High Limit Switch
A high-limit switch is a protective part of a furnace designed to keep the mechanism from overheating itself. A furnace is at a risk of overheating any time airflow is limited from passing through the heat exchanger. A clogged filter or squirrel cage can limit the airflow and, without a high limit switch, the furnace would continually cycle until it burned itself out. The high limit switch shuts down the furnace at a set limit so a clog doesn't burn out the system. Note that the limit switch has nothing to do with the thermostat. Rather, it is wired in series with the 24-volt control circuit of your furnace. A pair of wires is connected to the switch. When the furnace is cool enough, the switch closes, bringing the two wires together and setting the burner into action again.
Types Of High Limit Switches
When looking for a replacement, it's important to note that there are numerous limit switches used in many different applications. Users will find electromechanical and solid-state switches, as well as three possible choices for contact: momentary, maintained or positive-opening. Furnace limit switches are electromechanical devices with momentary contact.
Replacing A High Limit Switch
Most important, before attempting a replacement, make sure the switch actually is defective. Turn power off to the furnace (a must in any component replacement). Use an ohmmeter to measure resistance across the switch, which, if the temperature is below the setpoint, should be zero. When the proper replacement model is selected, mount the new control in the furnace. Then reconnect the wires, ensuring they're in the proper location. Turn the power back on and run the furnace through one complete heating cycle to verify that it's running appropriately. After this step, though, it should be verified that the furnace has proper air flow. This is done by measuring the outlet and return air temperatures. Compare the increase with the recommended temperature rise as noted on the furnace's label plate. If the rise falls outside of this range, obstructions should be sought in the blower motor, filter and duct system.
Who Makes High Limit Switches?
Among the companies manufacturing high limit switches are:
- ABB Low Voltage Products & Systems
- Carlton-Bates Co.
- Eaton's Cutler-Hammer Sensors
- Euchner-USA Inc.