English:Heat Pumps
Heat Pumps
Introduction
Heat Pumps is a vocational aiMOOC for apprentices, trainees, and vocational students who work with HVAC, refrigeration, plumbing, electrical systems, building services, or energy technology. You will learn how heat pumps move thermal energy, how the refrigeration cycle works, how different systems are selected and integrated, and how safe installation, commissioning, maintenance, and fault finding are carried out in professional practice.
A heat pump does not normally create all of its useful heat directly from electricity. It uses electrical energy to operate a refrigeration cycle that moves heat from a lower-temperature source to a higher-temperature sink. The same basic technology is used in refrigerators and air conditioners. Many heat pumps are reversible, so the same system can provide heating in winter and cooling in summer.
By the end of this course, you should be able to explain the main components of a heat pump, compare common heat-pump types, calculate and interpret simple performance values, identify important design and installation checks, apply safe working principles, and reason through basic commissioning and fault-finding situations.
| Vocational learning goal | What you should be able to do |
|---|---|
| System understanding | Trace the refrigerant cycle and explain the function of the compressor, heat exchangers, expansion device, controls, and reversing valve. |
| Design awareness | Relate building heat loss, source temperature, emitter temperature, airflow or water flow, and system sizing to performance. |
| Practical work | Plan installation and commissioning checks using manufacturer instructions, local regulations, and safe working procedures. |
| Diagnostics | Use measured temperatures, flow conditions, control status, operating history, and other approved service data to investigate faults logically. |
| Sustainability | Explain how electricity supply, refrigerant choice, building efficiency, and correct operation affect environmental performance. |
Heat Pump Fundamentals
Moving Heat Instead of Creating It
Heat naturally flows from a warmer region to a cooler region. A heat pump uses work, usually supplied by an electric compressor, to move heat in the opposite useful direction. In heating mode, the system extracts low-temperature heat from outdoor air, the ground, water, ventilation exhaust, or another source and delivers it to the building.
This is why the useful heat delivered by a heat pump can be several times greater than the electrical energy used by the compressor and auxiliaries. The International Energy Agency explains that a typical household heat pump can have a coefficient of performance around four under suitable conditions. This does not violate energy conservation: part of the delivered heat comes from the environment, while electricity supplies the work needed to move and upgrade that heat.
The Refrigeration Cycle
Most building heat pumps use a vapour-compression refrigeration cycle. The refrigerant repeatedly changes pressure, temperature, and often phase as it circulates through the system.
| Stage | Main component | What happens | Vocational point |
|---|---|---|---|
| Compression | Compressor | Low-pressure refrigerant vapour is compressed to a higher pressure and temperature. | Compressor current, speed, sound, discharge conditions, and control signals can provide useful diagnostic information. |
| Heat rejection | Condenser | The hot refrigerant transfers useful heat to indoor air or heating water and condenses. | Adequate airflow or water flow is essential for stable heat transfer. |
| Expansion | Expansion device | Refrigerant pressure drops as flow is metered into the low-pressure side. | Electronic or thermostatic expansion devices must be assessed according to the manufacturer's service procedure. |
| Heat absorption | Evaporator | The cold refrigerant absorbs heat from the source and evaporates. | Restricted airflow, poor source flow, ice, dirt, or low source temperature can reduce capacity. |
In a reversible air-source heat pump, a reversing valve changes which coil acts as the evaporator and which acts as the condenser. The labels outdoor coil and indoor coil are therefore often clearer than assuming one physical coil always has one thermodynamic role.
Pressure, Temperature, and Phase
Refrigeration technicians connect pressure and temperature because refrigerant saturation temperature depends on pressure. The refrigerant must be cold enough at the evaporator to absorb heat from the source and hot enough at the condenser to release heat to the sink. The compressor creates the pressure difference that makes these temperature levels possible.
Superheat describes vapour temperature above its saturation temperature at the same pressure. Subcooling describes liquid temperature below its saturation temperature at the same pressure. These quantities can support commissioning and diagnosis, but correct targets depend on system design, operating mode, refrigerant, electronic controls, and manufacturer instructions. Do not use a generic target value when the manufacturer provides a specific procedure.
Coefficient of Performance
For heating, a simple instantaneous coefficient of performance can be written as:
COP = useful heating output divided by electrical input
If a heat pump delivers 6 kW of heat while using 2 kW of electrical power, its COP at that operating point is 3.0. COP changes with source temperature, sink temperature, compressor speed, defrost operation, fan and pump power, and other conditions. A single test-point COP is therefore not the same as seasonal performance.
SCOP is a seasonal coefficient of performance used in some rating systems. SPF or seasonal performance factor is often used for measured or system-level seasonal performance. Cooling rating terms vary by region, so always identify the standard behind a label before comparing equipment.
Why Temperature Lift Matters
The temperature lift is the difference between the source temperature and the temperature required at the heat sink. A larger lift generally makes the compressor work harder and reduces COP. This is one reason why good building insulation, generous heat emitters, well-designed ductwork, and low heating-water temperatures can improve real-world performance.
For a hydronic system, lowering the required flow temperature can be highly valuable. An oversized radiator or underfloor-heating circuit may deliver the required room heat at a lower water temperature than a small high-temperature emitter. The correct design is based on room-by-room heat loss, emitter output data, and the selected heat pump's capacity and performance curves.
Main Heat Pump Types
Air-to-Air Heat Pumps
An air-to-air heat pump transfers heat between outdoor air and indoor air. Ductless mini-splits use one or more indoor fan-coil units. Ducted systems use an air handler and duct network. Many provide both heating and cooling.
Air-source systems are widely used because the heat source is readily available and no ground loop is needed. Their capacity and efficiency vary with outdoor temperature, and the outdoor coil may need defrosting in cold, humid weather.
Air-to-Water Heat Pumps
An air-to-water heat pump extracts heat from outdoor air and transfers it to a water circuit. The heated water can serve underfloor heating, fan coils, suitably sized radiators, buffer or thermal stores where required by the design, and sometimes domestic hot-water production through an appropriate cylinder or heat exchanger.
System performance depends strongly on required water temperature and hydraulic design. The highest possible water temperature is not automatically the best operating point. Weather-compensated control can reduce flow temperature when outdoor conditions allow, helping the heat pump operate more efficiently.
Ground-Source Heat Pumps
Ground-source systems exchange heat with the ground through buried horizontal collectors, vertical boreholes, or other approved ground-loop arrangements. The ground often provides a more stable source temperature than outdoor air, which can support steady winter performance. Installation usually requires more site work and higher initial cost than a simple air-source system.
The ground circuit is commonly a closed loop containing water and an approved antifreeze mixture, often called brine in the trade. Ground-loop design must consider ground conditions, borehole or trench design, fluid properties, pump energy, pressure drop, environmental rules, and long-term heat balance.
Water-Source and Other Heat Pumps
Water-source heat pumps use a water body, groundwater system, shared water loop, or other approved water source or sink. Some buildings use exhaust-air heat pumps that recover heat from ventilation exhaust. Large heat pumps can use industrial waste heat, wastewater, data-centre heat, or district-energy networks.
These applications use the same thermodynamic principles but require different source-side engineering, permissions, corrosion or fouling control, pumping arrangements, and monitoring.
Comparing System Types
| Type | Typical source | Typical heat delivery | Main strengths | Main design concerns |
|---|---|---|---|---|
| Air-to-air | Outdoor air | Indoor air | Heating and cooling, simple retrofit options, zoning | Outdoor-temperature effects, airflow, defrost, condensate, noise |
| Air-to-water | Outdoor air | Hydronic water | Works with floor heating, fan coils, radiators, and hot-water systems | Flow temperature, emitter sizing, hydraulics, defrost, cylinder design |
| Ground-source | Ground loop | Air or hydronic water | Stable source temperature and strong seasonal potential | Ground works, loop sizing, pumping energy, capital cost |
| Water-source | Water or shared loop | Air or hydronic water | Can use stable water temperatures or recovered heat | Water quality, pumping, permissions, fouling, source availability |
Components and Controls
Outdoor Unit and Refrigerant Circuit
A modern outdoor unit can contain the compressor, fan, outdoor heat exchanger, electronic expansion device, four-way reversing valve, sensors, power electronics, control board, accumulators or separators, and service connections. Layout varies by manufacturer and model.
The photograph shows why professional servicing requires both refrigeration and electrical competence. Components are tightly integrated, and live electrical parts, rotating fans, hot surfaces, cold surfaces, and pressurised refrigerant may be present.
Compressors and Inverters
Many current heat pumps use variable-speed compressors driven by inverters. Modulating capacity can reduce on-off cycling, improve comfort, and let the heat pump follow changing building load. However, a variable-speed system still needs correct sizing, airflow or water flow, sensors, controls, and installation.
Common compressor types include scroll, rotary, and reciprocating designs. The correct service method depends on the equipment. A compressor should never be diagnosed from one symptom alone: control demand, supply voltage, inverter status, temperatures, pressures where authorised, and system flow conditions must be considered together.
Heat Exchangers
The indoor and outdoor heat exchangers transfer thermal energy between refrigerant and air or water. Heat-transfer performance drops when air coils are dirty, airflow is blocked, water flow is too low, strainers are blocked, circuits contain air, or scale and fouling develop.
A useful diagnostic habit is to ask two questions: Is enough fluid moving through the heat exchanger? and Is the temperature difference consistent with the measured load and operating condition? These questions apply to both air and water systems.
Expansion Devices
The expansion device controls refrigerant flow and creates the pressure drop between high and low sides. Electronic expansion valves can be controlled from multiple sensors and operating maps. A valve position displayed by software is not proof that the physical refrigerant flow is correct, so diagnosis must combine control data with approved measurements.
Reversing Valves and Defrost
A reversible heat pump uses a four-way valve to redirect refrigerant flow. In heating mode, frost can build on an outdoor air coil when its surface is below freezing and moisture is present. The control system may start a defrost cycle, temporarily changing operation so heat melts the frost.
Some steam or water near the outdoor unit during or after defrost can be normal. Heavy persistent ice, blocked drainage, a failed fan, sensor faults, insufficient airflow, or control problems require investigation. Never chip ice from a coil in a way that can damage fins or refrigerant tubing.
Sensors and Control Logic
Heat pumps commonly monitor outdoor temperature, coil temperature, refrigerant temperature, water flow and return temperature, room demand, pressure or pressure-derived values, compressor speed, and protective conditions. Modern controls may include weather compensation, zone control, demand response, backup-heat logic, domestic-hot-water priority, and remote monitoring.
Good fault finding begins by understanding what the controller is trying to do. Read the operating mode, demand, setpoints, sensor values, alarms, and recent history before changing settings.
Building and System Design
Heat Loss and Sizing
Correct sizing starts with a building heating-load calculation at the local design condition. The calculation should account for transmission through the building envelope, ventilation and infiltration, indoor design temperatures, and other relevant loads. Room-by-room heat loss is especially useful when checking radiator, underfloor, fan-coil, or air-distribution capacity.
Oversizing can increase cost and may cause poor low-load operation. Undersizing can lead to inadequate comfort or excessive reliance on auxiliary heat. The selected heat pump must be checked against manufacturer capacity data at the expected source and sink temperatures, not only against a nominal catalogue rating.
Building Fabric First
Reducing heat loss through insulation, airtightness work, and suitable glazing can reduce the required heating capacity and allow lower emitter temperatures. This can improve comfort and reduce operating energy. Building upgrades and heat-pump design should therefore be considered together rather than as unrelated projects.
Hydronic Emitter Design
For air-to-water and ground-to-water systems, the heat pump must be matched to the heating-water circuit. Important factors include design flow temperature, return temperature, water flow, emitter output, pipe sizing, pump head, balancing, system volume, air removal, expansion provision, water quality, and freeze protection where relevant.
A basic hydronic heat-output estimate is:
Heat output in kW ≈ mass flow of water in kg/s × 4.18 kJ/kgK × temperature difference in K
For example, 0.10 kg/s of water with a 5 K temperature drop transfers about 2.09 kW. This simple relationship is useful for sense-checking measured flow and temperature difference. Real measurements have uncertainty, and glycol mixtures have different properties from pure water.
Weather Compensation
Weather compensation adjusts heating-water temperature in response to outdoor temperature. When weather is mild, the system can often use a lower flow temperature. When outdoor conditions become colder, the controller raises the target according to a heating curve.
The best curve is not simply the hottest curve. It should deliver stable comfort at the lowest practical water temperature for the building and emitters. Large manual temperature setbacks and frequent thermostat interruptions can sometimes conflict with efficient steady heat-pump operation, depending on the building and control strategy.
Airflow and Duct Design
Air-to-air and ducted heat pumps depend on correct airflow. Filters, coil cleanliness, fan speed, duct resistance, register position, zoning dampers, and indoor-unit placement all affect capacity, noise, and comfort. Low airflow can create abnormal coil temperatures and protective shutdowns.
Do not judge a ducted heat pump only by supply-air temperature. Heat output depends on both temperature rise and airflow. A moderate air-temperature rise with adequate airflow can provide the required room heat efficiently.
Thermal Storage and System Integration
Thermal storage can be useful in some systems for hydraulic separation, minimum system volume, domestic hot water, load shifting, or integration with other heat sources. It is not automatically required in every installation, and unnecessary storage can add standing losses or complexity.
A designer should identify the purpose of each buffer, cylinder, low-loss header, plate heat exchanger, pump, valve, and control signal. Every added component should solve a defined hydraulic, control, comfort, or energy-management requirement.
Installation Practice
Site Survey
Before installation, confirm the design load, equipment selection, available electrical supply, distribution system, source conditions, access, drainage, noise constraints, structural support, service clearances, pipe and cable routes, and local permissions. Check manufacturer documentation before deciding final unit positions.
For outdoor air-source units, avoid locations where discharge air can recirculate directly back into the coil. Keep required clearances, protect service access, and plan for condensate and defrost water so it does not create ice, dampness, or a slip hazard.
Mounting and Mechanical Installation
Units must be securely supported, level where required, protected from avoidable vibration, and positioned for safe service access. Pipework and cables should be supported without transmitting damaging strain to equipment connections.
Wall penetrations should be weather-sealed and arranged so water cannot track into the building. Refrigerant lines and hydronic pipes require suitable insulation, physical protection, and correct routing. Follow the manufacturer's limits for pipe size, length, height difference, oil-return requirements, bends, and additional charge.
Refrigerant Circuit Work
Opening, joining, evacuating, recovering, and charging refrigerant circuits may be legally restricted to trained or certified personnel. Use only approved tools, recovery equipment, leak-testing methods, vacuum procedures, and refrigerant identified by the manufacturer and local rules.
Moisture, air, dirt, incompatible oils, incorrect refrigerant mass, and poor joints can damage performance and reliability. A sealed system should therefore be kept clean and dry during installation. Pressure testing and evacuation must follow the manufacturer's procedure and applicable safety standards.
Never use oxygen to pressure-test a refrigeration circuit. Refrigerants and oils may present pressure, frostbite, asphyxiation, decomposition, and fire hazards. Some modern refrigerants are mildly or highly flammable, so ignition control, ventilation, charge limits, leak procedures, and equipment suitability must be taken seriously.
Electrical Installation
Heat pumps can contain mains voltage, high-current compressor circuits, inverter electronics, crankcase heaters, pumps, fans, control wiring, and communication buses. Electrical work must comply with local rules and manufacturer requirements for isolation, protective devices, cable sizing, earthing, polarity, phase arrangement where applicable, and electromagnetic compatibility.
Inverter equipment can retain hazardous DC voltage after power is isolated. Observe the manufacturer's stated discharge time and verification method. Lockout and tagout procedures should be used where required by the workplace.
Hydronic Installation
Flush and prepare hydronic circuits according to the equipment and local water-quality requirements. Provide correct expansion accommodation, pressure control, air separation, filling arrangements, strainers or dirt separation where specified, and freeze protection where needed.
Flow switches and minimum-flow requirements protect the heat pump. A bypass, buffer, low-loss header, or hydraulic separator may be required in some designs, but these components must be selected for a defined purpose and commissioned correctly.
Condensate and Drainage
Indoor cooling coils produce condensate, and outdoor coils can produce substantial water during heating and defrost. Drains need the correct fall, trapping where required, insulation or frost protection where appropriate, and a safe discharge point. A drainage fault can cause water damage, hygiene problems, nuisance icing, or system shutdown.
Workplace Safety
Main Hazards
Heat-pump work combines hazards from electricity, pressure, refrigerants, hot and cold surfaces, rotating fans, lifting, work at height, sharp sheet metal, drilling, brazing or hot work where used, and water systems. Your risk assessment should match the actual task and equipment.
Before removing panels or beginning service work, identify all energy sources and follow the workplace isolation procedure. Do not bypass protective devices or interlocks to force operation outside an approved manufacturer service method.
Use appropriate personal protective equipment and ventilation for the task. Refrigerant contact can cause cold burns, and a large leak in a confined space can displace oxygen. Flammable refrigerants require additional controls. When a procedure is outside your training, legal authorisation, or employer scope, stop and involve a qualified person.
Safe Learning for Trainees
Training rigs and supervised practical work should be designed so learners can measure temperatures, electrical values, airflow, water flow, and control signals without unnecessary exposure to live conductors or pressurised refrigerant. Simulated faults are often better for training than deliberately creating dangerous real faults.
A professional apprentice knows the limits of their competence. Safe escalation is a technical skill, not a failure.
Commissioning
A Systematic Commissioning Sequence
Commissioning proves that the installed system is safe, complete, correctly configured, and capable of delivering the intended service. Exact steps vary, but a structured sequence normally includes visual inspection, electrical checks, source-side checks, distribution-side checks, control setup, refrigerant-circuit checks by authorised personnel, operation in relevant modes, performance measurements, documentation, and user handover.
Do not treat first start-up as the end of commissioning. Record design settings and measured values so future technicians have a baseline.
| Commissioning area | Examples of evidence |
|---|---|
| Installation quality | Correct mounting, clearances, weather sealing, insulation, support, drainage, labels, guards, and service access |
| Electrical | Supply within specification, protective devices, earthing, communication, safe isolation, correct sensor and actuator connections |
| Air or water side | Clean filters or strainers, design flow available, valves set correctly, circuits vented, pumps and fans operating, emitters or ducts balanced |
| Controls | Correct date and time, operating mode, heating curve, zones, hot-water settings, backup-heat logic, schedules, and sensor readings |
| Refrigerant circuit | Installation and charge confirmed using the manufacturer's approved procedure by authorised personnel |
| Performance | Stable operation, expected temperature changes, no abnormal noise, no unexplained alarms, and capacity consistent with conditions |
| Handover | User understands controls, maintenance, normal defrost behaviour, filter cleaning, warning signs, and who to contact for service |
Functional Testing
Test the modes that are relevant to the installation, such as space heating, cooling, domestic hot water, zone operation, auxiliary heat, and defrost logic where conditions allow or the manufacturer provides a service test. Confirm that valves, pumps, fans, and thermostatic or zone controls interact correctly.
A common commissioning mistake is to adjust many parameters at once. Change only justified settings, document the original and final values, and confirm the effect.
Fault Finding
A Measurement-Led Method
Fault finding should move from evidence to hypothesis, not from guess to parts replacement. Start with the customer's description and operating history. Confirm the fault, read alarms and controller data, inspect the system, check basic flow and demand conditions, then take approved measurements.
Ask whether the fault is on the source side, refrigerant circuit, sink or distribution side, electrical supply, controls and sensors, or building load. This structure prevents tunnel vision.
| Symptom | Possible causes to investigate | Useful first checks |
|---|---|---|
| Low heating output | Low airflow or water flow, low source temperature, high requested flow temperature, control limit, refrigerant issue, undersized emitter, excessive building load | Confirm demand, mode, filters or strainers, fan or pump operation, flow temperatures, source conditions, and alarm history |
| High electrical consumption | High temperature lift, auxiliary heater operation, poor building fabric, incorrect schedule, repeated defrost, flow problem, sensor or control issue | Compare operating hours, backup-heat status, setpoints, outdoor conditions, flow temperature, and delivered comfort |
| Frequent cycling | Oversizing at current load, poor zoning, insufficient system volume, thermostat interaction, flow interruption, control settings | Observe run time, load demand, zone status, flow stability, and controller limits |
| Outdoor coil heavily iced | Normal frost not clearing, airflow restriction, fan problem, drainage icing, sensor fault, defrost control issue, refrigerant fault | Distinguish normal frost from persistent ice, inspect airflow and drainage, read sensor data and defrost history |
| Noise or vibration | Mounting, pipe contact, fan damage, ice, compressor operating condition, water velocity, cavitation, loose panels | Identify exact location and operating mode, inspect supports, clearances, fan, pipework, and pump conditions |
| Room not reaching temperature | Building heat loss exceeds output, emitter too small, flow temperature too low, airflow imbalance, thermostat location, schedule, closed valve | Compare room demand with emitter operation, water or air delivery, system setpoint, and calculated load |
Sensor Plausibility
A sensor value can be wrong even when no sensor fault code is present. Compare related readings. For example, an outdoor sensor should be plausible for actual weather, and a flow-temperature sensor should respond when the system starts heating. Sudden impossible changes may indicate wiring, connection, placement, or sensor problems.
Do not replace a sensor only because a single reading looks unusual. Check the service documentation, compare with an independent calibrated measurement where permitted, and inspect wiring and connections.
Maintenance and Service
Routine Maintenance
Routine maintenance depends on system type and local requirements. Typical tasks include checking filters, coils, fans, drains, strainers, water pressure, antifreeze condition where relevant, insulation, electrical connections according to the approved service procedure, operating history, alarms, controller updates where supported, and evidence of refrigerant leakage.
Outdoor units need free airflow. Leaves, debris, snow, stored objects, and vegetation can obstruct the coil. Cleaning must protect delicate fins and electrical parts. Indoor filters should be maintained at the interval specified for the equipment and site conditions.
Documentation
Good documentation is part of technical quality. Record model and serial numbers, refrigerant and factory charge data, line or pipe details, settings, design temperatures, measured values, test results, software or control versions where relevant, maintenance actions, and any deviations from standard configuration.
A clear commissioning sheet makes later fault finding faster and reduces unnecessary component replacement.
Energy, Cost, and Environmental Performance
Energy Efficiency in Context
The U.S. Department of Energy describes heat pumps as systems that transfer heat rather than generate it directly and notes that modern air-source heat pumps can be effective across a wide range of climates. The International Energy Agency identifies heat pumps as an important technology for efficient electrification of heating.
Actual energy use depends on climate, building heat loss, system sizing, temperature lift, controls, user settings, defrost, auxiliary heat, maintenance, and electricity supply. A high laboratory rating does not guarantee a high seasonal result if installation or operation is poor.
Refrigerants and Climate Impact
A heat pump's climate impact includes electricity used during operation and any refrigerant emissions over manufacture, installation, service, leakage, and end-of-life recovery. Refrigerants differ greatly in global warming potential and safety classification.
The industry is moving toward lower-global-warming-potential refrigerants, but lower GWP does not remove the need for careful safety engineering. Some alternatives have higher flammability, different pressures, or different service-tool requirements. Always use equipment, procedures, ventilation, and recovery methods approved for the specific refrigerant.
Grid Flexibility and Thermal Storage
Smart controls and thermal storage can allow some heat demand to shift away from electricity-system peak periods without reducing comfort. A well-insulated building also stores heat in its fabric. However, control strategies must respect comfort, hygiene requirements for hot water, freeze protection, and equipment limits.
The International Energy Agency's 2026 Heat Pump Monitor also highlights the need for qualified technicians as the market grows. For vocational learners, this means heat-pump competence combines refrigeration, electrical, hydraulic, control, and building-performance skills.
Vocational Calculations
Worked Example: COP
A heat pump is measured at 8.4 kW useful heating output and 2.4 kW total electrical input.
COP = 8.4 divided by 2.4 = 3.5
At that operating point, the system delivers 3.5 units of heat for each unit of electrical energy used. This value should be recorded together with outdoor or source temperature, flow temperature, operating mode, and other relevant test conditions.
Worked Example: Hydronic Heat Output
A water circuit has a measured mass flow of 0.20 kg/s and a flow-to-return temperature difference of 4 K.
Heat output ≈ 0.20 × 4.18 × 4 = 3.34 kW
If the heat pump controller claims 8 kW but the water-side estimate is only about 3.3 kW, do not immediately assume a refrigerant fault. First check whether the flow measurement is accurate, sensors are correctly positioned, glycol properties differ, bypass flow is present, the system is still stabilising, or the controller value represents a different quantity.
Worked Example: Temperature Lift
Two air-to-water systems serve the same building on the same 5 °C outdoor day. System A requires 35 °C water, while System B requires 55 °C water. System B has a much larger temperature lift, so it will generally require more compressor work and is likely to operate at a lower COP.
The practical lesson is that emitter and building design affect the heat pump as strongly as the heat pump model itself.
Workplace Case Study
Training Workshop Retrofit
A vocational training workshop has a calculated design heat loss of 12 kW. The existing radiators were originally selected for high-temperature boiler water. The proposed air-to-water heat pump can provide 12 kW only at a lower water temperature than the radiators currently need during design weather.
A weak approach would be to select the heat pump by nominal capacity alone and simply raise the water setpoint later. A better approach is to examine the room heat-loss calculation, radiator output at lower mean water temperature, possible emitter upgrades, building-envelope improvements, pipe and pump capacity, electrical supply, defrost drainage, outdoor-unit location, noise, and backup-heat strategy.
Your goal is to make the whole system work efficiently, not merely to install a machine.
Professional References
The following open web resources are useful for checking technical principles and current guidance. Manufacturer service manuals and local regulations remain essential for work on a specific product or site.
- U.S. Department of Energy: Heat Pump Systems: Overview of operating principles and common system types.
- International Energy Agency: How a heat pump works: Explanation of heat-pump operation, efficiency, applications, and system integration.
- International Energy Agency: Heat Pump Monitor 2026: Current market, workforce, efficiency, refrigerant, and energy-system context.
- ENERGY STAR: Air-Source Heat Pumps: Consumer and installation information, including the importance of correct sizing.
Interactive Tasks
Quiz: Test Your Knowledge
What is the main energy function of a vapour-compression heat pump in heating mode? (It transfers heat from a lower-temperature source to a warmer building) (!It converts all electricity directly into resistance heat) (!It destroys heat outside the building) (!It stores all heat permanently inside the compressor)
What is the main function of the compressor in the refrigerant cycle? (It raises the pressure and temperature of refrigerant vapour) (!It removes dust from the indoor air) (!It lowers water pressure in the heating circuit) (!It drains condensate from the outdoor unit)
What is the main function of the expansion device? (It meters refrigerant and creates a pressure drop) (!It raises refrigerant pressure) (!It drives the outdoor fan) (!It measures building heat loss)
What happens in the evaporator during normal heat absorption? (The refrigerant absorbs heat and evaporates) (!The refrigerant releases all heat and freezes) (!The compressor motor stops permanently) (!The heating water is electrically isolated)
What does a heating COP of 4 mean at the stated test condition? (The system delivers four units of useful heat per unit of electrical input) (!The system uses four units of electricity per unit of heat) (!The compressor runs for exactly four hours) (!The water temperature is always four degrees higher)
What is the usual efficiency effect of reducing hydronic flow temperature while still meeting the heating load? (It generally improves heat-pump efficiency) (!It always doubles compressor speed) (!It prevents all defrost cycles) (!It eliminates the need for building insulation)
What is the thermal source for a ground-source heat pump? (The ground or a ground-coupled loop) (!Only direct solar radiation) (!Only indoor exhaust air) (!Only an electric resistance element)
Why does an air-source heat pump use a defrost cycle in cold humid conditions? (To remove ice from the outdoor heat exchanger) (!To dry the indoor filter permanently) (!To raise the building heat loss) (!To charge the refrigerant circuit automatically)
What should determine heat-pump capacity selection for a building? (A calculated design heating load and equipment performance data) (!The colour of the outdoor unit) (!The largest model that fits through the door) (!The previous boiler nameplate rating alone)
What is an essential part of professional commissioning? (Verifying safe operation and documenting measured results) (!Changing every factory setting) (!Bypassing protective controls) (!Leaving without testing the distribution system)
Memory Game
| Compressor | Raises refrigerant vapour pressure and temperature |
| Evaporator | Heat exchanger where refrigerant absorbs heat |
| Condenser | Heat exchanger where refrigerant releases heat |
| Expansion valve | Meters refrigerant and lowers its pressure |
| COP | Ratio of useful heating output to electrical input |
| Defrost | Operating mode that removes ice from an outdoor coil |
| Weather compensation | Control that adjusts water temperature with outdoor conditions |
| Ground loop | Buried circuit that exchanges heat with the ground |
Drag and Drop
| Match the correct terms. | Topic |
|---|---|
| High-pressure vapour leaving the compression stage | Compressor |
| Useful heat released to the building side | Condenser |
| Pressure reduction and refrigerant metering | Expansion device |
| Heat absorbed from the source | Evaporator |
| Flow direction changed for reversible operation | Reversing valve |
Crossword Puzzle
| Compressor | Which component raises refrigerant vapour pressure and temperature? |
| Evaporator | Which heat exchanger absorbs heat into the refrigerant? |
| Condenser | Which heat exchanger releases useful heat from the refrigerant? |
| Refrigerant | What working fluid circulates through the sealed heat-pump circuit? |
| Defrost | What operating process removes ice from an outdoor coil? |
| Inverter | What electronic drive allows many compressors to vary speed? |
LearningApps
Cloze Text
Open-Ended Tasks
Easy
- Heat pump photo survey: Find a heat-pump installation at your training centre, workplace, or neighbourhood and create an annotated photo or sketch identifying the outdoor unit, indoor distribution, pipe or cable route, drainage, and service access without opening equipment.
- Refrigeration cycle drawing: Draw your own four-stage heat-pump cycle and explain in clear English where pressure rises, where pressure falls, where heat is absorbed, and where heat is released.
- COP calculation practice: Create four realistic examples of heating output and electrical input, calculate COP for each one, and explain why the operating condition must be recorded with the result.
- Technician interview: Interview an HVAC or refrigeration technician about one common heat-pump installation mistake and one commissioning habit that prevents callbacks, then summarise the answers.
Standard
- Heat pump site survey: Produce a site-survey checklist for a small air-source heat-pump installation covering heat load, outdoor-unit position, noise, drainage, pipe routes, electrical supply, emitters or ducts, access, and local compliance.
- Temperature monitoring experiment: On a safe training rig, log source, flow, return, and room temperatures during a supervised operating cycle, graph the results, and explain what the changes tell you about heat transfer.
- Heat pump explainer video: Produce a three-minute training video that explains compressor, condenser, expansion device, evaporator, reversing valve, and defrost operation to a new apprentice.
- Maintenance checklist: Create a maintenance sheet for either an air-to-air or air-to-water heat pump and separate tasks that a user may safely do from tasks reserved for trained service personnel.
Advanced
- Heat-loss and emitter project: Analyse a small building with a supplied room-by-room heat-loss schedule, select suitable emitter outputs at low water temperature, and justify a heat-pump capacity using manufacturer performance data.
- Commissioning plan: Write a complete commissioning plan for a hypothetical installation, including safety, electrical, airflow or hydronic checks, controls, authorised refrigerant checks, functional tests, baseline measurements, and customer handover.
- Fault diagnosis case study: Develop a measurement-led diagnosis for a heat pump that has high electricity use and poor comfort, showing how you would distinguish building-load, control, flow, defrost, auxiliary-heat, sensor, and refrigerant-related causes.
- Lifecycle comparison: Compare two heating options for the same building using energy use, electricity or fuel source, refrigerant risk, maintenance, equipment lifetime assumptions, and likely emissions, then present the limits of your comparison.
Learning Assessment
- System explanation assessment: Explain a complete heating-mode refrigerant cycle from evaporator inlet to evaporator inlet and connect each pressure and temperature change to the component that causes it.
- Design transfer assessment: A building needs 45 °C water on a cold day but another needs 60 °C for the same outdoor temperature; reason which heat-pump system is likely to operate more efficiently and identify building or emitter changes that could reduce temperature lift.
- Commissioning evidence assessment: Given a commissioning sheet with missing airflow, water-flow, control, and handover data, identify which conclusions can and cannot be made and specify the measurements or records needed.
- Fault-finding assessment: A system has frequent cycling, several closed zone valves, and no current alarm; construct a safe diagnostic sequence before considering refrigerant-circuit work.
- Cold-weather assessment: Compare normal frost and defrost behaviour with abnormal persistent icing and explain which source-side, drainage, sensor, fan, control, and refrigerant possibilities should be investigated.
- Energy calculation assessment: Use supplied heat-output, electrical-input, water-flow, and temperature data to calculate COP and hydronic heat output, then explain why the two estimates may not agree exactly.
- Professional judgement assessment: Review a proposed outdoor-unit location beside a bedroom window and a narrow path, identify performance, noise, service-access, drainage, and safety concerns, and recommend a better design.
Evidence of Learning
Evidence of learning should show that you can connect theory with safe professional decisions, not just repeat vocabulary.
| Evidence area | Strong evidence |
|---|---|
| Knowledge | Accurate explanation of heat transfer, refrigeration cycle, system types, COP, temperature lift, defrost, and controls |
| Technical skills | Correct interpretation of schematics, measurements, heat-loss information, airflow or hydronic data, and manufacturer performance information |
| Safety | Consistent use of isolation, risk assessment, competence limits, refrigerant rules, and safe escalation |
| Practical products | Annotated system drawing, site survey, maintenance checklist, commissioning record, fault-finding flow, calculation sheet, or training video |
| Reasoning | Diagnosis based on multiple measurements and system interactions rather than one symptom or guessed component |
| Transfer | Ability to adapt the principles to an unfamiliar building, climate, emitter system, or source type and explain the limits of the conclusion |
| Communication | Clear handover notes, professional documentation, and explanations understandable to customers, supervisors, and other trades |
OERs on the Topic
The English Wikipedia article provides a broad open reference on heat-pump principles, history, types, applications, performance, and refrigerants.
The International Energy Agency's The Future of Heat Pumps material is published under an open licence and provides further technical and policy context. The U.S. Department of Energy heat-pump pages provide additional freely accessible explanations for learners and instructors.
Linked Learning Areas
Heat pumps connect several vocational fields. Refrigeration explains the sealed circuit, thermodynamics explains energy transfer, electrical engineering covers power and controls, plumbing and hydronics cover water distribution, building physics determines heat load, and digital controls coordinate efficient operation. A skilled technician must understand these links because many real faults occur at the boundary between trades rather than inside one component.
