Parts of a Heat Pump System

A heat pump system moves heat between indoor and outdoor spaces to provide both heating and cooling with high efficiency. This article explains the parts of a heat pump system, how each component works, common configurations, troubleshooting tips, and maintenance best practices for homeowners and technicians.

Component	Primary Function	Location
Compressor	Circulates refrigerant and raises pressure/temperature	Outdoor Unit
Evaporator Coil	Absorbs heat during heating or cooling mode	Indoor Unit/Air Handler
Condenser Coil	Releases heat to outside or inside depending on mode	Outdoor/Indoor Unit
Expansion Device	Reduces refrigerant pressure to enable heat absorption	Line Set/Indoor Unit
Reversing Valve	Switches refrigerant flow to change mode	Outdoor Unit
Auxiliary/Emergency Heat	Provides supplemental heat when needed	Indoor Unit/Backup

Overview Of Heat Pump System Types And Basic Operation

Heat pumps come in air-source, ground-source (geothermal), and water-source variants. Air-source heat pumps are the most common in U.S. homes and use outdoor air as the heat exchange medium. Geothermal systems use buried loops and deliver higher efficiency but higher installation cost.

All heat pumps share the same thermodynamic cycle: the refrigerant evaporates at low pressure to absorb heat and condenses at high pressure to release heat. Components work together to move heat from the source to the sink, reversing direction when switching between heating and cooling.

Major Components Of A Heat Pump System

Compressor

The compressor is the system’s heart. It raises refrigerant pressure and temperature so the refrigerant can release heat at the condenser. Scroll, reciprocating, and rotary compressors are common types; scroll compressors dominate residential applications for reliability and efficiency.

Evaporator Coil

The evaporator coil allows refrigerant to absorb heat and evaporate. In heating mode the evaporator is the outdoor coil for air-source systems, while in cooling mode the indoor coil becomes the evaporator. Coil design, fin density, and airflow influence performance.

Condenser Coil

The condenser coil releases heat to the surrounding air or ground loop. In a split system the outdoor coil commonly acts as the condenser during cooling and as the evaporator during heating when the system reverses. Coil cleanliness affects heat transfer efficiency significantly.

Expansion Device (Metering Device)

The expansion device lowers refrigerant pressure so it can evaporate and absorb heat. Types include capillary tubes, thermostatic expansion valves (TXV), and electronic expansion valves (EEV). TXVs and EEVs provide better control and efficiency in variable-load conditions.

Reversing Valve

Unique to heat pumps, the reversing valve changes refrigerant flow direction to switch between heating and cooling. It is a solenoid-actuated valve typically located in the outdoor unit. Proper operation of the reversing valve is essential for reliable mode changes.

Air Handler And Indoor Unit

The indoor unit contains the evaporator coil (in cooling mode), blower, air filter, and often the auxiliary electric heat strips. The air handler manages airflow through ductwork and is a critical component for comfort and indoor air quality. Blower motor type (PSC vs ECM) affects energy use and controllability.

Ductwork And Registers

Ducts distribute conditioned air throughout the building. Properly sized and sealed ductwork minimizes losses and ensures balanced airflow. Registers and grilles control distribution; dampers may be installed for zoning. Duct insulation matters in unconditioned spaces to prevent thermal loss.

Thermostat And Controls

The thermostat governs setpoints and mode choices and communicates with the heat pump control board. Modern thermostats may support heat pump-specific features like balance points, auxiliary heat control, and compressor lockout. Smart thermostats can optimize schedules and monitor runtime for efficiency gains.

Refrigerant Lines And Service Ports

Two copper refrigerant lines (suction and liquid) connect indoor and outdoor units. Service ports allow technicians to measure pressures and add or recover refrigerant. Proper insulation of the suction line prevents loss of capacity and reduces condensation.

Accumulator And Filter-Drier

An accumulator in the suction line protects the compressor from liquid slugging by retaining excess liquid refrigerant. The filter-drier removes moisture and particulates that could damage components and degrade refrigerant performance. These components prolong system life and maintain reliability.

Auxiliary And Emergency Heat

Heat pumps use auxiliary heating—electric resistance strips or a gas furnace—when outdoor temperatures are too low or defrost cycles require supplemental heating. The thermostat switches to auxiliary heat when needed. Understanding when auxiliary heat engages helps control energy costs.

Auxiliary Subsystems And Optional Components

Defrost Control And Sensors

Air-source heat pumps use defrost cycles in cold weather to remove ice buildup from the outdoor coil. A defrost control board and temperature sensors determine when to reverse to cooling briefly or energize electric heat to shed frost. Efficient defrost strategies minimize runtime penalty.

Zoning Dampers And Modulating Equipment

Zoning systems use motorized dampers and multiple thermostats to control different areas independently. Modulating compressors and variable-speed fans match output to load, improving comfort and reducing short-cycling. Variable refrigerant flow (VRF) systems offer advanced multi-zone control without traditional ducts.

Indoor Air Quality Add-Ons

Optional components include electronic air cleaners, UV germicidal lights, humidifiers, and dehumidifiers. These devices integrate with the air handler to improve air quality and moisture control, which affects occupant health and comfort.

How Components Work Together Across Operating Modes

In heating mode the compressor compresses refrigerant, the outdoor coil becomes the evaporator absorbing outdoor heat, refrigerant returns warmer to the indoor coil which acts as the condenser releasing heat to the living space.

In cooling mode the reversing valve flips flow so the indoor coil is the evaporator absorbing indoor heat and the outdoor coil releases heat. The expansion device reduces pressure before the evaporator in both modes to enable heat absorption.

Common Failures And Troubleshooting Tips

Frequent issues include refrigerant leaks, compressor failures, frozen coils, malfunctioning reversing valves, and airflow restrictions. Symptoms often overlap so a systematic diagnostic approach is necessary.

• Low Refrigerant Causes reduced capacity and icing; check for leaks and pressure anomalies.
• Compressor Short Cycling Often due to improper charge, electrical faults, or control issues; prolongs wear.
• Frozen Outdoor Coil May indicate low refrigerant, poor airflow, or defrost control problems.
• Reversing Valve Stuck Prevents mode change; verify electrical control and replace valve when necessary.
• Poor Airflow From clogged filters, failing blower motors, or duct leaks; reduces efficiency and comfort.

Installation Considerations And Sizing

Correctly sizing a heat pump requires a Manual J load calculation to determine heating and cooling loads. Oversizing leads to short cycling and humidity control problems; undersizing fails to meet load and may rely on auxiliary heat frequently.

Proper refrigerant charge, line set length and pitch, electrical circuit sizing, and placement of the outdoor unit with adequate clearance affect long-term performance. Geothermal systems need ground loop design matched to soil conditions and climate.

Maintenance Best Practices

Regular maintenance preserves efficiency and extends equipment life. Key tasks include replacing air filters, cleaning coils, checking refrigerant charge, inspecting electrical connections, testing safety controls, and verifying defrost operation.

• Replace filters every 1–3 months depending on use and indoor air quality.
• Clean outdoor coil and remove debris seasonally to prevent capacity loss.
• Schedule annual professional tune-ups focusing on refrigerant, compressor, and control checks.
• Monitor thermostat operation and program schedules to avoid unnecessary auxiliary heat.

Energy Efficiency And Performance Metrics

Heat pump performance is measured by SEER (cooling efficiency) and HSPF (heating efficiency) for air-source systems. Higher SEER and HSPF values indicate better efficiency. For ground-source systems, COP (coefficient of performance) and EER may be quoted.

Smart controls, variable-speed compressors, and improved coil designs have increased real-world efficiency. Proper installation and matched components are as important as rated efficiency for actual energy savings.

Safety And Environmental Considerations

Refrigerants have evolved from high-global-warming-potential (GWP) compounds to lower-GWP options like R-410A alternatives. Local regulations may affect allowable refrigerants and handling practices.

Technicians should follow EPA Section 608 requirements for refrigerant recovery and handling. Homeowners should avoid DIY refrigerant work and contact certified professionals for leaks and major repairs.

Choosing Replacement Parts And Upgrades

When replacing components, matching refrigerant type, compressor capacity, metering device, and coil compatibility is critical for performance. Upgrading to variable-speed blowers or thermostats can improve comfort and reduce energy use without replacing the entire system.

Consider whole-system upgrades like adding zoning or switching to a heat pump-compatible furnace for hybrid systems. Rebates and incentives often offset upgrade costs and improve payback.

Key Takeaways And Practical Advice

Understanding the parts of a heat pump system helps identify issues early and communicate effectively with service technicians. Regular maintenance, proper sizing, and choosing compatible replacement parts maximize efficiency and lifespan.

For safety and compliance, refrigerant service should be performed by certified HVAC professionals. Homeowners can improve reliability by changing filters, keeping outdoor units clear, and using programmable thermostats to minimize unnecessary auxiliary heating.

Resources For Further Reading

Authoritative resources include the Air-Conditioning, Heating, and Refrigeration Institute (AHRI), U.S. Department of Energy (DOE) heat pump guidance, and EPA refrigerant regulations. Manufacturer installation manuals and local utility rebate pages provide product-specific and incentive information.