Air Source Heat Pumps: How They Work, Costs, and the Boiler Upgrade Scheme

Air Source Heat Pumps: How They Work, Costs, and the Boiler Upgrade Scheme

Air source heat pumps (ASHPs) have moved from a niche renewable technology to the central plank of the UK government's heating decarbonisation strategy. With gas boiler sales for new homes banned from 2025 and a phase-down of gas boilers in existing homes planned for 2035, understanding how heat pumps work — and how to advise customers on them — is increasingly essential for anyone working in heating.

This guide covers the operating principles of air source heat pumps, the differences between monobloc and split systems, performance metrics (COP and SCOP), the Boiler Upgrade Scheme grant, system requirements, and what an existing property needs to make heat pump installation viable.

How Air Source Heat Pumps Work

A heat pump moves heat rather than generating it. The thermodynamic cycle that drives it is the same vapour-compression refrigeration cycle used in a domestic refrigerator — but reversed in purpose. Where a fridge extracts heat from the food compartment and rejects it to the kitchen, a heat pump extracts heat from the outdoor air and delivers it to the heating system inside the building.

The cycle operates through four stages:

1. Evaporation: Refrigerant at low pressure and temperature circulates through an outdoor coil (the evaporator). Even when outdoor air is at -5°C, it contains sufficient thermal energy to evaporate the refrigerant (which may have a boiling point of -15°C or lower at that pressure). The refrigerant absorbs heat from the air and vaporises.
2. Compression: The refrigerant vapour passes to the compressor, which increases its pressure and temperature — significantly. A refrigerant that entered the compressor at -10°C and low pressure may leave at 60–80°C and high pressure.
3. Condensation: The hot, high-pressure refrigerant vapour passes through the condenser (a heat exchanger in contact with the heating system water), where it gives up heat to the water circuit and condenses back to liquid.
4. Expansion: The liquid refrigerant passes through an expansion valve, dropping back to low pressure and temperature, and the cycle repeats.

The critical point: the electrical energy consumed drives only the compressor. The heat delivered to the building is the compressor energy plus the heat extracted from the outdoor air. This is why the heat output is greater than the electrical input — typically two to four times greater.

COP and SCOP: Understanding Efficiency

The efficiency of a heat pump is expressed as the Coefficient of Performance (COP). A COP of 3.0 means the heat pump delivers 3 kW of heat for every 1 kW of electricity consumed. COP varies with outdoor temperature (lower outdoor temperature → lower COP) and flow temperature (higher heating system temperature → lower COP).

Because COP changes throughout the year, the Seasonal Coefficient of Performance (SCOP) is the more useful metric. SCOP is calculated over a full heating season for a specific climate zone. UK heat pumps are typically rated to the H1 (Strasbourg) climate zone. A good domestic ASHP in the UK climate achieves SCOP values of 2.5–3.8 depending on the manufacturer and system design.

The flow temperature is the most significant variable under the installer's control. At 35°C flow temperature (appropriate for underfloor heating or oversized radiators), a modern ASHP might achieve COP 4.0. At 55°C flow temperature (required by undersized radiators or a direct cylinder immersion), COP may fall to 2.0–2.5. This is why system design — particularly radiator sizing and flow temperature optimisation — is critical to heat pump performance.

Monobloc vs Split Systems

Monobloc

A monobloc ASHP contains the entire refrigerant circuit (compressor, evaporator, condenser, expansion valve) within a single outdoor unit. Only water pipework connects the outdoor unit to the indoor cylinder and heating circuit. The installer does not handle refrigerant — only water pipework connections are required indoors.

This makes monobloc installation accessible to a broader range of heating engineers, as F-Gas certification for refrigerant handling is only required if the refrigerant circuit is broken (e.g., for servicing). Most domestic ASHP installations use monobloc units. Leading UK monobloc models include the Vaillant aroTHERM Plus, Worcester Bosch ACS, Daikin Altherma 3, Mitsubishi Ecodan (monobloc range), and Panasonic Aquarea.

Split System

A split ASHP separates the outdoor unit (compressor and evaporator) from an indoor unit (condenser and controls) connected by refrigerant pipework. The indoor unit can be positioned in an airing cupboard or utility room, which reduces noise in the living space and can improve performance in extreme cold (the indoor unit can provide supplementary heat more efficiently). Split installation requires F-Gas certified engineers to handle the refrigerant pipework.

The Boiler Upgrade Scheme (BUS)

The Boiler Upgrade Scheme replaced the Renewable Heat Incentive (RHI) in April 2022. It provides an upfront capital grant for eligible heat pump and biomass boiler installations in England and Wales.

Current BUS grant values (as of 2024/25):

• Air source heat pump: £7,500
• Ground source heat pump: £7,500
• Biomass boiler: £5,000 (rural properties off the gas grid only)

The grant is paid directly to the MCS-certified installer, who deducts it from the customer's invoice. The customer pays the balance — typically £5,000–£12,000 after grant for an ASHP in a well-insulated 3-bedroom property.

BUS Eligibility Requirements

• The property must be in England or Wales (Scotland has the Home Energy Scotland scheme)
• The property must have an existing heating system being replaced (not a new build)
• The property must have a valid EPC (Energy Performance Certificate) with no outstanding recommendations for loft or cavity wall insulation (these must be installed first)
• The heat pump must be installed by an MCS-certified installer to MCS standards
• The heat pump model must be listed on the MCS product database
• The installation must meet MCS 020 (heat pump installation standard) requirements, including heat loss calculation and heat pump sizing

MCS Certification

The Microgeneration Certification Scheme (MCS) is the quality assurance framework for small-scale renewable technology installations in the UK. MCS 020 specifically covers heat pump installations. An MCS-certified installer must carry out a heat loss calculation (to BS EN 12831) for the property, size the heat pump to meet the design heat load, and issue an MCS certificate on completion. The certificate is required to claim the BUS grant and also satisfies planning and building control notification requirements.

Property Requirements for Viable Heat Pump Installation

Insulation

A heat pump delivers heat at lower flow temperatures than a gas boiler (35–55°C vs 70–80°C). A poorly insulated house losing heat faster than a heat pump can supply it at those temperatures will result in a cold, expensive-to-run system. The design heat loss (calculated per BS EN 12831) is the starting point — the heat pump must be sized to meet it at the design outdoor temperature (typically -3°C for most UK locations).

As a rough guide, properties with a heat loss above 8–10 kW will require a larger (and more expensive) heat pump, and the economics may be marginal. Most modern semi-detached and terraced houses with loft and cavity wall insulation have design heat losses of 5–8 kW — well within the range of standard domestic ASHPs (5 kW, 7 kW, 9 kW, 12 kW, 14 kW are common sizes).

Radiators and Underfloor Heating

Existing radiators sized for a 70/50°C (flow/return) boiler system may be undersized for a heat pump operating at 45/40°C. At 45°C mean water temperature instead of 60°C mean water temperature, a radiator delivers approximately 50% of its rated output. To compensate, radiators may need to be upgraded to larger panels (double or triple panel), or additional radiators added to rooms.

Underfloor heating (UFH) systems are ideal for heat pumps — they naturally operate at 35–45°C flow temperatures, maximising COP. Properties with UFH throughout typically achieve the best heat pump performance.

Hot Water

A heat pump cannot supply mains-pressure hot water directly (unlike a combi boiler). An unvented hot water cylinder (typically 200–300 litres) is required, heated by the heat pump via an indirect coil. The cylinder requires a dedicated electrical supply for its immersion heater (used as a backup or for legionella pasteurisation cycles) and must be installed with the standard G3 unvented safety devices.

Outdoor Space

An ASHP outdoor unit requires a firm, level base (concrete pad or proprietary anti-vibration feet) with adequate clearance around it — typically 300 mm sides and rear, 1,000–2,000 mm front clearance for airflow, depending on the manufacturer. Planning permission is not usually required for domestic ASHPs (they are permitted development in most cases), but there are restrictions in conservation areas and listed buildings.

Electrical Supply

A domestic ASHP draws 2–5 kW of electrical power during operation. Most single-phase 100 A supplies can accommodate this without upgrade. However, if the property also has an EV charger and electric cooking, the cumulative demand should be checked against the supply capacity. A dedicated 32 A MCB and circuit for the heat pump is standard.

Running Costs: Is a Heat Pump Cheaper to Run Than a Gas Boiler?

This depends on the SCOP achieved, electricity and gas tariff rates, and the property's heat demand. With electricity typically costing 2.5–3× the unit price of gas in the UK, a heat pump must achieve a SCOP of at least 2.5–3.0 to match the running cost of a gas boiler at 90% efficiency.

A well-designed system in a well-insulated property with low-temperature radiators or UFH will achieve SCOP 3.0–3.5, making running costs comparable or lower than gas. A poorly designed system at high flow temperature may achieve SCOP 2.0–2.2, resulting in higher running costs.

The Ofgem Heat Pump Ready programme and Time of Use tariffs (e.g. Octopus Cosy, Agile) allow smart heat pumps to shift demand to cheaper overnight electricity, improving the economics significantly.

Summary

Air source heat pumps are a proven, deployable technology for UK domestic heating. The government's £7,500 BUS grant makes the capital cost competitive with a high-specification boiler replacement in many cases. The critical factors for a successful installation are: correct heat loss calculation and heat pump sizing (MCS 020 required), adequate radiator or UFH emitter sizing for low flow temperatures, and a correctly specified unvented cylinder. A heat pump installed into a system designed for gas — with undersized radiators and high flow temperatures — will underperform and disappoint. One installed into a correctly designed system will reliably heat a well-insulated property at lower running cost than gas for its 20+ year service life.

At APM Plumbing & Electrical we stock heating controls and accessories and underfloor heating for UK trade professionals.