Smart heating controls have become central to retrofit strategies across the UK. However, many retrofit professionals struggle to interpret exactly what PAS2035 demands when it comes to heating system upgrades and controls installation. This guidance clarifies the standard's requirements and helps teams implement compliant solutions.

What PAS2035 Says About Heating Controls

PAS2035:2019 (and its successor PAS2035:2021) treats heating controls as a key element of retrofit design. The standard doesn't prescribe specific products or brands, but instead sets performance and design principles that must be met. This distinction matters—compliance comes through meeting functional requirements, not ticking boxes with particular manufacturers.

The standard requires that heating controls in retrofit projects must:

• Be designed to deliver measurable energy savings aligned with the retrofit specification
• Integrate into a coherent heating system strategy, not exist as isolated upgrades
• Account for the building's thermal characteristics and occupancy patterns
• Provide reliable, user-friendly operation without creating maintenance burdens
• Be properly commissioned and tested before handover

The Role of Controls in Retrofit Design

PAS2035 positions heating controls within a wider retrofit narrative. Controls alone don't constitute a retrofit—they work alongside fabric improvements, ventilation upgrades, and heating system changes. The standard requires that retrofit teams assess how controls interact with these other improvements.

For example, a property receiving wall insulation and a new heat pump will need heating controls designed specifically for heat pump operation, not controls specified for a gas boiler. The standard expects this logic to be explicit in design documentation.

Assessing Control Suitability

PAS2035 requires retrofit teams to assess existing controls during the survey phase. This means evaluating whether current controls can effectively manage the new heating system, or whether replacement is necessary. The assessment should consider:

• Compatibility with the new heating technology
• Whether controls support the building's thermal strategy
• User capability and expected interaction patterns
• Integration with any smart metering or monitoring systems
• Resilience and failure modes

Smart Controls and Building Performance

The standard distinguishes between basic heating controls and smarter systems with enhanced capability. While PAS2035 doesn't mandate smart controls, it recognises their value in delivering retrofit benefits.

Smart heating controls typically offer:

• Weather compensation—adjusting heating output based on external temperature
• Room temperature targeting—zone-based control reducing overheating
• Time scheduling—occupancy-responsive heating patterns
• Remote access—allowing occupants to adjust settings and monitor consumption
• Fault diagnostics—identifying system problems before they worsen

However, PAS2035 emphasises that enhanced capability only creates value if occupants understand and use the controls effectively. Specifying a complex smart system that homeowners find confusing creates retrofit failure, not success.

Controls and Building Simulation

PAS2035 requires that predicted energy savings are modelled during design. Heating controls significantly influence these predictions. The standard expects teams to account for realistic occupant behaviour when modelling—assuming people will actively manage smart controls, when evidence suggests many will simply leave them on default settings.

Modelling should reflect:

• Expected control setpoints and schedules
• Typical occupant interaction patterns
• Seasonal variation in heating demand
• Impact of controls on overall system efficiency

Commissioning and Handover Requirements

PAS2035 places significant emphasis on proper commissioning. Controls must be tested to verify they operate as designed and that occupants receive clear guidance on their use.

Commissioning should include:

• Functional testing of all control features
• Verification that controls integrate properly with the heating system
• User training and provision of clear documentation
• Testing of fault conditions and system response
• Recording of commissioning results for compliance evidence

Documentation and Compliance Evidence

The standard requires comprehensive documentation demonstrating how heating controls support retrofit objectives. This includes design rationale, compatibility assessments, commissioning records, and user guidance.

Effective documentation should explain to the building owner why specific controls were chosen, how they contribute to energy savings, and how to use them effectively. This creates accountability and helps occupants realise retrofit benefits.

Key Takeaways for Retrofit Teams

PAS2035 requires heating controls to be thoughtfully integrated into retrofit design rather than added as an afterthought. Controls must support the building's overall thermal strategy, be properly commissioned, and be selected with realistic user interaction in mind.

Compliance means demonstrating clear logic in control selection, proper testing before handover, and ensuring occupants understand their heating system. This approach delivers genuine retrofit benefits and helps buildings perform as intended.