Heat Pump Fundamentals, Refrigerant Cycle, and System Components
This domain tests how a heat pump moves heat rather than creating it, how the vapor-compression cycle changes between heating and cooling, and what each major component does. Expect questions on reversing-valve operation, defrost, balance point, and why capacity falls as outdoor temperature drops.
Core rules and concepts
- The cycle has four functions: the compressor raises pressure and temperature; the condenser rejects heat; the metering device drops pressure; the evaporator absorbs heat.
- A reversing valve swaps coil roles. In heating, the indoor coil is the condenser and the outdoor coil is the evaporator.
- COP is heat output divided by electrical input in compatible units. Rated capacity and COP fall at lower outdoor temperatures; published ratings at 47°F and 17°F are common.
- The balance point is where heat-pump capacity equals the building load. Below it, auxiliary or emergency heat makes up the difference.
- Key components include the compressor, suction accumulator, crankcase heater, reversing valve and solenoid, defrost board and sensors, outdoor fan, filter drier, and TXV or EEV.
- During defrost the outdoor fan stops, the valve shifts to cooling, and auxiliary heat tempers the supply air.
Worked example
A 3-ton unit delivers 36,000 Btu/h at 47°F while drawing 3.0 kW. COP = 36,000 ÷ (3.0 × 3,412) = 3.5. At 17°F it delivers 22,000 Btu/h at 2.8 kW: COP = 22,000 ÷ 9,554 = 2.3. If the building load is 25,000 Btu/h, the balance point lies between these temperatures and auxiliary heat covers the 3,000 Btu/h gap at 17°F.
Traps that cost marks
Treating a heat pump as a 100%-efficient furnace; forgetting that auxiliary heat runs during defrost; confusing the suction accumulator with a receiver; assuming rated capacity at every outdoor temperature; and judging charge during a defrost cycle.