Introduction

Heat pump installation has become increasingly central to UK decarbonisation targets, particularly within social housing retrofit programmes. However, the relationship between heat pump deployment and PAS2035 compliance remains an area of confusion for many retrofit practitioners. This guidance clarifies how heat pumps fit within the standard's holistic approach to building performance and energy efficiency.

PAS2035 and the Hierarchy of Measures

PAS2035:2019 Code of Practice for the assessment and reporting of building performance in relation to air quality and energy performance in existing dwellings establishes a clear hierarchy for retrofit interventions:

• Reduce overall heat demand through fabric improvement
• Improve system efficiency
• Integrate renewable energy systems

Heat pumps fall within the second and third categories. Critically, PAS2035 emphasises that heat pump installation should not substitute for fabric upgrades. A property with poor insulation, air leakage, or thermal bridging will require significantly oversized heat pump capacity to maintain comfort, reducing system efficiency and increasing capital costs.

Heat Demand Assessment and Sizing

The importance of accurate modelling

Correct heat pump sizing depends on realistic heat loss calculations. PAS2035 requires:

• Detailed U-value assessments of all building elements
• Air permeability testing or conservative estimation
• Thermal bridging evaluation
• Internal conditions specification (temperature and humidity profiles)

Many retrofit projects have encountered oversized, inefficient heat pump installations resulting from inadequate heat loss modelling. Installers must work from reliable fabric assessment data. Where this data is unavailable, conservative assumptions should be documented and justified.

Part L Building Regulations alignment

From 2023, new retrofit regulations require fabric-first approaches before low-carbon heating systems are specified. This regulatory shift reinforces PAS2035 principles and means heat pump projects increasingly require complementary insulation, window, or ventilation work.

Integration with Building Services Design

Heating system selection

Heat pump viability depends on several factors PAS2035 assessment should address:

• Emitter specification: Air source heat pumps operate most efficiently with low-temperature heat distribution (45–55°C flow temperature). Existing radiator systems may require upsizing or replacement. Floor heating offers superior efficiency but higher capital cost.
• Hot water provision: Heat pump water heating efficiency diminishes as target temperature increases. Thermal storage cylinders with immersion heating provide cost-effective peak demand management.
• Control strategies: Effective weather compensation, room thermostats, and zone control are essential to realise modelled performance gains.

Ventilation requirements

Heat pump retrofits often trigger mechanical ventilation requirements, particularly where fabric improvements reduce natural air leakage. PAS2035 assessment must account for heat recovery ventilation (MHRV) system efficiency, installation quality, and maintenance protocols. Poorly commissioned MHRV systems can undermine heat pump performance gains significantly.

On-Site Quality and Commissioning

Reducing performance gap

The difference between modelled and actual heat pump performance typically results from:

• Incorrect heat emitter sizing or balance
• Poor commissioning of controls and weather compensation
• Inadequate system documentation for occupants
• Lack of post-installation monitoring and adjustment

PAS2035 requires clear specification of commissioning protocols, operating parameters, and occupant guidance. Installers should provide documented evidence of system performance testing, including flow/return temperatures and heat output verification.

Building logbooks

PAS2035 mandates creation of building logbooks for retrofit projects. These documents must include heat pump specifications, control settings, expected performance parameters, and maintenance schedules. Clear occupant guidance significantly improves operational performance and reduces unnecessary service calls.

Renewable Energy Certification

Heat pumps qualify for renewable energy incentives under the Renewable Heat Incentive (RHI) and future schemes. PAS2035 assessment and reporting can support certification applications by providing detailed energy performance baselines, making retrofit documentation practically valuable beyond compliance.

Key Takeaways for Installers

• Verify heat loss calculations before sizing heat pumps; oversizing reduces efficiency and increases costs
• Specify compatible heat distribution systems and control strategies during design
• Commission systems rigorously and document performance testing
• Create comprehensive building logbooks with clear occupant guidance
• Plan mechanical ventilation integration early in retrofit planning

Heat pumps represent appropriate technology within a structured, fabric-informed retrofit approach. PAS2035 compliance ensures this integration delivers genuine carbon savings and improved building performance.