Air source heat pumps (ASHPs) have become central to the UK's decarbonisation agenda, particularly within retrofit programmes. However, their suitability varies significantly depending on building characteristics, and installation must follow strict procedural requirements under PAS 2035. For retrofit coordinators and housing associations, understanding both the technical assessment criteria and compliance framework is essential.

Why ASHPs Matter in Retrofit

Traditional gas boilers remain the dominant heating source across UK housing stock, contributing significantly to carbon emissions. ASHPs offer an alternative by extracting heat from outside air—even at low temperatures—and transferring it indoors. They are increasingly favoured in retrofit projects because they require no fuel storage, have no direct emissions, and can reduce heating costs substantially where conditions are suitable.

However, ASHPs are not universally appropriate. Installation requires careful site assessment, and poor specification can lead to underperformance, occupant dissatisfaction, and wasted investment.

Technical Suitability Assessment

Building Fabric Quality

ASHP efficiency depends heavily on building fabric performance. Poor insulation and high heat loss mean the system works harder and less efficiently. The following factors should be evaluated:

• Wall insulation levels (solid brick vs. cavity vs. external wall insulation)
• Loft insulation depth and quality
• Window specification and air-tightness
• Overall estimated heat loss rates

Where heat loss is excessive, building fabric improvement should typically precede or accompany ASHP installation. PAS 2035 explicitly requires this sequencing consideration.

Heating System Compatibility

ASHPs work best with low-temperature heating systems. Traditional radiators designed for 70–80°C water temperatures are less efficient with ASHPs, which typically deliver 35–55°C water. Assessment must determine:

• Existing radiator size and specification
• Whether the system can operate at lower temperatures
• Need for radiator upsizing or replacement
• Suitability for underfloor heating alternatives

Properties with underfloor heating or modern low-temperature radiators have clear advantages.

Outdoor Space and Noise

Physical installation feasibility requires adequate external space for the outdoor unit. Considerations include:

• Available wall or ground space suitable for the unit dimensions
• Proximity to boundaries and sensitive receptors (neighbours)
• Potential for noise complaints (modern units are quieter, but still generate sound)
• Drainage requirements for condensation discharge
• Planning and building regulation implications

Climate and Performance

ASHPs perform reasonably across UK climates but are more efficient in milder southern regions than colder northern areas. Expected Seasonal Coefficient of Performance (SCOP) should be modelled based on location and system specification.

PAS 2035 Retrofit Coordinator Requirements

PAS 2035 is the UK standard for retrofit of non-domestic buildings and buildings other than dwellings. For residential retrofit (including housing association stock), the standard provides a procedural framework that retrofit coordinators must follow.

Key PAS 2035 Steps

1. Information Gathering and Review

Collect as-built information, occupancy patterns, utility data, and current system performance. This baseline is essential for recommending appropriate measures.

2. Building Characterisation

Conduct or review surveys to establish thermal performance, structural condition, and existing systems. For ASHP suitability, this includes detailed heating system and fabric assessment.

3. Measure Identification and Sequencing

PAS 2035 requires a systematic approach: fabric improvements should generally be prioritised before or alongside low-temperature heating systems. The standard discourages installing ASHPs without corresponding fabric or radiator upgrades where feasible.

4. Performance Modelling and Design Specifications

Use recognised models (such as SAP or dynamic thermal simulation) to predict outcomes. Specify system capacity based on actual heat loss, not arbitrary sizing rules. Oversized systems reduce efficiency and increase cost.

5. Quality Assurance and Commissioning

Ensure installation quality through site inspections and competent installer selection. Commissioning must verify system settings, controls, and occupant training.

Common Coordinator Responsibilities

• Coordinating assessments across multiple disciplines (thermal, structural, services)
• Ensuring sequencing of measures aligns with PAS 2035 logic
• Documenting design rationale and performance predictions
• Managing stakeholder expectations about realistic outcomes
• Verifying installer competency (MCS accreditation or equivalent)
• Ensuring building regulations compliance and sign-off

Best Practice Summary

Successful ASHP retrofit requires early suitability assessment, integration with wider building improvements, and strict adherence to PAS 2035 procedural standards. Retrofit coordinators should view ASHPs not as standalone solutions but as components of holistic retrofit strategies tailored to individual building characteristics. When specified correctly and installed competently, ASHPs deliver substantial carbon reductions and long-term value.