Understanding Air Source Heat Pumps in Retrofit

Air source heat pumps (ASHPs) have become central to the UK's retrofit strategy for decarbonising domestic heating. Unlike fossil fuel boilers, they extract thermal energy from outside air and transfer it indoors via a refrigeration cycle, delivering space heating and hot water with significantly lower carbon emissions.

For retrofit projects, ASHPs offer practical advantages: they require no fuel storage, have minimal on-site emissions, and can operate efficiently across a range of external temperatures. However, their performance and suitability vary considerably depending on property characteristics, insulation standards and building fabric conditions.

Assessing Property Suitability

Thermal Performance and Fabric First

PAS2035 emphasises a fabric-first approach, requiring retrofit coordinators to assess and improve a building's envelope before specifying heat pump systems. This is not merely best practice—it is a defined requirement under the standard.

Key fabric considerations include:

• Wall insulation (cavity fill, external render or internal boarding)
• Roof and loft insulation levels
• Window and door condition and thermal performance
• Air tightness and draught reduction
• Thermal bridging at junctions and details

Properties with poor thermal performance will require ASHPs with higher output capacity to maintain comfort, reducing efficiency gains and increasing running costs. PAS2035 requires documented evidence that fabric improvements have been considered and justified before heat pump sizing is finalised.

Heating System and Distribution

ASHPs typically operate most efficiently with lower flow temperatures (35–45°C) compared to conventional boilers (60–80°C). Properties with traditional radiator systems designed for high-temperature heat may require replacement or supplementation with larger radiators, underfloor heating, or fan convectors.

Assessors must evaluate:

• Existing radiator sizes and their adequacy at lower temperatures
• Pipework layout and thermal losses
• Hot water cylinder compatibility and storage capacity
• Controls and zoning capability

Where radiator replacement is necessary, this cost must be factored into the retrofit scope and programme.

Technical Assessment Requirements Under PAS2035

Energy Assessment and Modelling

PAS2035 requires formal energy modelling to predict performance. This involves:

• Detailed survey of building dimensions, construction, U-values and air permeability
• Modelling current energy use and carbon emissions
• Scenario testing with proposed interventions (fabric measures and heat pump system)
• Calculation of predicted energy and carbon savings
• Verification that the retrofit achieves meaningful decarbonisation

The assessment must demonstrate that the ASHP sizing is proportionate to the building's heat demand after fabric improvements, not oversized for the original poor condition.

Site-Specific Constraints

Practical installation factors must be documented:

• Outdoor space: ASHPs require external unit placement with adequate clearance and access for maintenance
• Noise: External units generate sound (typically 40–50 dB); proximity to neighbours and sensitive rooms must be evaluated
• Electrical supply: Single-phase supply may require upgrade for larger units; three-phase availability affects installation costs
• Gas infrastructure: Where mains gas exists, removal costs and any retained boiler for backup must be considered
• Planning and building control: Some installations require conservation area consent or building regulation approval

Installation and Commissioning Standards

PAS2035 does not specify installation methodology but requires that systems are installed by competent engineers certified to relevant standards (typically MCS, FGAS, or equivalent). Documentation must include:

• System design calculations and sizing justification
• Hydraulic diagrams and commissioning records
• FGAS certification for refrigerant handling
• Building control completion certificates
• Commissioning reports confirming system performance
• User guidance and maintenance schedules

Post-Installation Monitoring

PAS2035 introduces a monitoring and feedback phase lasting typically 12–36 months. During this period, actual performance data is collected and compared against predictions. Significant variances must be investigated and addressed, with lessons documented for the retrofit programme and future projects.

This approach ensures accountability and continuous improvement across the retrofit sector.

Conclusion

Air source heat pumps are an important decarbonisation technology, but their success depends on rigorous suitability assessment aligned with PAS2035 principles. Fabric improvement, thermal modelling, site constraints evaluation and proper commissioning are not optional extras—they form the technical foundation of compliant retrofit design. By following these requirements, retrofit coordinators and installers can deliver ASHPs that perform as predicted and generate genuine carbon reductions for building occupants.