1. Low Ambient Controls
- Definition: Low ambient controls maintain a minimum head pressure in an AC system when outdoor temperatures drop.
- Head Pressure: Pressure generated by the AC compressor.
- Fan Cycle Control:
- Function: Pressure switches that turn the condenser fan off when head pressure drops to a set point (cut-out).
- Cut-In: The fan restarts when head pressure rises above a certain point.
- Importance: Ensures proper pressure drop across the metering device and proper metering of liquid refrigerant into the evaporator.
- Multi-Speed Condenser Fans: Adjust fan speed based on outdoor temperatures to maintain head pressure.
2. Bimetal Temperature Controls
- Function: Use two dissimilar metals fused together, which expand at different rates when heated, causing curvature.
- Applications: High limit temperature switches in furnaces.
- Operation: Make or break electrical contacts to control electrical flow.
- Maintenance: Limited control life; verify operation annually.
3. Electronic Fossil Fuel Backups (Hybrid Systems)
- Definition: Dual fuel systems that use multiple energy sources to meet heating demands.
- Components: Connect electric HVAC systems to solid fuel furnaces, providing auxiliary or supplemental heat during cold periods.
- Operation:
- Auxiliary Heating: Replaces the air handler with a furnace for fossil fuel heat.
- Staging Process:
- First Stage: Heat compression from the heat pump.
- Second Stage: Activated when outdoor temperature drops below the balance point (usually around 40 degrees).
- Balance Point: The lowest outdoor temperature at which the heat pump alone can meet the heating needs without supplemental heat.
- Fossil Fuel Backup: Controls the heat pump and furnace signals, cycling them based on temperature sensors' readings.
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