Heat Pump Terminology 101
Your heat pump terminology cheat sheet: A to Z terms explained in simple language. Understand components, efficiency metrics, and settings for smarter buying and better comfort.
A quick, skimmable A–Z of heat-pump terms for homeowners. Click a letter to jump, then tap “Back to top.”
A A
- Accumulator: Small tank in the suction line that protects the compressor from liquid refrigerant
- AHRI (Air-Conditioning, Heating & Refrigeration Institute): Independent body that certifies and lists equipment performance
- Air handler: Indoor unit with blower and coil that moves conditioned air through ducts
- Air-source heat pump (ASHP): System that moves heat between outdoor air and your home
- Automatic/adaptive defrost: Control logic that defrosts only when needed to save energy
- Auxiliary heat (AUX): Automatic electric or gas backup that kicks in when the heat pump can’t meet demand
B B
- Backup heat: Any secondary heat source used with a heat pump, such as heat strips or a furnace
- Balance point: Outdoor temperature where heat pump output equals the home’s heat loss
- Base pan heater: Electric heater under an outdoor unit to melt ice in cold climates
- Blower (ECM): Variable-speed indoor fan motor that adjusts airflow efficiently
- Branch box: Refrigerant distribution manifold for multi-zone mini-splits
- Brazing: High-temperature joining method used on copper refrigerant lines
- BTU (British Thermal Unit): Heating/cooling capacity measure; 12,000 BTU equals 1 ton
- Buffer tank: Small water tank that stabilizes temperatures in air-to-water systems
- Bivalent point: Temperature where a dual-fuel system switches from heat pump to furnace
C C
- Capacity: Amount of heat a unit can move, expressed in BTU/h or tons
- CEE tiers: Utility program efficiency tiers used for rebates and incentives
- CFM (Cubic Feet per Minute): Airflow rate through ducts and equipment
- Charge (refrigerant): The precise amount of refrigerant in a sealed system
- Clearances: Required space around equipment for airflow, service, and safety
- Coefficient of Performance (COP): Heating efficiency ratio of heat output to electrical input
- Cold-climate heat pump (CCHP): Unit designed to maintain capacity and efficiency in very low temperatures
- Compressor (scroll/rotary/inverter): The “pump” that raises refrigerant pressure and temperature
- Condensate drain: Tubing that removes water from the indoor coil during cooling or dehumidifying
- Condenser coil (outdoor coil): Coil that rejects or absorbs heat outside, depending on mode
- Crankcase heater: Warms compressor oil to prevent refrigerant migration when off
D D
- Defrost cycle: Temporary reversal in winter to melt frost off the outdoor coil
- Demand response: Utility signal that lets your system reduce load during peak hours
- Desuperheater: Heat-recovery loop that preheats domestic hot water from compressor waste heat
- Ducted mini-split: Mini-split system connected to small ductwork
- Ductless mini-split: Zoned system with wall, floor, or ceiling cassettes and no ducts
- Dual fuel: Heat pump paired with a gas furnace for backup or extreme cold
E E
- EEV (Electronic Expansion Valve): Precise, motor-driven metering device for refrigerant flow
- EER (Energy Efficiency Ratio): Cooling efficiency at a single test condition
- Electric resistance heat: Simple electric backup heat using heat strips
- Emergency heat (EM heat): Manual mode that forces backup heat only
- Energy Star: U.S./Canada label for products meeting high efficiency standards
- Expansion valve (TXV/EEV): Device that drops refrigerant pressure before the evaporator coil
- External static pressure (ESP): Resistance to airflow from ducts and components
F F
- Fan coil: Indoor coil and fan assembly for hydronic or refrigerant systems
- Filter (MERV rating): Air filter that protects equipment and indoor air quality
- Flare fitting: Mechanical connection on line sets, common in mini-splits
- Frost sensor: Sensor that triggers defrost to protect the outdoor coil
G G
- Glide (refrigerant temperature glide): Temperature range during phase change in blended refrigerants
- Ground-source heat pump (GSHP): Uses buried loops or wells for stable ground temperatures
H H
- Heat exchanger: Surface where refrigerant and air or water trade heat
- Heat pump water heater (HPWH): Uses a small heat pump to heat domestic hot water
- Heat strip (heat kit): Electric resistance elements installed in the air handler
- HRV/ERV (Heat/Energy Recovery Ventilator): Ventilation devices that exchange heat and, for ERV, moisture
- HSPF2 (Heating Seasonal Performance Factor 2): Newer seasonal heating efficiency metric for air-source heat pumps
- Humidity control: Ability to remove moisture in cooling or use reheat strategies
- Hyper-heat/low-ambient: Maker terms for models that keep strong capacity in sub-freezing temps
I I
- Indoor unit (head): The room-side unit in a ductless system
- Intelligent defrost: Algorithm that reduces unnecessary defrost cycles
- Inverter-driven compressor: Variable-speed compressor for comfort and efficiency
K K
- kWh (kilowatt-hour): Unit for measuring electrical energy use on your utility bill
L L
- Latent heat: Moisture removal part of cooling; separate from sensible change
- Line set: Insulated copper lines carrying refrigerant between indoor and outdoor units
- Load calculation: Engineering estimate of a home’s heating and cooling needs
- Lockout temperature: Set temperature where backup heat is enabled or disabled
- Low ambient kit: Accessories that allow cooling or reliable operation in cold outdoor temps
M M
- Manual D: ACCA method for designing duct systems
- Manual J: ACCA method for residential load calculations
- Manual S: ACCA method for proper equipment selection
- MERV (Minimum Efficiency Reporting Value): Filter rating scale from 1 to 16
- Micron gauge: Tool that verifies deep vacuum quality during install/service
- Microchannel coil: Thin, all-aluminum coil design with low refrigerant volume
- Mini-split: Modular heat pump system serving one or more zones
- Monobloc: All-in-one outdoor air-to-water heat pump unit
- Multi-split: One outdoor unit serving several indoor heads or zones
N N
- NEEP Cold Climate list: Regional list of heat pumps that meet cold-climate criteria
- Nitrogen purge: Flow of nitrogen during brazing to prevent oxidation
- Noise rating (dBA): Sound level specification for indoor or outdoor units
O O
- O/B reversing valve wire: Thermostat signal that switches between heat and cool
- Outdoor reset (weather compensation): Control that adjusts output based on outdoor temperature
- Outdoor temperature sensor: Sensor used for defrost, lockouts, and efficiency controls
- Outdoor unit: Weather-exposed section containing the compressor and coil
- Oversizing: Choosing equipment with too much capacity, causing short cycling and discomfort
P P
- Plenum: Main supply or return air chamber attached to the air handler
- Pressure test: Nitrogen pressurization step to verify leak-free refrigerant piping
Q Q
- Quick-connect line set: Pre-charged or mechanical connections marketed for simplified installs
R R
- R-32: Lower-GWP refrigerant increasingly used in residential heat pumps
- R-410A: Common residential refrigerant being phased down due to higher GWP
- R-454B: Newer lower-GWP replacement for R-410A in many systems
- Radiant floor (emitter): Hydronic floor system paired with air-to-water heat pumps
- Reheat dehumidification: Method that cools then reheats air to remove moisture without over-cooling rooms
- Refrigerant: Working fluid that absorbs/releases heat as it changes state
- Reversing valve: Four-way valve that switches the system between heating and cooling
- Return air: Air pulled back to the air handler to be conditioned
S S
- SCOP (Seasonal COP): Seasonal efficiency metric common outside North America
- SEER2 (Seasonal Energy Efficiency Ratio 2): Updated U.S./Canada seasonal cooling efficiency metric
- Sensible heat ratio (SHR): Share of cooling that lowers temperature versus removing moisture
- Setback strategy: Thermostat scheduling that changes setpoints when you’re away or asleep
- Short cycling: Rapid on/off operation caused by oversizing or control issues
- Sizing (right-sized): Matching equipment capacity to the home’s load
- Snow hood: Shield that keeps outdoor coils clear in snowy sites
- Split system: Standard setup with separate indoor and outdoor sections
- Static pressure: Resistance to airflow in ducts; critical for proper airflow
- Subcooling: Liquid refrigerant temperature below saturation for reliable feeding of the valve
- Superheat: Vapor refrigerant temperature above saturation to protect the compressor
- Supply air: Air leaving the air handler headed to rooms
T T
- Thermostat (heat pump): Control that properly handles O/B, AUX, and staging for heat pumps
- Time-of-use rates (TOU): Utility pricing that varies by time; useful for shifting operation
- Ton: 12,000 BTU/h of cooling or heating capacity
- Two-stage/variable-speed: Equipment that modulates output for comfort and efficiency
U U
- UL/CSA: Safety certification marks for electrical/mechanical equipment
- Utility rebate: Cash or bill credits offered for qualifying high-efficiency installations
V V
- Variable-speed drive (VFD): Electronics that control motor speed in compressors or fans
- Vapor injection: Advanced compressor tech that boosts cold-weather heating capacity
- Ventilation (ERV/HRV): Fresh-air systems that pair well with tight, efficient homes
- VRF/VRV (Variable Refrigerant Flow/Volume): Multi-zone systems with advanced controls
W W
- Water-source heat pump (WSHP): Uses a water loop, well, or lake for the heat source/sink
- Weather compensation: See outdoor reset; adjusts output to outdoor conditions
- Wind baffle: Accessory that stabilizes airflow across the outdoor coil in gusty locations
Y Y
- Y1/Y2 calls: Thermostat signals that request first or second stage of cooling/heating
Z Z
- Zone control: Dampers and controls that split a ducted home into independent zones
- Zoning (ductless): Using multiple indoor heads so each room has its own setpoint
References:
- U.S. Department of Energy: Heat Pump Systems
- ENERGY STAR: Air-Source Heat Pumps
- NREL: Cold-Climate Heat Pumps
- Mitsubishi Electric: Hyper-Heating (H2i) Overview
- Daikin: Inverter Technology
Conclusion:
This glossary is heat pump specific and intentionally overlaps with our full HVAC glossary. Use them together: this page for heat-pump terms, the full glossary for broader HVAC lingo. If we missed a term you need, please let us know and we’ll add it.
- View the Full HVAC Glossary
- Browse all Heat Pump Guides on DIY Home Comfort
Scott Harding
Scott is the main author of DIY Home Comfort. He's also an experienced HVAC technician that enjoys home renovation and spending time with his family. You can find out more about him here.
