Air Source Heat Pumps: Is Your Property Suitable?

Air source heat pumps (ASHPs) are increasingly specified in retrofit programmes, but not every property is equally suited to this technology. A thorough suitability assessment prevents costly oversizing, poor performance and occupant dissatisfaction. This guide sets out the key criteria to evaluate.

Building Fabric and Thermal Efficiency

Heat pump performance is fundamentally dependent on building fabric quality. The better insulated your property, the lower the heating demand and the more efficiently the heat pump operates.

• Wall insulation: Properties with solid walls or poor cavity fill present higher heat loss. Consider fabric upgrades (external wall insulation or internal insulation) before specifying an ASHP, or size accordingly.
• Roof and loft: Uninsulated or poorly insulated roofs significantly increase heating load. Loft insulation to 270mm minimum is standard expectation.
• Windows and doors: Single glazing or poor seals increase demand. Double glazing or better is advisable.
• Air tightness: Draughty properties require larger heat pump capacity. Air tightness testing (or visual assessment) helps quantify this.

A heat loss calculation, informed by building survey data, is essential. Properties with poor fabric may require parallel insulation work to make ASHPs economically viable and to achieve performance warranties.

Heating System and Radiators

Heat pumps operate at lower water temperatures than gas boilers. Compatibility with existing heating distribution is critical.

Radiator Performance

Most older radiators are oversized for gas boiler operation and perform poorly at the 40–50°C flow temperatures typical of heat pumps. Options include:

1. Retention of existing radiators with larger ASHP capacity (increases running costs)
2. Radiator replacement or supplementation with larger models
3. Conversion to low-temperature distribution (underfloor heating or fan coils)

A radiator heat output assessment at design temperature (typically 45°C flow) must be conducted. If existing radiators cannot meet room heat loss at these temperatures, replacement or hybrid systems should be specified.

Heating Controls

Heat pumps require robust thermostatic and weather-compensated controls to operate efficiently. Properties with poor or absent heating controls are poor candidates unless controls are upgraded as part of the retrofit.

Space and Installation Constraints

Physical space requirements and access determine installation feasibility.

• External unit placement: ASHPs require outdoor space. Ground level with clear air flow is ideal; wall-mounting is possible but less efficient in some configurations.
• Clearances: Minimum 1 metre clearance on intake side; avoid positioning near windows, doors or neighbouring properties (noise).
• Indoor space: Cylinder and pump room access for routine maintenance is necessary. Tight airing cupboards or under-stairs spaces may limit options.
• Pipework routing: Long refrigerant or water pipe runs reduce efficiency and increase cost. Assess route feasibility early.
• Structural support: Ground conditions and wall robustness must support external unit weight and vibration.

Noise and Neighbour Relations

ASHPs emit noise (typically 40–50 dB from the outdoor unit). In semi-detached or terraced housing, this is a material consideration.

• Proximity to neighbouring bedrooms or shared boundaries increases complaint risk.
• Acoustic fencing or distance (minimum 1–2 metres from boundaries) reduces transmission.
• In apartment blocks or flats, rooftop or courtyard units require careful placement and may need enhanced insulation.
• Obtaining written neighbour consent is recommended for shared or closely spaced properties.

Electrical Infrastructure

ASHPs require reliable mains electricity supply. Properties with:

• Single-phase 60A supply or less may need DNO upgrade for larger heat pump models (typically 8–12 kW units).
• Unstable or frequent power outages are unsuitable without backup generation or storage.
• Limited space for new consumer unit or cabling runs may increase installation cost.

Early liaison with the local Distribution Network Operator (DNO) is advisable for rural properties or those with constrained supply.

Hot Water and Storage

Most ASHP retrofits require a hot water cylinder. Suitability considerations include:

• Cylinder space: Properties with no airing cupboard or tight plant room space may struggle to accommodate a 200–250-litre cylinder.
• Water pressure: Unvented cylinders require mains pressure; low-pressure systems may require pressure vessel addition.
• Legionella risk: Cylinders must achieve 60°C regularly. Properties with low heating demand may require immersion heaters or heat pump sizing adjustments.

Economic and Tenure Suitability

Beyond technical factors, consider:

• Rent or purchase: In rental properties, ASHP payback periods are longer. Regulatory framework and rent control must be assessed.
• Occupancy stability: Properties with frequent tenant turnover complicate long-term maintenance and performance monitoring.
• Meter and billing: Separate electricity metering for heating is advisable; multi-occupancy properties may need consumption allocation strategies.

Checklist for Suitability Assessment

1. Conduct heat loss calculation; identify fabric upgrade needs
2. Survey existing radiators; assess replacement/supplementation requirements
3. Verify external space for outdoor unit and maintenance access
4. Evaluate noise risk; document neighbour proximity
5. Confirm electrical supply capacity; contact DNO if uncertain
6. Assess hot water cylinder space and unvented system feasibility
7. Review heating controls and upgrade if necessary
8. Confirm occupancy and tenure stability

A systematic suitability assessment, documented in writing and shared with stakeholders, ensures ASHP retrofits deliver reliable performance and occupant acceptance.