1. Disc-Type Temperature Limit Controls
- Function: Operates similarly to bimetal controls, used as high limit temperature devices.
- Location: Embedded in the plenum or heat exchanger areas.
- Operation: Shuts down the furnace if the temperature setting is exceeded.
- Additional Function: In older furnaces, a second device may operate the blower fan when the temperature around the heat exchanger rises.
2. High and Low Pressure Controls
- Diaphragm Function: Contains a diaphragm that deflects according to specific pressures.
- High Pressure Safety Control:
- Operation: Opens switch contacts when the pressure rises above the maximum setting.
- Example: Shuts off the compressor if the condenser coil is dirty.
- Low Pressure Safety Control:
- Operation: Opens switch contacts when the pressure drops to its set point.
- Example: Shuts off the compressor when refrigerant is low or there is excessive frost on the outdoor coil.
- Heat Pumps: Uses low pressure switches to sense refrigerant pressure drops and initiate defrost.
3. Vapor Charged Controls
- Sensing Bulbs: Contain gas that expands and contracts with temperature.
- Operation: Opens and closes switch contacts based on temperature changes.
4. Motor Overloads
- Function: Sense motor current.
- Operation: Shuts down the motor by opening and breaking power to the motor to prevent overheating and damage.
5. Fuses and Fusible Links
- Function: Provide overcurrent protection in electrical circuits.
- Application: Used in compressors, evaporator coils, motors, and wiring configurations.
- Operation: Break the electrical circuit when detecting an electrical current greater than the circuit's designed limit.
- Reaction: Once activated, they cannot be reset and must be replaced.
- Specifications:
- Action Voltage and Amperage Rating: Defines the operational limits.
- Physical Size: Must conform precisely to the original part installed or specified by the manufacturer.
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