Good Guys, what is an open high limit error on a furnace?

Forced-air furnaces powered by natural gas,LP, or heating oil use a small but important device called the high-temperature limit switch.The fan limit switch is used to turn the fan on and off during the heating cycle of the furnace, and to shut it down if the interior temperature gets too high.

The limit switch can be accessed by removing the cover panel.A limit switch has a long temperature sensor probe attached to it.The probe goes through the wall of the furnace and the plate goes above the heat exchanger.Control wires for the blower fan and the furnace's gas valve are sent to the mounting plate.

It's possible that your furnace has limit switches that check temperatures at different parts of the furnace.Each of the switches may have their own mounting plate, or they may be part of a single combination unit.

The limit switch is mounted on the outside of the furnace rather than inside the service panel.The wire connections are protected by a rectangular cover plate.

Every normal cycle of the furnace has a limit switch.The heat exchanger is heating when the thermostat calls for it.The limit switch keeps the blower off when the air above the heat exchanger isn't warm enough to heat the house.When the air in the plenum reaches the lower setting on the limit switch, the switch goes on and off the blower fan, which circulates air through the heat exchanger on its way to the house, and into the furnace.

The limit switch on the blower keeps it running while the burners are turned off when the house temperature reaches the thermostat setting.The limit switch shuts off the fan when the air in the supply plenum drops back to the lower setting.

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