Heat Pump Fundamentals and Refrigeration Cycle
Heat Pump Fundamentals and Refrigeration Cycle
This domain tests how a heat pump moves heat rather than creates it, and how the refrigeration cycle changes when the reversing valve switches between heating and cooling. Expect questions on component function, pressure-temperature relationships, superheat and subcooling, capacity balance, and defrost.
Core rules
- A heat pump has four major components: compressor, condenser, metering device, and evaporator. In heating, the outdoor coil is the evaporator and the indoor coil is the condenser; the reversing valve changes refrigerant flow.
- Heat flows from warmer to cooler. The compressor raises pressure and temperature; the metering device drops pressure and temperature.
- Superheat is measured at the evaporator outlet or compressor inlet and protects the compressor from liquid. Subcooling is measured at the condenser outlet and proves a liquid seal at the metering device.
- Capacity and COP vary with outdoor temperature. The balance point is where heat pump capacity equals building load; below it, auxiliary heat makes up the difference.
- Defrost is needed when the outdoor coil frosts. It reverses cycle or uses demand defrost, and auxiliary heat tempers supply air.
Worked example
A heat pump delivers 36,000 Btu/h heating output while drawing 3.0 kW. Convert kW to Btu/h: 3.0 x 3,412 = 10,236 Btu/h. COP = output/input = 36,000/10,236, or about 3.5. If the same unit draws 4.0 kW, COP falls to about 2.6, showing why auxiliary heat raises operating cost.
Traps
Do not call the outdoor coil a condenser in heating. Do not confuse gauge pressure with absolute pressure when using P-T charts. Do not measure superheat at the condenser outlet; that is subcooling. Do not assume a heat pump is low on charge because head pressure is low in cold weather.