The blower on a forced-air gas furnace is controlled by the fan limit switch. This switch has three settings: FAN ON, FAN OFF and LIMIT.
The FAN ON setting ensures the blower won’t turn on until there’s warm air in the supply air plenum, preventing the blower from sending cold air into your home.
The FAN OFF setting keeps the blower operating even after the burner has turned off. The blower will run until the heat exchanger has cooled down and there’s no more warm air to send to your rooms.
The LIMIT setting, a safety control, turns the burner off if the air in the warm air plenum reaches an unsafe temperature. This rarely occurs, but may happen due to blocked ducts or improperly operating system controls.
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