High Temperature Limit Control on a Furnace

A furnace's high temperature limit controls play an important role in keeping homeowners and residents safe. These limit controls are typically used to keep the furnace from overheating, as an overheated furnace can be a fire hazard. High temperature limit controls can help stave off any potential fires and are, therefore, placed in various parts of the furnace's control system.

  1. Controls

    • High temperature limit controls in furnaces operate on fairly simple principles and differ little from one switch to the next. These switches generally serve as a sort of circuit breaking mechanism that cuts off the furnace's electrical supply when the temperature gets too high. A high limit set point is usually predetermined, so when the temperature is reached, the switch breaks the circuit and prohibits the furnace from operating.

    Fan Limit Control

    • The fan limit control in some furnaces is also based on the same principle. This control consists of two switches that will turn the furnace's blower on and off based on the temperature of the furnace's heat exchanger. When the temperature reaches a certain level, the fan will turn on and distribute the warmed air throughout the house. These controls also have a high limit setting which shuts off the fan if the furnace becomes too hot.

    Vent Controls

    • A high temperature control limiter is generally placed in the vent or exhaust system itself. The furnace must be able to vent any noxious gases, such as carbon monoxide, outside of the home. It will generally do this at a fairly constant temperature. If the temperature begins to rise too high, this may be an indication that the internal temperature has risen to an unacceptable level. The high limit switch will kick in and cause the system to shut down as necessary.

    Resetting

    • If your furnace shuts down because of a potentially high temperature problem, you may need to reset your temperature control limit switch. Doing so will indicate to your system that it should continue to operate as normal, according to the settings that you or the manufacturer have indicated. Resetting the switch involves removing the service panel and finding the switch inside the furnace. The switch is located on the temperature control setting and is usually pulled out when it needs to be reset.

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