Air Conditioning, Heat Pumps, and Refrigerant Systems Installation
Questions focus on installation, evacuation, charging, and heat-pump-specific components. Know how to use superheat and subcooling, and how heat-pump mode changes charging and defrost.
Installation and charging
Purge with nitrogen while brazing, pressure-test, then evacuate to 500 microns and hold. Weigh in refrigerant when the line set exceeds the manufacturer's length or when charging is required. TXV systems are charged by subcooling; fixed-orifice systems by superheat. Heat pumps are typically charged in cooling mode; if outdoor temperature is low, use the manufacturer's winter-charge method.
Worked example
An R-410A TXV system has a high-side saturation temperature of 110°F and a liquid-line temperature of 98°F. Subcooling = 110 − 98 = 12°F. The manufacturer specifies 10°F. The system is slightly overcharged; recover refrigerant rather than venting. If subcooling were 4°F, suspect undercharge or a liquid-line restriction.
Heat pumps
The reversing valve swaps evaporator and condenser. Defrost controls ice on the outdoor coil; auxiliary heat maintains supply temperature. Check the defrost board, sensor, and reversing valve solenoid. A heat pump that heats poorly may have a stuck reversing valve or failed auxiliary heat, not just low charge.
What costs marks
- Skipping evacuation or failing to hold a vacuum.
- Charging by “beer-can cold” suction lines.
- Adding refrigerant without weighing or checking subcooling/superheat.
- Testing a heat pump only in heating mode when the manufacturer requires cooling-mode charge.
- Mismatching indoor and outdoor units or using undersized line sets.