Heat Pump Fundamentals and System Components
This domain tests how a heat pump moves heat rather than creates it, and how each component changes the refrigerant state. Expect questions on the refrigeration cycle, reversing-valve operation, metering devices, accumulators, crankcase heaters, defrost hardware, and the relationship between outdoor temperature, capacity, and balance point.
Cycle and components
- The compressor raises pressure and temperature; the outdoor coil is the evaporator in heating and the condenser in cooling.
- The reversing valve changes refrigerant direction. In most residential systems the solenoid is energized in heating; losing the solenoid signal typically drives the valve to cooling.
- A bi-flow TXV or EEV maintains superheat in both modes; fixed-orifice systems use check valves and pistons and rely on indoor/outdoor coil matching.
- The accumulator protects the compressor from liquid return in heating; the crankcase heater keeps oil warm when the compressor is off.
- Heat-pump capacity falls as outdoor temperature falls, while building load rises. Their intersection is the balance point; below it, auxiliary heat makes up the difference.
Worked example
At 35°F outdoor, a home's heat loss is 18,000 Btu/h and the heat pump's published capacity is 14,000 Btu/h. The balance point is above 35°F, so auxiliary heat must supply 4,000 Btu/h at that condition. The exam may ask for the auxiliary kW: 4,000 ÷ 3,412 ≈ 1.17 kW of resistance heat, ignoring fan heat and defrost margin.
Traps
Do not assume the reversing valve is energized in cooling; verify the manufacturer's O/B logic. Do not apply cooling-mode superheat targets to a heating check. Do not confuse balance point with economic balance point, and remember defrost reduces net heating output.