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

Quick Answer: Air source heat pumps (ASHPs) extract heat from outdoor air and deliver it to the heating system at an efficiency (COP) typically between 2.5 and 4.0 — meaning they produce 2.5-4kW of heat per 1kW of electricity consumed. They must be installed by an MCS-certified installer to qualify for the Boiler Upgrade Scheme (BUS) grant of £7,500 (from April 2024). System temperatures of 35-45°C mean larger radiators or underfloor heating are usually needed.

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

Quick Reference Table

Quoting a heating job? squote turns a 2-minute voice recording into a professional quote.

Try squote free →
SCOP Range Efficiency Annual Running Cost (vs 30p/kWh elect) Notes
Below 2.5 Poor Higher than a gas boiler Indicates poor installation or system design
2.5-3.0 Acceptable Broadly comparable to gas Minimum acceptable performance
3.0-3.5 Good 15-25% cheaper than gas (at current tariffs) Well-designed installation
3.5-4.0+ Excellent 25-40% cheaper than gas Optimal system design; low flow temperatures
Flow Temperature Typical COP System Requirement
35°C 4.0-5.0 Underfloor heating only; large radiators
45°C 3.0-4.0 Oversized radiators (×1.5-2 standard size)
55°C 2.2-3.0 Standard radiators may work; efficiency reduced
65°C+ 1.5-2.0 Poor efficiency; defeats purpose
Heat Pump Size Typical Application Property Type
5-7kW Well-insulated 2-3 bed New build/highly insulated
7-10kW Average 3-4 bed Semi with loft and cavity insulation
10-14kW Larger or poorly insulated 4+ bed or stone/solid wall
14-20kW Large property Large detached; may need 2 units

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):

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):

Radiator Rating (70°C) Output at 45°C (Δt20) Output at 35°C (Δt15)
1000W ~540W ~380W
2000W ~1080W ~760W
3000W ~1620W ~1140W

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:

Property requirements:

Claiming the grant:

Hybrid Heat Pump Systems

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

Hybrids are a practical solution where:

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