Air Source Heat Pumps: How They Work, Sizing, COP Values & BUS Grant

Summary

Air source heat pumps are the UK government's primary technology for decarbonising domestic heating, with targets for 600,000 installations per year by 2028. They work on the same refrigeration cycle as a fridge or air conditioning unit, but in reverse: a refrigerant extracts heat from outdoor air (even at temperatures as low as -15°C), compresses it to raise the temperature, and delivers the heat to the heating system.

The key difference between a heat pump and a gas boiler is the flow temperature. A gas boiler produces flow temperatures of 65-80°C; a heat pump works most efficiently at lower flow temperatures of 35-50°C. This has major implications for system design: existing radiators sized for 70°C flow water will be undersized when the system runs at 45°C. This is the most common reason for poor heat pump performance — the heat distribution system was never redesigned when the heat pump was installed.

MCS certification is essential. Installing heat pumps is a regulated activity (Gas Safe is not required, but MCS is), and MCS certification is the route to the BUS grant, which is significant at £7,500 per installation. MCS certification requires training (City & Guilds Level 3 Diploma in Heat Pump Systems or equivalent), quality management, and regular auditing.

Key Facts

• COP (Coefficient of Performance) — Ratio of heat output to electricity input; COP of 3.0 = 3kW heat per 1kW electricity
• SCOP (Seasonal COP) — Average efficiency over the full heating season; typically 2.5-3.5 for UK ASHP installations
• Flow temperature — Heat pumps work best at 35-45°C; performance drops significantly above 50-55°C
• Design flow temperature — System should be designed for 45°C maximum flow for good SCOP; 35°C is ideal
• BUS grant — Boiler Upgrade Scheme; £7,500 for ASHP installations (£7,500 also for GSHP); check current OZEV/DESNZ guidance
• MCS certification — Required for installer; all products must appear on the MCS Product Register
• Minimum COP for MCS — Heat pump must achieve a minimum SCOP of 2.5 under MCS standards
• Outdoor unit — Installed outside the property; typically 600-900mm high; noise level important (45-55dBa typical)
• Permitted development — Most ASHPs are permitted development; some restrictions in conservation areas, flats, and listed buildings
• Planning limitations — One unit per property; not on front elevation in conservation areas (England rules apply; Scotland/Wales have different rules)
• Hot water cylinder — ASHPs require a separate hot water cylinder (they do not heat domestic hot water directly); minimum 180-250 litres
• Legionella pasteurisation — The cylinder must be able to reach 60°C periodically for Legionella pasteurisation; most ASHP systems include a built-in immersion heater for this
• Backup/auxiliary heater — Many ASHPs include or are paired with an electric resistance heater for backup in very cold weather
• Noise regulations — External unit must not exceed MCS noise requirements (50dBa at 1m in most areas; lower in some conservation areas)
• Refrigerant — Modern units use R32, R410A, or newer low-GWP refrigerants; F-Gas regulations apply

Quick Reference Table

Detailed Guidance

How an ASHP Works

The refrigeration cycle in reverse:

1. Evaporator — Refrigerant at low pressure and temperature (-15 to +5°C depending on outdoor conditions) flows through the outdoor evaporator coil. Even cold outdoor air contains heat energy; the refrigerant boils (evaporates) as it absorbs this heat.

2. Compressor — The gaseous refrigerant is compressed by an electric compressor, raising its temperature and pressure (to +40-70°C depending on design).

3. Condenser — The hot, high-pressure refrigerant passes through the indoor condenser (heat exchanger). Here, the refrigerant condenses back to liquid, releasing its heat into the heating system water.

4. Expansion valve — The refrigerant passes through an expansion valve, dropping back to low pressure and low temperature, ready to repeat the cycle.

The key insight is that the compressor uses relatively little electricity to create a large pressure differential, which enables a significant temperature lift from ambient outdoor air to useful space heating temperatures.

Sizing the Heat Pump

Correct sizing is critical. Unlike boilers, where oversizing is a minor inefficiency, oversizing a heat pump causes "cycling" (frequent on/off), which reduces efficiency and component life.

Sizing method:

1. Calculate the property's heat loss (W/°C) using the BS EN 12831 method or simplified SAP calculation
2. Determine the design outdoor temperature (typically -3°C to -4°C for most of England, colder in Scotland/Northern England)
3. Design temperature inside: 21°C living areas, 18-19°C bedrooms
4. Total heat loss at design conditions = heat pump required output

Rule of thumb sizing (for budget estimation only):

• Well-insulated new build: 30-50W/m²
• Average semi-detached (cavity wall, loft insulation): 50-70W/m²
• Older solid wall property: 80-120W/m²

A typical 100m² well-insulated 3-bed house might have a heat loss of 5-7kW at design conditions, suggesting a 7kW ASHP.

MCS requirement: Heat pump system design must use a formal heat loss calculation; a "rule of thumb" is not MCS-compliant.

Radiator Sizing and System Design

This is the most common failure point in heat pump installations. At 45°C flow temperature, a standard radiator outputs roughly 50% of its rated output (which is calculated at 70°C flow):

This means that in practice, radiators need to be 1.5-2x the standard size for a 45°C heat pump system, or the system needs to run at a higher flow temperature (with reduced efficiency).

Options for system design:

1. Replace all radiators with appropriately sized ones (most effective, most expensive)
2. Add additional radiators to the existing circuit (practical where room allows)
3. Install underfloor heating (UFH) throughout (ideal for new builds or full refurbishments)
4. Accept some rooms being slightly underheated; focus on principal living areas
5. Retain gas boiler for backup ("hybrid" system)

MCS Certification and BUS Grant

The Boiler Upgrade Scheme (BUS) offers £7,500 for ASHP installations. To qualify:

Installer requirements:

• Must be MCS-certified for heat pump installation
• Must be a member of an appropriate competent person scheme for associated electrical and plumbing work
• Heat pump must appear on the MCS product register
• System design must follow MCS 020 (heat pump installation standard)

Property requirements:

• Property in England or Wales (separate schemes in Scotland and NI)
• Existing heating system must be a gas, oil, or electric storage heater (you're replacing a fossil fuel system)
• Property must have a valid EPC (Energy Performance Certificate) not more than 10 years old
• EPC must not recommend loft or cavity wall insulation as a high priority measure (or those measures must have been installed)

Claiming the grant:

• Installer applies for the grant voucher on behalf of the customer before installation
• Voucher is issued; installation proceeds
• Installer claims the grant after installation; it is deducted from the customer's invoice
• Grants are time-limited and budget-capped; check current DESNZ/OZEV guidance for current availability

Hybrid Heat Pump Systems

A hybrid system pairs an ASHP with an existing gas boiler:

• The heat pump runs as the primary heat source during milder weather (when COP is highest)
• The gas boiler takes over when outdoor temperatures drop below a set threshold (heat pump becomes inefficient) or when demand exceeds heat pump capacity
• The control system automatically switches between sources based on efficiency

Hybrids are a practical solution where:

• The heating system isn't being upgraded (existing radiators too small for heat-pump-only)
• The property is a poor candidate for a heat pump (very high heat loss, poor insulation)
• The customer wants reduced gas consumption but not full electrification

BUS grant eligibility for hybrid systems has changed; check current DESNZ guidance.

Frequently Asked Questions

Do I need to replace the radiators when fitting a heat pump?

Probably yes, at least in some rooms. Most existing radiators were sized for a boiler running at 65-80°C. At the lower flow temperatures a heat pump uses, they'll output significantly less heat. A proper heat loss calculation will tell you which rooms are marginal. In practice, for a system designed for 45°C, radiators need to be roughly double the size of a 70°C system — so upgrading to larger radiators (or adding extra panels) is essential for comfort.

Can a heat pump heat domestic hot water?

Yes, but it does so less efficiently than it heats the space heating system. DHW requires higher temperatures (typically 60°C for Legionella prevention), which means a lower COP for that cycle. Most ASHP installations use a dedicated hot water cylinder with an immersion heater for the daily Legionella pasteurisation cycle and rely on the heat pump for the bulk of DHW heating at a somewhat lower efficiency than the space heating.

How noisy is an ASHP?

Modern units are typically 45-55dB(A) at 1 metre — roughly the level of a normal conversation or a refrigerator. MCS noise limits require the unit not to exceed 42dB(A) at the nearest neighbour's habitable room window (under MCS requirements). Site the unit carefully: away from neighbour's bedrooms, ideally in a corner where walls provide some noise deflection, not directly outside the customer's bedroom window. Vibration isolation feet should be used on the base.

What electricity tariff is best for heat pumps?

Standard tariffs (around 24-30p/kWh in 2026) make heat pump economics marginal vs. gas. Economy 7 or time-of-use tariffs (like Octopus Go or Cosy Octopus) offer cheap overnight electricity (7-8p/kWh) that significantly improves the economics when the thermal mass of the heating system can be charged overnight and released during the day.

Regulations & Standards

• MCS 020 — Heat pump installation standard; governs MCS-certified ASHP installations

• Building Regulations Part L — Conservation of fuel and power; heat pump installations must comply

• F-Gas Regulations (EU 517/2014 as retained UK law) — Refrigerant handling; only F-Gas certified engineers can handle refrigerants

• BS EN 14825 — Testing of heat pumps; defines COP and SCOP measurement

• Permitted Development Rights (England) — Schedule 2, Part 14; conditions for ASHP installation without planning permission

• MCS (Microgeneration Certification Scheme) — MCS certification and heat pump standards

• Boiler Upgrade Scheme (DESNZ) — BUS grant guidance and application

• Heat Pump Association — UK industry body for heat pump installation guidance

• radiator sizing — Radiator sizing for heat pump systems

• underfloor heating — UFH as preferred heat distribution for heat pumps

• boiler selection — Comparison with gas boiler options