Airtightness Testing

Heat Pump Sizing & Low-Carbon Building Performance

The UK construction industry is entering a major transition in how homes are heated. With updated regulations now preventing many new dwellings from connecting to the gas grid, heat pumps are rapidly becoming the standard heating solution for modern homes.

However, a heat pump is not simply a direct replacement for a traditional gas boiler.

To achieve good real-world performance, low running costs, and comfortable indoor conditions, the entire building must work together as a complete system. Proper heat pump performance depends heavily on accurate heat loss calculations, airtightness, ventilation strategy, insulation quality, emitter sizing, and domestic hot water design.

At airtightness uk, we help developers, contractors, and retrofit teams ensure buildings are properly prepared for low-temperature heating systems.

Why Heat Pump Sizing Matters

Oversized or undersized systems can create serious long-term problems.

An undersized heat pump may struggle to maintain comfortable temperatures during colder periods, while an oversized system can cycle excessively, reducing efficiency and increasing wear on components.

Correct sizing requires accurate, property-specific calculations including:

• Building heat loss

• Air leakage rates

• Ventilation losses

• Insulation performance

• Window and door performance

• Internal design temperatures

• Domestic hot water demand

• Flow temperature requirements

Low-temperature heating systems rely heavily on a well-performing building envelope. Excessive air leakage or poor ventilation commissioning can dramatically reduce system efficiency and increase operating costs.

The Importance of the System Criteria Document

The updated Domestic Heating Design Guide 2026, now referenced within Approved Document L, has significantly increased the level of accountability around heating system design.

Designers and installers are now expected to provide a detailed System Criteria Document outlining:

• Site-specific heat loss calculations

• Heat pump sizing methodology

• Emitter sizing

• Flow temperatures

• Control settings

• Domestic hot water sizing

• Indoor design temperatures

This creates a clear compliance trail under the Building Safety Act and places greater importance on accurate design and verified building performance.

Part L, Part G & Real-World Performance

Modern homes must now balance increasingly strict energy efficiency requirements alongside tighter water efficiency targets.

Simply installing low-flow fixtures to satisfy Part G water calculations is not always enough. Heat pumps deliver heat differently to traditional boilers and require a joined-up domestic hot water strategy to avoid performance issues.

Poor coordination between hot water demand, cylinder sizing, and heat pump output can lead to:

• High running costs

• Excessive immersion heater use

• Resident complaints

• Poor hot water recovery

• Oversized plant equipment

This is why integrated building performance design is becoming increasingly important across both new build and retrofit projects.

The Role of Airtightness & Ventilation

A highly efficient heat pump system requires a building that is:

• Properly airtight

• Correctly ventilated

• Thermally consistent

• Professionally commissioned

Our services help ensure the “as-built” performance of the property matches the intended design.

We provide:

• Air Tightness Testing

• Air Leakage Diagnostics

• Part F Ventilation Testing

• MVHR Commissioning

• Thermal Imaging Surveys

• Smoke Testing & Leak Investigation

By identifying uncontrolled air leakage and ventilation issues early, we help improve heat pump efficiency, reduce energy waste, and support compliance with modern building regulations.

Supporting Better Low-Carbon Buildings

At airtightness uk, we work closely with developers, builders, retrofit coordinators, and energy consultants to help create buildings that genuinely perform in real-world conditions.

As the industry moves further towards low-carbon heating, building performance testing is becoming more important than ever.

A properly tested and commissioned building helps deliver:

• Lower energy bills

• Improved comfort

• Better heat pump efficiency

• Reduced condensation risk

• Improved indoor air quality

• Greater occupant satisfaction

• Better long-term system reliability

For more information about our airtightness and ventilation testing services, contact our team today.