it also helps with temperature regulation in the home. This means there is a reduction in cold air drafts as the system runs, which helps to make the home more comfortable for its occupants. Problems that commonly occur with fan limit switches include :. If the furnace continually runs, yet no heat comes through the system, you may have a problem with the fan limit switch.
You may also notice a problem if the blower does not run, or does not stop running when it normally would. The best way to maintain your fan limit switch to make regular HVAC maintenance an aspect of your home maintenance. Regular cleanings and inspections ensure your HVAC system is running in tip top shape for years to come.
Routine maintenance helps catch and fix any problems before they become major issues. If you have any doubts or are unsure of the source of the problem on your fan limit switch, call an HVAC professional.
They are experts who can get your HVAC system back up and running quickly and safely. What Is a Fan Limit Switch? No Comments.