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Furnace Limit Switch Troubleshooting

Modern furnaces are equipped with safety devices that are designed to stop problems before they can occur (or at least accumulate). The furnace limit switch is one such helpful device. By understanding how the limit switch works and how you can take care of it, your furnace stands a better chance of working well longer.

How does the limit switch work in a furnace?

Depending on the type of furnace you have, the limit switch can be a single combination switch or be two separate switches—fan control limit and high limit. After your furnace begins a heat cycle, the burners require some time to heat up. The air handler needs to wait to turn on during this period so it is not delivering lukewarm or cool air throughout your house. The fan control limit switch is what communicates this message from the furnace’s supply plenum to its air handler. The switch is located under the supply plenum, and it reads the plenum’s temperature. When the plenum reaches the appropriate temperature, the limit switch will send a signal to the air handler. The handler will then begin to run and deliver warm air to the rooms in your house.

Additionally, when the plenum gets too hot, the high limit switch will shut off the gas supply to the furnace burners. This safety feature prevents your furnace from overheating. Some furnaces provide extra safety features by wiring two or three high limit switches in a series that check the temperature in the furnace at different points.

So, to put it simply—the limit switch tells the furnace fan when to turn on and when to shut off, and it tells the furnace to shut down if it is overheating. Each of these functions are tied to a different temperature setting, all of which are factory set to match the capacity and safety guidelines of the furnace.

What happens if the furnace limit switch is not properly working?

After years of use, the limit switch on your furnace can develop issues. Often, the problems result from airflow difficulties within your system, such as when it operates with a dirty filter. Without proper airflow, the heat exchanger will repetitively overheat, which can wear out your limit switch and eventually cause the switch to malfunction. When your limit switch is not working correctly, your furnace can overheat without shutting down as it should. Also, a malfunctioning switch may not send signals to the air handler successfully, which may cause the handler to never turn on.

How to test and replace the fan limit switch on a furnace

Things to check before replacing the limit switch

If your furnace has a circuit board with an LED light, start by checking whether it is blinking. Sometimes when a problem occurs, the light blinks a code (short, long, etc., like Morse Code) that corresponds to a code chart, which diagnoses the issue (for example—limit switch is open).

If your LED light is not blinking or your furnace does not have this feature, there are still a couple more things to consider before you presume the fault lies with the furnace limit switch. Check for the following issues:

  • Clogged furnace filter—A dirty or clogged filter blocks the air flow into the furnace, which may raise the inside temperature of the unit. When the temperature gets too high, the high limit switch turns off the gas to the furnace. It is important to regularly replace your air filters (monthly or bi-monthly) to ensure the furnace has good air flow. This task is often forgotten or minimized, but it is the cheapest, easiest, and best way to maintain a good working furnace.
  • Faulty blower motor—If the motor is not working, no air can be blown through the unit, which can cause it to overheat, and the high limit switch will turn off. You can check whether the motor is working by switching your thermostat Fan setting to On (rather than Auto). If air blows out from the vents, then the blower motor is working and something else (potentially the limit switch) is causing the problem.

If the filter and blower motor are both in good working order, it is time to test the limit switch. First, shut off the furnace, wait a few seconds, then turn it back on. Then, follow the instructions based on the type of limit switch your furnace has:

  • Electromechanical—These switches have a circular dial with mechanical high and low set points that can be adjusted. One side will have 24 volts for the gas valve, and the other side will have 120 volts for the fan. Use a multimeter to test whether there is continuity when the furnace reaches the preset temperatures on the high and low sides.
  • Solid state—These circular switches are smaller and sometimes feature a reset button. Use a multimeter to check whether the switch has continuity.

How to perform a furnace limit switch reset

It is best to consult the manual for your furnace if you want to reset the limit switch. The instructions differ depending on whether you have a combination switch or separate high and fan control limit switches, as well as on the specific model you have. However, the typical temperature settings are as follows:

  • Fan control limit—90 and 130 degrees Fahrenheit
  • High limit—200 degrees Fahrenheit

And the limit switch should be pulled out. If the switch is pushed in, the fan will run automatically disregarding the temperature settings.

How to find a limit switch replacement

If you do need to replace a faulty limit switch, your first step is to write down all the important info about your furnace—manufacturer, model number, serial number, voltage ratings, etc. You can usually find this information on the rating plate of the furnace unit or part. For limit switches, you also need to know the temperature range. And, if you have an older furnace, be sure to write down the length of the rod protruding from the back. When shopping online, use your detailed list to check whether a replacement matches up with your furnace’s needs. When shopping in a brick and mortar store, take the old limit switch and your list of details with you so the store clerk knows exactly what you need.

How to replace a limit switch

Most modern limit switches only have two wires and two screws to mess with, so it is not overly complicated to replace the switch. Older switches, however, can be more complex—with three wires to connect and a tab that may or may not need to be snapped off. If your furnace uses an older model of limit switch, consult the manual to ensure you handle the removal and replacement correctly.

If you have a simpler modern furnace limit switch, you can replace it quite easily by following these steps:

  • Shut off the power to the furnace. You can do this by turning off the furnace service switch or by shutting off the breaker at the main panel (doing both is the safest option).
  • Pay close attention to how the faulty limit switch is wired in (taking a picture with your phone can be helpful). Then, remove the switch.
  • Wire the new limit switch into the furnace in the same way as the old one (refer to your photo, if you took one).
  • When the new switch is secured, turn the power to the furnace back on. Then, set the thermostat to heat, and check whether the furnace is working correctly again.
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