Get to Know Your HVAC: The High Limit Switch

HVAC systems consist of complex equipment with a lot of delicate parts. That’s why it’s so important to have your system installed and maintained by highly-trained and skilled technicians. Let’s take a look at one of the small but important components in your gas furnace. This is the high limit switch.

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What is a High Limit Switch?

The high limit switch in your gas furnace is a small part with two very important jobs. This switch plays a role in the normal operation of your furnace.  Turning the fan motor on and off is the high limit switch’s main job. This is the role it plays in every cycle of your furnace’s normal operation. But that’s not the only thing that the high limit switch does. It’s also safety device that helps prevent your furnace from overheating.

The high limit switch consists of a long temperature-sensing probe attached to a metal mounting plate. From the mounting plate, the switch is wired to the blower fan motor and to the gas valve. The switch is generally located just past the heat exchanger. This is so that it can detect the temperature of the supply air. In other words, the air that’s about to be blown into your home.

See Also: Upgrading Your Gas Furnace with New Technology

The High Limit Switch’s First Job

During normal operation, the high limit switch’s main job is to control when the blower fan turns on and off. The blower fan needs to be controlled by the limit switch rather than the thermostat. This is because you don’t actually want the blower fan to turn on and off at the same time as the gas burner.

When the burner first comes on, it takes a while to heat up the heat exchanger. Then for that heat to transfer into the supply air waiting inside the furnace takes longer. If the blower fan turned on at the same time as the burner, it would be blowing cold air through the vents into your home. Nobody wants a blast of arctic air in the winter time!

So the high limit switch detects the temperature of the supply air. When that air reaches a warm enough temperature, the switch tells the blower fan to come on. Ever notice there’s a two to three minute delay between when your thermostat switches the heat on and when the warm air actually comes out of the vents? That’s from the fan waiting for the limit switch to send its signal.

Now, when your thermostat detects that the air in your home has reached its set temperature, the thermostat tells the gas burner in your furnace to turn off. But the blower fan keeps running for a little while.

That’s because it’s blowing all the heated air that’s already in your supply ducts on into your home. The energy used to heat that air isn’t wasted. Sometimes the temperature of the supply air inside the furnace drops below a certain temperature. This is when the high limit switch signals the blower motor to turn off.

See Also: Thermostats

The High Limit Switch’s Second Job

The high limit switch is also an extremely important safety device inside your furnace. When the furnace begins running, the switch’s role is to turn on the blower fan when the supply air gets hot enough.

However, if the temperature of the supply air gets too high, indicating that the inside of the furnace is overheating, then the high limit switch performs its second job—shutting off the gas valve to stop the heating.

If the high limit switch is tripped by overheating several times in a row, then the furnace will go into “lockout” mode, where it will not turn back on at all until it’s serviced and reset.

This is a very important safety feature because if the furnace overheats, it can cause the heat exchanger to crack. And if the heat exchanger cracks, it can allow poisonous carbon monoxide gas to enter your home.

By shutting off the gas valve, the high limit switch helps to keep your family safe, as well as helping to prevent costly repair or replacement of your system.

See Also: Protecting Your Home From an Air Conditioner Fire

Signs That a High Limit Switch Needs to Be Replaced

Like any other part in your HVAC system, the high limit switch can wear out over time. One of the most common issues that happens when a high limit switch goes bad is that it fails to signal the blower fan to turn off. If your fan keeps running continuously (and you’ve checked that your thermostat is set to AUTO rather than ON), that could be a sign of a bad high limit switch.

High limit switches can also weaken over time so that they begin tripping at a lower temperature. For instance, a high limit switch that’s supposed to trip at 155F might begin tripping at 130F instead. This can lead to your furnace going into lockout mode even though it wasn’t really overheating.

In either of these cases, the high limit switch will need to be replaced. However, it’s important to make sure that if the limit switch tripped repeatedly, it was really due to the switch being bad and not due to actual overheating.

See Also: How Important is Regular Maintenance

Protecting Your High Limit Switch

When a high limit switch gets tripped due to overheating, that contributes to wearing out the switch faster. One of the most common causes of furnace overheating is a clogged furnace filter. So, in addition to regular annual maintenance on your furnace, regularly replacing your furnace filter is the best thing you can do to help your high limit switch last as long as possible.

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