{"id":2738,"date":"2022-11-28T11:00:00","date_gmt":"2022-11-28T16:00:00","guid":{"rendered":"https:\/\/www.purlsheetmetal.com\/blog\/?p=2738"},"modified":"2022-11-28T16:11:05","modified_gmt":"2022-11-28T21:11:05","slug":"what-does-a-furnace-limit-switch-do","status":"publish","type":"post","link":"https:\/\/www.purlsheetmetal.com\/blog\/what-does-a-furnace-limit-switch-do\/","title":{"rendered":"What Does a Furnace Limit Switch Do?"},"content":{"rendered":"
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\"furnace-with-pipes\"<\/a><\/figure>\n<\/div>\n\n\n

Gas furnaces work on a basic principle: lit gas flames heat air that is blown through the furnace and into the rooms through ductwork. But gas furnaces are complex devices that have several safety features and mechanisms that allow them to work correctly and without creating hazards. <\/p>\n\n\n\n

One of the key components for the operation of a furnace is the furnace limit switch. You don\u2019t need to know all the details about this part, but understanding the basics will help you know more about how your furnace works\u2014and why it sometimes doesn\u2019t work.<\/p>\n\n\n\n\n\n\n\n

The Basic Function of the Furnace Limit Switch<\/strong><\/h2>\n\n\n\n

The furnace limit switch, sometimes called the furnace fan limit switch, is a switch that turns on and off the furnace blower assembly. The switch senses temperatures within the furnace and activates or deactivates the blower based on them. The limit switch is located just below the plenum where it can easily monitor the interior furnace temperature.<\/p>\n\n\n\n

The purpose of the furnace limit switch is to coordinate the blower assembly with the burners in the furnace, which prevents uneven heating and drafts indoors, and to shut the furnace off in case of overheating. Stopping the furnace from overheating isn\u2019t only to prevent danger to people in the house, but also prevent damage to the furnace\u2019s other components. <\/p>\n\n\n\n

The Furnace Limit Switch in Operation<\/strong><\/h2>\n\n\n\n

Let\u2019s go through a basic furnace heating cycle to show the important job of the limit switch during standard operation.<\/p>\n\n\n\n

When the thermostat makes a request for heating to the furnace, the control board activates the gas flow to the burners and the ignition system to light them. At this point, the fan won\u2019t have yet come on. The limit switch monitors the heat in the plenum until it has reached 20\u00b0 more than current room temperature. The switch then activates the blower assembly and begins pushing air into the furnace and around the heat exchanger. The heat exchanger contains all the hot gas drawn from the gas jets and transfers this heat from its metal surface to the air. The heated air continues into the ductwork.<\/p>\n\n\n\n

Once the thermostat registers the house interior has reached its target temperature, it signals the control board to shut off the gas to the burners. The blower fan will remain on, however, as the air still needs to help the heat exchanger cool down. The limit switch detects when the exchanger has reached a safe temperature, then shuts off the blower assembly.<\/p>\n\n\n\n

The standard temperature settings for a furnace limit switch:<\/p>\n\n\n\n