Electrician Red Seal · Question
A furnace fan motor in a commercial building is controlled by a thermostat and a fan/limit switch. The fan/limit switch has a 'FAN ON' setting, a 'FAN AUTO' setting, and a 'LIMIT' setting which acts as a high-temperature cutout. If the fan runs continuously regardless of the thermostat setting, and the furnace is not actively heating, which component is MOST likely faulty?
If the fan runs continuously when it shouldn't, especially with the furnace not heating, it usually points to the fan control contacts within the fan/limit swit
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Question: A furnace fan motor in a commercial building is controlled by a thermostat and a fan/limit switch. The fan/limit switch has a 'FAN ON' setting, a 'FAN AUTO' setting, and a 'LIMIT' setting which acts as a high-temperature cutout. If the fan runs continuously regardless of the thermostat setting, and the furnace is not actively heating, which component is MOST likely faulty?
Answer options:
- The thermostat.
- The furnace gas valve. ✅ The fan/limit switch's fan control contacts are stuck closed.
- The motor's start capacitor.
Correct answer: The fan/limit switch's fan control contacts are stuck closed.
Explanation: If the fan runs continuously when it shouldn't, especially with the furnace not heating, it usually points to the fan control contacts within the fan/limit switch being stuck closed. These contacts are responsible for turning the fan on and off based on temperature or the 'FAN ON/AUTO' setting. The thermostat controls the call for heat, not directly the fan for continuous operation independent of heating. The gas valve is for combustion, and the motor's start capacitor affects motor starting, not continuous unsolicited running.
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