What’s Inside a Heat Pump?
Heat pumps all share the same core idea: move heat from one place to another using a refrigeration cycle.
This article shows what’s inside a heat pump, what each component does, and where it lives in different system types:
- Air Source Heat Pumps (ASHP – air-to-water)
- Ground Source Heat Pumps (GSHP)
- Air-to-Air Heat Pumps
- VRF / VRV multi-split systems
These simplified diagrams show where the main components are located in different heat pump system types.
Air Source Heat Pump (Air-to-Water)
Ground Source Heat Pump (GSHP)
Air-to-Air Heat Pump (Split System)
VRF / VRV Multi-Split System
Every heat pump – ASHP, GSHP, air-to-air and VRF – is built around the same core refrigeration loop.
CompressorEngine
Technical: Increases refrigerant pressure and temperature, providing the driving force that pushes refrigerant around the circuit. In inverter systems it modulates speed to match the load.
Simple: The compressor is the “engine” of the heat pump. It squeezes the refrigerant and makes it hot enough to heat your home.
Location: ASHP: outdoor (monobloc) or indoor/outdoor (split) • GSHP: indoor cabinet • Air-to-air: outdoor unit • VRF/VRV: outdoor condensing unit.
Evaporator CoilHeat Collector
Technical: Low-pressure refrigerant absorbs heat and boils in the evaporator. This is the “source” side: air, ground or water supplies energy to the refrigerant.
Simple: The evaporator is where the heat pump “collects” heat from outside air or the ground.
Location: ASHP & air-to-air: outdoor coil • GSHP: internal heat exchanger connected to brine loop • VRF/VRV: outdoor coil in heating mode, indoor coil in cooling mode.
Condenser Coil / Plate Heat ExchangerHeat Giver
Technical: Hot, high-pressure refrigerant rejects heat and condenses back to liquid. This can be an air coil (air-to-air) or a plate heat exchanger transferring heat to water (air-to-water / GSHP).
Simple: The condenser is where the heat pump “gives” heat to your radiators, underfloor heating, hot water or indoor air.
Location: ASHP monobloc: outdoors; ASHP split: indoor hydrobox; GSHP: inside cabinet; Air-to-air: indoor fan coil; VRF/VRV: each indoor unit has its own coil.
Expansion Valve (TEV / EEV)Pressure Drop
Technical: Creates a pressure drop between condenser and evaporator. Controls superheat and refrigerant flow so the evaporator operates efficiently.
Simple: A clever valve that lets the refrigerant expand and go cold again, ready to pick up more heat.
Location: ASHP: indoors or outdoors depending on design • GSHP: inside cabinet • Air-to-air: usually in outdoor unit • VRF/VRV: multiple electronic expansion valves (one per indoor unit or branch).
Reversing Valve (4-Way Valve)Mode Switch
Technical: Changes the direction of refrigerant flow so the system can operate in heating or cooling. Often controlled by a coil energised by the PCB.
Simple: Like a railway junction – it sends the hot and cold sides one way for heating, the other way for cooling.
Location: ASHP & air-to-air & VRF/VRV: outdoor unit • GSHP: inside indoor cabinet.
AccumulatorCompressor Protector
Technical: A vessel on the suction line that separates liquid from gas, ensuring mostly vapour enters the compressor and preventing liquid slugging.
Simple: A safety bottle that catches unwanted liquid so it doesn’t smash the compressor.
Location: In the suction line close to the compressor, normally in the same outdoor or indoor unit as the compressor.
ReceiverRefrigerant Store
Technical: Stores liquid refrigerant on the high-pressure side to stabilise charge and allow varying load and mode without starvation.
Simple: A storage tank that holds spare refrigerant so the system always has the right amount available.
Location: Common in larger or VRF systems, usually in the outdoor unit on the liquid line.
These components only appear on systems that heat water: air-to-water ASHPs and GSHPs.
Primary Circulation PumpHeating Water Pump
Technical: Circulates heating water between the heat pump and the distribution system (radiators, underfloor, buffer tank). Ensures correct flow rate for heat transfer and defrost.
Simple: The main water pump that pushes hot water around your heating system.
Location: ASHP monobloc: often integrated in outdoor unit • ASHP split: in indoor hydrobox • GSHP: in indoor cabinet or mounted nearby.
Ground Loop Pump(s)Brine Pump
Technical: Pumps antifreeze solution (brine) through horizontal loops or vertical boreholes to collect low-grade heat from the ground.
Simple: A pump that sends fluid out into the ground loops to pick up heat and bring it back to the heat pump.
Location: Indoors, near the GSHP; may be built into the unit or in a separate manifold station.
3-Way Diverting ValveDHW vs Heating
Technical: Motorised valve that directs the hot water from the heat pump either to the space-heating circuit or to the domestic hot water (DHW) cylinder coil.
Simple: A traffic director for hot water: it decides whether the heat pump is topping up the cylinder or heating the house.
Location: Usually inside the indoor hydrobox or mounted close to the cylinder on ASHP/GSHP systems.
Filters & StrainersSystem Protection
Technical: Mesh or magnetic devices on the water side that capture debris and sludge, protecting pumps, valves and plate heat exchangers from blockage and wear.
Simple: Filters that catch dirt and sludge so they don’t block the heat pump or pipes.
Location: In the heating circuit, typically on the return line to the heat pump or just before sensitive components.
Expansion Vessel (Water Side)Pressure Buffer
Technical: A sealed vessel with an air cushion that absorbs changes in water volume as it heats and cools, preventing excessive pressure rise.
Simple: A springy tank that stops the system pressure from going too high when the water heats up.
Location: On the water circuit, often inside the hydrobox or mounted on a nearby wall.
DHW Cylinder & Coil / Plate Heat ExchangerHot Water Store
Technical: Insulated cylinder that stores domestic hot water. Heat is transferred from the heat pump via an internal coil or dedicated plate heat exchanger with pump set.
Simple: The hot water tank that holds your showers, baths and taps water, warmed by the heat pump instead of (or alongside) a boiler.
Location: Always indoors, usually near the hydrobox or GSHP unit.
Backup / Immersion HeaterBackup Heat
Technical: Electric element used for legionella pasteurisation, emergency backup or short-term peak load, controlled by the system PCB.
Simple: An electric heater inside the cylinder or unit that can top up the temperature if needed.
Location: Often inside the cylinder and/or integrated into the hydrobox; occasionally in the outdoor unit on some monoblocs.
Air-to-air and VRF systems deliver heat directly to air, and some air-to-water systems use fan coils indoors.
Indoor Fan Coil UnitIndoor Heater/Cooler
Technical: Contains the indoor heat exchanger (coil), fan and air filters. In heating mode it acts as the condenser (air-to-air) or as a local terminal unit in VRF.
Simple: The indoor “head” that blows warm (or cool) air into the room.
Location: Indoors – wall-mounted, ducted, cassette or floor console. Each VRF indoor unit serves a separate zone.
Outdoor Fan & Coil AssemblyOutdoor Heat Exchanger
Technical: The outdoor coil plus fan draw air across the refrigerant heat exchanger, enabling evaporation (heat pickup) or condensation (heat rejection) depending on mode.
Simple: The big fan and grille outside that either takes heat from the air or dumps heat into it.
Location: Outdoor unit for ASHP, air-to-air and VRF/VRV systems.
Air FiltersAir Quality
Technical: Mesh or fine filters mounted in indoor fan coils that capture dust and particles, protecting the coil and improving indoor air quality.
Simple: The washable filters behind the front cover that catch dust and keep the unit clean.
Location: Inside indoor fan coil units; some outdoor units also have guards or coarse filters.
These components don’t move heat directly, but they decide how the system behaves, and they keep it safe.
Main Controller PCBBrain
Technical: Electronic control board that runs algorithms for compressor speed, expansion valve position, defrost cycles, pump control, weather compensation and safety logic.
Simple: The brain of the heat pump – it reads all the sensors and tells every component what to do.
Location: Inside the outdoor unit on monoblocs and air-to-air; inside indoor cabinet/hydrobox on splits and GSHP; in VRF, often multiple PCBs in the outdoor and each indoor unit.
Room & Weather ControllerUser Interface
Technical: Wall-mounted or integrated controller allowing setpoint adjustment, schedules, modes, curves and diagnostics. Communicates with main PCB via bus.
Simple: The control panel or thermostat where the user sets temperatures and times.
Location: Indoors, typically in a representative room or near the plant area.
Temperature SensorsThermistors
Technical: NTC or similar devices measuring outdoor air, indoor air, flow/return water, refrigerant suction/discharge and defrost-related coil temperatures.
Simple: Tiny electronic thermometers that tell the system how hot or cold everything is.
Location: Distributed across coils, pipes and casing – outdoor coil sensors, indoor water sensors, cylinder sensors, room sensors and more.
Pressure Sensors & SwitchesSafety Limits
Technical: Transducers and high/low pressure switches monitoring refrigerant pressures. They provide feedback for control and trigger lockouts if safe limits are exceeded.
Simple: Devices that check if the refrigerant pressure is too high or too low and shut things down if there’s a problem.
Location: On the refrigerant lines inside the heat pump casing, usually near compressor and heat exchangers.
Flow Switches & Flow SensorsWater Safety
Technical: Confirm water circulation through the heat exchanger. If flow is inadequate, the system will reduce output or lock out to prevent freezing or overheating.
Simple: Check that water is actually moving before the heat pump starts working hard.
Location: In the water circuit close to the heat pump connections.
Water-Side Safety ValvesPressure Relief
Technical: Spring-loaded safety valves (typically 3 bar for heating, 6 bar for DHW) that discharge water if pressure rises above safe limits.
Simple: Last-resort valves that let water escape if the pressure gets dangerously high.
Location: On heating circuit near the heat pump and on the DHW cylinder, usually with tundish and discharge pipework.
This table summarises where the core components are located in different types of heat pump systems.
| Component | ASHP (Air-to-Water) | GSHP | Air-to-Air | VRF / VRV |
|---|---|---|---|---|
| Compressor | Outdoor (monobloc) or indoor/outdoor (split) | Indoor cabinet | Outdoor unit | Outdoor unit (often multiple) |
| Evaporator | Outdoor coil | Indoor HX, connected to ground loop | Outdoor coil | Outdoor coil in heating (indoor in cooling) |
| Condenser / Plate HX | Outdoor (monobloc) or indoor (split) | Inside unit | Indoor fan coil | Each indoor fan coil |
| Expansion Valve | Indoor or outdoor | Inside unit | Usually outdoor | Multiple EEVs (per indoor/branch) |
| Circulation Pump | Yes – heating water | Yes – heating water & brine | No (refrigerant only) | No (air terminals) |
| Ground Loop Pump | Not used | Yes – brine pump | Not used | Not used |
| Outdoor Fan | Yes | No (ground source) | Yes | Yes |
| DHW Integration | Yes – via cylinder / PHE | Yes – via cylinder / PHE | No (air only) | Typically no, unless hybrid system |
- Where those parts are physically located (inside vs outside vs plant room)
- Whether the system gives heat to water (ASHP/GSHP) or directly to air (air-to-air/VRF)
- Whether there are one or many indoor units (single split vs VRF)