The humble furnace limit switch. It is present on every furnace, yet most consumers give it little thought, and less attention. Year after year, season after season, it does its job keeping you safe and comfortable. But one day it might stop doing its job. Perhaps the limit switch on your furnace has failed, and that’s why you’ve found this PickHVAC furnace troubleshooting FAQ guide.
Following is everything you ever wanted to know about your furnace limit switch, and more. We will help you locate it and discuss how to replace it if necessary – It’s a fairly easy DIY repair for a careful, moderately experienced homeowner.
A Glossary of terms is below intended to assist the Do It Yourselfer in finding your way around the inside of your furnace, understanding its operation and carrying out maintenance and repair items like how to replace a furnace limit switch – discussed below.
The furnace limit switch is also known as a high limit switch, fan limit switch, high temperature limit switch, or simply, limit switch. This article will refer to the part as a “limit switch.”
What the Limit Switch Does – Safety and Comfort in One Switch
The furnace high limit switch helps to ensure your safety and the comfort of your home. Pretty handy for a part that costs as little as $4.
It senses the furnace temperature in the plenum, and tells the blower fan to turn on and off as needed, according to pre-set temperature settings. It can also shut down the furnace is it overheats due, for example, to a very dirty air filter restricting airflow, so the created heat can’t get out of the furnace. That’s a very useful safety feature.
When the thermostat calls for heat, the limit switch causes the furnace burners to ignite and begin to warm the furnace as super-hot combustion gases travel through the heat exchanger on their way out of the house.
Comfort: At this point, if the furnace blowers and furnace burners were to come on at the same time, the system would pump cold air through the house. Air in the plenum isn’t hot enough yet and must be heated, so the limit switch keeps the blower fan off. This optimizes indoor comfort.
When air in the plenum reaches the high limit setting on the limit switch, the switch signals the control board to start the blower fan, which circulates air across the heat exchanger for warming, then on to the rest of the house through ductwork. Cool air simultaneously pulled from the house returns through air intakes, and circulates into the furnace to be heated. We talk about this as if it is a two-part process – warm air is pushed out, and cool air is pulled in. However, this is really continuous circulation in a forced air heating system.
When the thermostat reports the indoors temperature has reached its designated setting, the furnace burners turn off. The fan blower stays on briefly to distribute the heat still stored in the heat exchanger. When the limit switch senses that the plenum air supply has reached its lower temperature setting, the limit switch shuts off the blower fan so cool air won’t circulate through the ducts.
Safety: As a safety device, the role of a limit switch is to shut off the gas supply to the furnace burners, to prevent furnace and heat exchanger overheating.
The limit switch shuts down the furnace burners if the heat exchanger gets too hot, which can crack heat exchanger pipes and ruin the furnace. And when a heat exchanger cracks, deadly carbon monoxide can and will leak from it into the air in your home.
Overheating commonly happens for these reasons:
- The blower fan is not working right, so heated air isn’t getting forced out – and cool air isn’t being pulled in
- The furnace filter is dirty – really dirty – so that it is restricting air flow through both the furnace and the heat exchanger, causing heat exchanger cooling failure. Improper airflow causes the heat exchanger to overheat repeatedly, and excessively wears on the limit switch, eventually causing malfunction.
What does a limit switch look like? Where is the Furnace Limit Switch?
The furnace limit switch is attached to the furnace plenum inside the cabinet.
You will have to take off the furnace cabinet cover or door to locate it. This diagram shows what most of them look like and gives you an idea where to find it. Note that brands differ a little.
It features a long temperature sensor probe attached to a mounting plate. The mounting plate is fastened outside of the furnace plenum, but inside the furnace cover, and the probe goes through the furnace housing into the plenum. The mounting plate receives two control wires for the blower fan and gas valve. Location of the limit switch varies by make and model, but is usually attached to the plenum above the combustion chamber and/or heat exchanger.
Limit Switch Maintenance
Schedule a professional furnace tune-up every year, including limit switch and air flow inspection. Replace a worn out or malfunctioning limit switch with an identical new one.
Furnace Limit Switch Troubleshooting
Troubleshooting the most common limit switch problems takes knowing what to look for.
- The furnace blower fan doesn’t shut off, but the burners are off and the air exchanger is cool. What happened? The limit switch failed to turn on the furnace burners.
- The furnace won’t turn on? This is often indicative of a limit switch failure while in the OFF position.
Alternatively, if a limit switch senses high heat and has to shut the furnace burners off a number of times (see manufacturer for specs), the furnace computer may put the furnace into “hard shutdown” mode until the furnace receives professional service.
Do It Yourself Limit Switch Replacement
Can I replace a furnace limit switch? That’s a common question, and yes, it can be done.
Locate the limit switch as described above. Using a multimeter, check the limit switch for continuity. Infinite resistance on the multimeter means the limit switch is bad. Unscrew the switch plate, remove the malfunctioning limit switch, and install a matching replacement.
Here’s a handy find/remove/replace limit switch video that is less than 4 minutes long.
Furnace Limit Switch Replacement Cost
A limited online search for Furnace limit switch prices revealed a range of $3.98 to about $25, depending on manufacturer. All parts should be OEM, though universal switches might work in some furnaces.
Can a Furnace Limit Switch be bypassed?
Can you bypass furnace limit switch? Yes – but we do not recommend it.
Here is a straightforward response from an HVAC technician: “Why do you need to know how to bypass anything in a furnace? Planning to burn a house down? Bypassing most things in a home heating system would create a very real and very dangerous fire and safety hazard.”
There is no good reason for a consumer to bypass a furnace limit switch. Furnace switches are reliable, inexpensive to replace, and are designed for protection of the consumer. Bypassing a furnace limit switch or any home heating component can create dangerous safety and fire hazards. In limited instances, HVAC professionals may use the technique to isolate or diagnose specific system problems.
Glossary of Furnace and Furnace Limit Switch
These terms will assist you in finding, diagnosing and replacing a furnace limit switch.
Blower Fan: When engaged by the limit switch, the furnace blower fan circulates cool air across the heat exchanger and through the ductwork distribution system; the fan simultaneously circulates cool air from the house, which is received through air intakes.
Combustion Chamber: The part of the furnace where the gas is burned to generate heat.
Continuity (electrical): Testing of electrical current to determine resistance, and detect (limit switch) malfunction.
Forced Air: A blower system to “force” heated or cooled air through a house using ductwork. Alternative systems include hot water radiators, electric baseboards, and wood stoves and pellet stoves.
Furnace Filter: A replaceable/recyclable/disposable fiberglass filter that cleans the air going into the furnace for heating circulation.
Heat Exchanger: Also called the air exchanger, cool air is blown across the heat exchanger, where the air is warmed. Warm air is then blown through the plenum and ductwork.
High Limit (and Low Limit) Setting: The limit switch may shut down the furnace for repeatedly exceeding the dangerous “high limit” or “low limit” settings, which may indicate any of several possible part or system failures.
Multimeter (also, multi-meter): Measures electrical current, voltage, and resistance. It can be used to determine whether the limit switch is faulty.
Plenum: The sheet metal ductwork encasement above the heat exchanger, the plenum holds air for warming and circulation through the heating system, and serves as a central manifold for other ductwork.
Temperature Sensor Probe: The part of the limit switch that extends through the furnace housing and into the plenum to measure the air temperature, which determines subsequent actions of the limit switch.
Thermostat: Senses the temperature of a heating or cooling system, and maintains the system temperature near a desired level.