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Furnace High Limit Switch

This necessary element is required for a furnace to operate. Because all components ultimately fail, it is vital to be aware of the warning indications and call an HVAC expert as soon as possible if it malfunctions.

Do you know how your heater functions internally? Several crucial parts are housed in the heat-generating device. Your family may be in danger without a furnace limit switch, commonly referred to as a furnace high limit switch or a furnace fan limit switch.

Have you ever questioned the purpose of this tiny piece? Read on if you’re interested!

Explanation of the Furnace High Limit Switch

When the furnace reaches the desired temperature, a furnace limit switch turns on the blower. A furnace’s thermostat and limit switch must work together to regulate temperature. Standard parts of a furnace limit switch include an outside plate and an inside temperature probe.

It achieves two goals.

The limit switch is used to turn on and off the blower fan. The fan turns on to circulate hot air throughout the house. It permits the blower to continue working even after the furnace has turned off, giving heated air to the living areas.

To detect overheating, the limit switch additionally checks the supply air temperature. If the furnace overheats, components could be damaged by hot supply air. When temperatures become too high, limit switches trip, shutting down the system until it cools.

What Reminds You of a Limit Switch? Where Can You Find It?

Inside the cabinet is the furnace’s limit switch. It is hidden behind the furnace cabinet door. Numerous brands exist.

A lengthy temperature sensor probe is fastened to a plate. The probe can enter the furnace through the mounting plate of the furnace lid, which is situated outside the plenum. Two gas valves and the control wires for the blower fan are attached to the mounting plate.

The limit switch is ordinarily mounted on the plenum, which is located above the combustion chamber and heat exchanger.

Your furnace runs as follows when the limit switch is engaged:

  • The temperature is increased.
  • Furnaces’ burners warm the plenum air.
  • When the air is sufficiently heated, the system circulates it through your duct to the vents.
  • The limit switch keeps an eye on the temperature of the furnace.
  • If the furnace gets too hot, the limit switch turns off the burners and activates the blower fan.
  • The limit switch turns on the burners as soon as the furnace’s temperature returns to normal.

In ideal conditions, the limit switch monitors this daily cycle.

Types of Limit Switches

There are both manual and automatic limit switches. Depending on the temperature, automated limit switches adjust the setting automatically. On the other hand, manual limit switches need someone to turn the temperature control dial.

Furnace High Limit Switches: How Do They Regulate Temperature?

The thermostat activates the furnace when the heat is switched on or raised. The limit switch closes when the furnace heats up, completing the circuit and turning on the blowers. When the structure or establishment achieves the desired temperature, the furnace burners will shut off. However, the fan will keep running until the furnace has cooled to the bare minimum safe level (setting on a thermostat for the lowest acceptable air temperature).

The gadget will cycle, restart, or stay off depending on the indoor air temperature. Furnace efficiency is increased by switches with high limits.

The thermostat orders the furnace to restart as the home cools.

Limit Switch for the Furnace, a Safety Feature

If the furnace overheats, the limit switch shuts off the blowers and burners. A dangerously overheated furnace could have disastrous results.

The most frequent reasons for furnace overheating, which place unnecessary strain on the device, are a malfunctioning blower fan or a dirty furnace filter, which homeowners should be aware of.

By changing their air filters and regular furnace maintenance, homeowners can avoid damage and overheat.

What Function Does the Furnace High Limit Switch Serve?

A furnace high-limit switch is used to activate and deactivate the heating system. The switch will shut off when the furnace’s temperature reaches a risky level.

The limit switch is commonly attached to the blower motor temperature sensor in most residential structures. Thus, it successfully completes two essential tasks:

The high-limit switch manages the blower fan’s on-and-off cycle, which keeps your home cool. The temperature sensor tells the blower fans to turn on when the air is heated.

The temperature sensor in the high-limit switch detects when the furnace becomes too hot. Shutting down the system is a precaution.

High-limit switches that frequently trip signify an issue. The blower fans are turned off, and the system is locked by the high-limit switch until it cools to a safe temperature.

The Importance of High-Limit Switches

  • It protects health and furnaces. For the following reasons, high-limit switches are essential:
  • Safeguards the heat exchanger could become damaged in a very hot furnace.
  • Your home’s heat exchanger is broken, allowing flue gases to enter. Your air will be polluted with carbon monoxide.
  • The high-limit switch stops heat exchanger cracking at high temperatures.
  • The burners will be turned off if your heat exchanger overheats thanks to your high-limit switch. The heat exchanger is protected from cracking.
  • The blower can be turned on and off using the high-limit switch.
  • When the supply air temperature rises, the high-limit switch turns on the blower. Your home is heated by a blower.
  • The blower circulates the heat after it has heated. When the high-limit switch notices decreased temperatures, the blower stops.

Issues with Limit Switches

Limit switch failures are becoming a hassle. First, the blowers in the furnace might not shut off. The second sign is cool vent air.

Those are encouraging indications that the system is protecting you. However, the system needs to be inspected right away by Timberline Mechanical or another HVAC business.

Causes of Furnace Limit Switch Tripping

The switch may abruptly “trip” or shut off due to other circumstances. A temperature sensor is used in the limit switch. When that sensor becomes dirty, the limit switch can trip.

The furnace’s airflow issues could trip the switch. For instance, clogged furnace filters might stop airflow and trip a switch.

Your furnace filter needs to be changed periodically. Depending on the furnace, if an HVAC expert shows you how, you can clean the temperature sensor yourself.

You may save time, effort, and money by performing routine maintenance on your furnace and heating system. Like much other mechanical equipment, a furnace has used. The law stipulates a number of elements for home security. The most important component of the furnace is the “limit switch.”

When the temperature of a furnace rises above a risky level, a limit switch turns off the burners and turns on the blower fan. The burners will restart when the temperature drops to a reasonable level.

A heat exchanger could be damaged if a furnace overheats and releases carbon monoxide into your house or apartment.

1. Simple Cycling

The high-limit switch turns off the furnace if it detects a defect. The HLS could trigger early and cause temperature instability in your home when short cycling, which abruptly shifts between the heating and cooling cycles, is used.

Short cycling happens when heat builds up in air ducts too much or too little. Large furnaces typically do this. Excessive heating and cooling cause the high-limit switch to trip because it shuts and opens too quickly.

2. Dirty Air Filters

Unclean filters prevent airflow, which raises the furnace temperature. The switch trips when the heat exchanger becomes too hot.

A specialist should clean the air filters. Clean air filters that allow air to pass through them may assist the furnace in keeping a steady temperature.

3. Vents are Closed

Gases pass through the furnace vent fast. Gas cannot exit through a blocked vent, which suffocates the furnace and trips the high-limit switch.

Ensure the vent is clear after the flame has burned, then find a way to release the gas. A specialist can be consulted as well.

4. Excessive Heat

Overheating is a frequent problem with old or improperly maintained HVAC systems. Overheating can be caused by various things, including clogged filters, dirty sensor rods, unclean blowers, and dead pilot lights.

Furthermore, these are critical indicators that your heating system hasn’t been maintained. To resolve the problem, boost output, and reduce expenditures, you should have your furnace serviced as soon as possible if it is overheating.

Older machinery may develop overheating problems much more quickly. Regular maintenance will stop your heating system from overheating, tripping, and turning into a severe and expensive problem by allowing enough cold air to enter the vents.

5. A Shoddy Blower Motor

The switch will activate when the heat exchanger overheats due to insufficient air production from the blower motor.

The fan motor needs to be checked immediately soon. The blower motor can be repaired, but if the service specialist recommends replacing it, you must follow his advice. The blower motor must be replaced when it becomes too old.

6. Clogged ‘A’ Coil

The furnace system’s A-Coil functions as an AC unit as well. A furnace’s “A” coil can become blocked with dust and debris, making it useless. This could cause the heating system’s high-limit switch to trip.

The “A” coil can be cleaned to get the dirt out. When the A-coil is exposed to air, water, and moisture, it could be completely destroyed. If this happens, the coil can be changed.

7. A Worn-Out Limit Switch that is Malfunctioning

The limit switch is a crucial component of every furnace’s safety system. Say the temperature rises past a specific level. The heater’s electricity will then be cut off to prevent a possibly catastrophic house fire.

Limit switches can become less reliable if other gas furnace components are similarly compromised. Poor maintenance and overuse (when the HLS trips too frequently) are common causes of dysfunction.

Unreliable limit switches may prematurely trip even when the ducts do not show excessive heat. The HVAC system’s erroneous warnings could cause the limit switch to close when it shouldn’t.

How to Reset the Limit Switch on Your Furnace?

1. Disconnect the Power

Pull the plug at the panelboard to turn off the heating system. Electrical mishaps can be avoided in this way. You must switch the breaker marked “Heating System” or “Furnace” to the “off” position to turn off your furnace or heating system.

2. Use the Panel to Activate the Heater

Check the heating system’s thermostat. A downflow furnace or an up-flow furnace may have the control panel on the top, bottom, or both.

After that, remove the appliance’s control panel and put it away for the time being.

3. Finding the “Limit Switch”

Look for a bright red or yellow button that stands out from the rest of the panel when you open the dashboard’s control panel close to the ignition switch. Press and release the limit switch to restart your furnace manually.

How Can a High-Limit Switch Be Made to Stop Tripping Constantly?

Finding the source of the switch’s first trigger is necessary to fix a high-limit switch that continually trips. It is essential to make a complete system diagnosis before carrying out any of the fixes or solutions I’ve listed here.

1. The Flame Detection Rod Should Be Cleaned

90% of the time, a gas furnace’s unclean flame sensor rod has to be cleaned if it frequently goes on and off. If you know how to open the access panel, you can clean the sensor yourself; otherwise, you should seek expert assistance.

To clean the flame sensor head and get caking and debris out, remove the flame sensor rod and use an abrasive material, like fantastic sandpaper. If you contact it with greased hands, an issue with the sensor will arise after reinstalling it.

Please take caution when cleaning the flame sensor to avoid damaging it. To keep this rod in good shape and ensure it continues functioning as intended, you should clean it at least twice a year.

2. Making Limit Switch Adjustments

In the event that cleaning the flame sensor wasn’t enough to fix the issue, a high-limit switch that has aged poorly is probably to blame. The existing limit switch has to be replaced because it is damaged.

The gas furnace’s high-limit switch is connected to the burner or the surrounding area by two wires that return to the panel. You might hang it vertically or horizontally depending on how you view it. The maker of the new unit should have provided replacement instructions with the unit.

The equipment’s warranty could be voided if you don’t strictly adhere to the instructions. Replace this part every five years to guarantee your heating system’s safety and effective operation during the upcoming winter.

3. Dust off the Clogged Filters and the Fans

You should also clean the fans and filters on the gas furnace if they are unclean. We will start with the furnace filter, most likely at the bottom of your cabinet’s cabinet or a nearby compartment.

If this filter is spotless, the high-limit switch that quickly cycles your gas furnace on and off is probably not the problem. It should be disassembled and thoroughly vacuumed if there is evident dust.

Hazardous chemicals like dust and pollen won’t condense inside an air filter if it is kept clean. A dirty filter increases the chance of overheating and reduces airflow and lets poisonous air enter the house.

Now it’s time to move on to cleaning the blower fan. Thanks to this crucial function, your HVAC system ensures a constant flow of clean air. Blower fans may stop working as a result of accumulated dirt.

Just quickly clean them to see if they are rotating properly. If they often malfunction, you can get them fixed or replaced.

4. Adjust HVAC BTU Based on Home Size

Matching the HVAC BTU size to your heating and cooling requirements is the last remedy for a furnace high-limit switch that keeps tripping.

Due to their insufficient insulation, older homes require more BTUs than modern ones. Get a load calculation from a professional to determine how much system you need to heat or cool your home.

Your home needs to be appropriately heated during the winter. A higher wattage will stop it from cycling on and off, which is more gratifying but wasteful of energy.

5. Maintain the HVAC System as a Whole

Your HVAC system needs maintenance each year. The components should be cleaned, inspected, and tested as part of this servicing.

An expert will examine the gas furnace for irregularities to avoid high-limit switch trips and other problems.

If your high-limit switch is still tripping after trying all of these repairs, get a new one because the old one is damaged from years of use. After reinstalling this portion, examine each component to see if there are any new issues. Avoid neglecting this crucial maintenance task; a thorough check can prevent a winter energy disaster.

Recommendation: If the high-limit switch keeps tripping, do not turn it back on. When specific parts, like the heat exchanger, malfunction, dangerous carbon monoxide can leak into the home. You ought to have the gas furnace inspected, serviced, and repaired by a professional before turning it back on.

Can the Furnace’s High-Limit Switch be Fixed?

You can, but you shouldn’t most likely. A qualified electrician or HVAC technician would consider this a simple undertaking. It is not likely to be particularly safe for a homeowner.

Property owners should seek professional assistance if they think their furnace’s limit switch might be broken. You could prevent severe illness or injury by doing this, as well as having the HVAC system replaced.

How Much Would a Furnace High-Limit Switch Replacement Cost?

The limit switch for your furnace must be made by the same company. Depending on the manufacturer, this component may cost between $4 and $25. A universal switch might be available at the lowest price point.

Depending on the extent of the damage, hiring a professional to repair your furnace can cost you differently. The average cost of a furnace repair visit is $270, but charges can be as high as $1,000.


A part known as a furnace limit switch activates the furnace blower when the internal temperature reaches a predetermined threshold. A thermostat cannot maintain the proper temperature in a furnace with a faulty limit switch. A limit switch for a furnace typically consists of an inside temperature probe and an exterior plate.

To achieve the right temperature in your house, your furnace depends on several components operating in harmony. Change the limit switch with a replica if you intend to fix it yourself. Specifications for temperature, working range, and voltage are provided.

However, you shouldn’t risk damaging your system by attempting to fix it if you are unsure of your ability. Contact a professional who can repair your furnace so it can start up again.

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