The humble heating system limit switch. It is present on every furnace, yet most consumers provide it little thought, and less attention. Year after year, season after ~ season, the does that is job keeping you safe and also comfortable. Yet one work it could stop law its job. Probably the limit switch ~ above your heating system has failed, and also that’s why you’ve discovered this furnace troubleshooting FAQ guide.

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Following is every little thing you ever wanted to know about your heating system limit switch, and more. Us will aid you locate it and discuss just how to change it if essential – that a fairly easy DIY repair because that a careful, moderately competent homeowner.

A glossary of state is below intended to help the perform It Yourselfer in recognize your method around the inside of her furnace, understanding its operation and also carrying out maintenance and repair items like how to change a heating system limit move – questioned below.

The furnace limit switch is likewise known as a high border switch, fan limit switch, high temperature limit switch, or simply, limit switch. This article will describe the part as a “limit switch.”

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What the border Switch walk – Safety and also Comfort in One Switch

What the border Switch go – Safety and also Comfort in One Switch

The furnace high border switch help to ensure her safety and also the lull of your home. Pretty handy for a part that prices as little as $4.

It senses the furnace temperature in the plenum, and also tells the blower fan to rotate on and off as needed, follow to pre-set temperature settings. That can additionally shut under the furnace is that overheats due, because that example, come a an extremely dirty air filter restricting airflow, therefore the created heat can’t obtain out the the furnace. That’s a really useful security feature.

When the thermostat calls because that heat, the limit switch causes the heating system burners come ignite and begin to heat the heating system as super-hot combustion gases travel with the heat exchanger on their method out the the house.

Comfort: in ~ this point, if the heating system blowers and furnace burners to be to come on at the same time, the device would pump cold air v the house. Air in the plenum no hot sufficient yet and must be heated, for this reason the border switch keeps the blower pan off. This optimizes at home comfort.

When waiting in the plenum will the high limit setup on the limit switch, the switch signals the control board to start the blower fan, which circulates air across the warmth exchanger because that warming, then on come the remainder of the residence through ductwork. Cool air at the same time pulled from the home returns v air intakes, and circulates into the heater to be heated. Us talk about this as if it is a two-part process – warm air is driven out, and cool waiting is traction in. However, this is really consistent circulation in a forced air heater system.

When the thermostat reports the at home temperature has reached the designated setting, the furnace burners rotate off. The pan blower remains on summary to distribution the warmth still stored in the heat exchanger. When the limit switch senses the the plenum air supply has actually reached its reduced temperature setting, the border switch shuts off the blower pan so cool air won’t circulate with the ducts.

Safety: together a safety and security device, the role of a limit switch is come shut turn off the gas supply to the heating system burners, to avoid furnace and heat exchanger overheating.

The border switch shuts under the heating system burners if the warm exchanger gets too hot, which have the right to crack warm exchanger pipes and ruin the furnace. And when a warm exchanger cracks, deadly carbon monoxide can and also will leak from it right into the wait in your home.

Overheating frequently happens for these reasons:

The blower pan is not functioning right, therefore heated air no getting forced out – and cool air isn’t gift pulled inThe furnace filter is dirty – really dirty – so that it is restricting air circulation through both the furnace and also the warmth exchanger, causing heat exchanger cooling failure. Improper airflow causes the warm exchanger to overheat repeatedly, and excessively wears on the limit switch, ultimately causing malfunction.

What does a limit switch watch like? where is the furnace Limit Switch?

The furnace limit switch is attached to the heating system plenum within the cabinet.

You will have to take turn off the furnace cabinet covering or door to situate it. This diagram reflects what most of them look like and gives you an idea whereby to find it. Keep in mind that brand differ a little.


It attributes a lengthy temperature sensor probe attached to a mounting plate. The mounting plate is fastened exterior of the heater plenum, however inside the heating system cover, and the probe goes with the heating system housing right into the plenum. The mounting key receives two regulate wires for the blower fan and gas valve. Location of the limit switch varies by make and model, however is normally attached come the plenum over the burning chamber and/or warmth exchanger.

Limit switch Maintenance

Schedule a professional furnace tune-up every year, consisting of limit switch and also air circulation inspection. Change a worn the end or malfunctioning border switch with an identical new one.

Furnace border Switch Troubleshooting

Troubleshooting the most common limit switch difficulties takes discovering what come look for.

The heater blower fan doesn’t close up door off, however the burners room off and also the air exchanger is cool. What happened? The limit switch fail to revolve on the heating system burners.The furnace won’t revolve on? This is regularly indicative that a border switch failure while in the off position.

Alternatively, if a border switch senses high heat and also has come shut the furnace burners off a number of times (see manufacturer for specs), the furnace computer system may placed the heating system into “hard shutdown” mode until the heating system receives professional service.

Do the Yourself border Switch Replacement

Can I change a heater limit switch? that’s a common question, and also yes, it deserve to be done.

Locate the border switch as defined above. Using a multimeter, examine the border switch because that continuity. Unlimited resistance on the multimeter means the border switch is bad. Unscrew the move plate, remove the malfunctioning limit switch, and also install a corresponding replacement.

Here"s a handy find/remove/replace limit switch video that is less than 4 minute long.

Furnace border Switch replacement Cost

A restricted online search for heating system limit switch prices revealed a variety of $3.98 to around $25, depending on manufacturer. Every parts should be OEM, though universal switches might work in part furnaces.

Can a heating system Limit switch be bypassed?

Can girlfriend bypass furnace limit switch? correct – yet we execute not introduce it.

Here is a straightforward solution from an HVAC technician: “Why carry out you need to know exactly how to bypass something in a furnace? Planning to burn a home down? Bypassing many things in a home heating system would produce a an extremely real and an extremely dangerous fire and also safety hazard.”

There is no an excellent reason because that a customer to bypass a heater limit switch. Furnace switches are reliable, cheap to replace, and also are designed for protection of the consumer. Bypassing a heater limit move or any type of home heater component can create dangerous safety and fire hazards. In restricted instances, HVAC professionals may usage the method to isolate or diagnose specific system problems.

Glossary that Furnace and Furnace limit Switch

These terms will help you in finding, diagnosing and also replacing a heating system limit switch.

Blower Fan: When involved by the limit switch, the heating system blower pan circulates cool air throughout the warmth exchanger and through the ductwork distribution system; the fan at the same time circulates cool air from the house, i beg your pardon is obtained through air intakes.

Combustion Chamber: The part of the heating system where the gas is melted to generate heat.

Continuity (electrical): testing of electrical present to recognize resistance, and also detect (limit switch) malfunction.

Forced Air: A blower mechanism to “force” cook or cooled air with a home using ductwork. Different systems encompass hot water radiators, electric baseboards, and also wood stoves and also pellet stoves.

Furnace Filter: A replaceable/recyclable/disposable fiberglass filter the cleans the air going right into the heating system for heating circulation.

Heat Exchanger: also called the wait exchanger, cool waiting is blown throughout the warm exchanger, where the air is warmed. Warm air is climate blown v the plenum and ductwork.

High border (and low Limit) Setting: The border switch might shut down the furnace for consistently exceeding the attention “high limit” or “low limit” settings, which might indicate any kind of of number of possible part or mechanism failures.

Multimeter (also, multi-meter): Measures electric current, voltage, and also resistance. It deserve to be offered to determine whether the border switch is faulty.

Plenum: The sheet steel ductwork encasement over the heat exchanger, the plenum hold air because that warming and also circulation v the heating system, and serves as a central manifold for various other ductwork.

Temperature Sensor Probe: The component of the border switch that extends with the heating system housing and also into the plenum to measure the waiting temperature, i beg your pardon determines subsequent actions the the border switch.

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Thermostat: Senses the temperature that a heater or cooling system, and also maintains the device temperature near a desired level.