As buildings become more innovative, traditional energy assessment tools are being tested. One such challenge arises in properties that use mixed-mode ventilation systems—a blend of natural ventilation (like operable windows) and mechanical systems (such as fans, HVAC units, or demand-controlled ventilation).
The question is: how accurately do Energy Performance Certificates (EPCs) capture the energy dynamics of these hybrid ventilation models?
What Is Mixed-Mode Ventilation?
Mixed-mode ventilation combines the best of two worlds:
Natural ventilation: Fresh air through windows, vents, or passive stack systems.
Mechanical ventilation: Fans, air-conditioning, or heat recovery systems for comfort and control.
The system typically switches between modes based on season, occupancy, or indoor air quality (IAQ) sensors. This approach can reduce energy use—but only if managed effectively.
The EPC Assessment Challenge
EPCs in the UK are calculated using SAP (Standard Assessment Procedure) for homes or SBEM (Simplified Building Energy Model) for non-domestic properties. Both methods rely on assumptions that don’t always align with the complexity of mixed-mode ventilation.
Key Issues:
Simplified Modelling
EPC software tends to assume a dominant ventilation strategy rather than switching.
This can lead to overestimation or underestimation of actual energy use.
User Behaviour Uncertainty
Mixed-mode efficiency depends heavily on how occupants use windows, fans, and controls.
EPCs assume “typical” behaviour, which may not reflect reality.
Seasonal Variation
EPCs provide a single rating, but mixed-mode systems perform very differently across summer and winter.
Static assessments miss this seasonal performance gap.
Indoor Air Quality (IAQ)
While EPCs measure energy efficiency, they don’t fully capture the health benefits of better air quality provided by mixed-mode systems.
Why This Matters
As building codes push for net-zero performance, more properties are adopting mixed-mode ventilation. Yet if EPCs don’t properly reflect these systems, property owners may face:
Misaligned efficiency ratings, affecting valuation or marketability.
Compliance risks, especially under MEES regulations.
Tenant dissatisfaction, if EPC promises don’t match real-world performance.
Towards Smarter EPCs
For EPCs to remain credible in the age of advanced building systems, improvements may include:
Dynamic modelling that recognises multiple ventilation modes.
Integration of real-time sensor data from smart building systems.
Better occupant behaviour modelling through AI-driven predictions.
Expansion towards a Digital Energy Identity, where ventilation, heating, and cooling data feed into a living building profile.
What Property Owners Can Do Now
Even if EPCs don’t fully capture mixed-mode ventilation performance, landlords and owners must remain compliant:
Learn more about our process on the About Us page.
Arrange a Residential EPC for your property.
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Have a unique ventilation system and need tailored advice? Reach out via our Contact Us page.
Conclusion
EPCs remain the cornerstone of energy compliance, but mixed-mode ventilation systems highlight their limitations. By simplifying complex building behaviours, EPCs may not always reflect true performance.
As the industry moves towards digital, real-time energy identities, the next generation of EPCs will need to capture these hybrid systems with far greater accuracy. Until then, building owners should ensure compliance while also investing in monitoring and optimisation beyond what an EPC alone can provide.
