Heat Pump Fundamentals: Reverse-Cycle Refrigeration and System Overview

What a Heat Pump Does

A heat pump is a refrigeration system that can move heat in two directions. In summer it works exactly like a central air conditioner, moving heat from inside the building to the outdoors. In winter it reverses that flow, extracting heat from outdoor air and delivering it inside. This ability to reverse direction is what makes the heat pump unique and why it is more energy-efficient than electric resistance heating in most climates.

The key principle behind all refrigeration and heat-pump operation is the same: heat always flows from a warmer region to a cooler region, and a refrigerant can be made to absorb or release large amounts of heat as it changes phase between liquid and vapor.

The Refrigerant Circuit: Major Components

A split-system heat pump shares most of its parts with a standard air conditioner but adds two critical components: the reversing valve and the accumulator.

Compressor — The heart of the system. It raises refrigerant vapor from low pressure to high pressure. High-pressure vapor is hot; compressing gas adds heat to it. The compressor must always receive vapor, never liquid refrigerant, or it will be damaged.

Reversing valve (four-way valve) — A solenoid-operated valve that changes the direction of refrigerant flow so the indoor and outdoor coils can swap roles. In cooling mode the solenoid is usually energized; in heating mode it is de-energized (or vice versa, depending on manufacturer). Always confirm the specific valve's wiring with the service manual before diagnosing.

Outdoor coil — In cooling mode this coil is the condenser: hot high-pressure refrigerant releases heat to outdoor air and condenses to liquid. In heating mode this coil becomes the evaporator: low-pressure refrigerant absorbs heat from cold outdoor air and boils into vapor.

Indoor coil — In cooling mode this coil is the evaporator: low-pressure refrigerant absorbs heat from indoor air and the air is cooled. In heating mode this coil becomes the condenser: high-pressure refrigerant releases heat to indoor air, warming the space.

Metering device — Reduces refrigerant pressure from high-side to low-side and controls the flow rate into the evaporator. Common types are the thermostatic expansion valve (TXV/TEV) and the fixed-bore orifice. Many heat pumps use a bi-flow TXV or two check-valve-and-orifice assemblies so metering works correctly in both modes.

Accumulator — Located on the suction line just before the compressor inlet. It holds any liquid refrigerant that did not fully evaporate and allows only vapor to enter the compressor. The accumulator is especially important in heat pump systems because low outdoor temperatures can reduce evaporator efficiency, increasing the risk of liquid slugging.

High-side and low-side lines — The discharge line carries hot, high-pressure vapor from the compressor. The suction line carries cool, low-pressure vapor back to the compressor. The liquid line carries high-pressure liquid from the condenser to the metering device.

Refrigerant Flow Direction in Each Mode

In cooling mode the flow path is: compressor → reversing valve → outdoor coil (condenser) → metering device → indoor coil (evaporator) → reversing valve → accumulator → compressor.

In heating mode the reversing valve shifts and the path becomes: compressor → reversing valve → indoor coil (condenser) → metering device → outdoor coil (evaporator) → reversing valve → accumulator → compressor.

When annotating a refrigerant circuit diagram, use arrows and clearly label each line as high-pressure or low-pressure in each mode. A common mistake is assuming the outdoor coil is always the condenser — in heating mode it is the evaporator.

Efficiency Ratings: COP and HSPF

Coefficient of Performance (COP) is the ratio of useful heat energy delivered to the electrical energy consumed at any given moment. A COP of 3 means the system delivers three units of heat energy for every one unit of electrical energy used. This is possible because the system is moving heat, not generating it. COP varies with outdoor temperature: as outdoor temperature falls, COP drops because there is less heat in the outdoor air to extract.

Heating Seasonal Performance Factor (HSPF) is a seasonal average efficiency rating for heat pumps in heating mode, calculated over an entire heating season under standardized conditions. It is expressed in BTU per watt-hour. Higher HSPF values mean greater seasonal efficiency. HSPF allows consumers and engineers to compare different heat pump models on a consistent basis. The current minimum federal standards and the threshold for ENERGY STAR recognition are updated periodically — always verify current requirements with the DOE or AHRI at the time of equipment selection.

Safety Points

Refrigerant handling requires EPA 608 certification. Never vent refrigerant to atmosphere — it is illegal under Section 608 of the Clean Air Act. Always recover refrigerant before opening the system.

Before servicing any electrical component including the reversing valve solenoid or compressor terminals, follow lockout/tagout (LOTO) procedures per OSHA standards. Verify the circuit is de-energized with a calibrated meter before touching terminals.

Wear appropriate PPE: safety glasses when working near refrigerant lines, insulated gloves when working near electrical components, and hearing protection near running compressors as required.

High-pressure refrigerant can cause freeze burns on contact with skin. Treat refrigerant lines under pressure with respect and use only rated hoses and fittings.