Characterising the actual performance of domestic mechanical ventilation and heat recovery systems

This paper describes the findings and recommendations of a meta-study examining the actual in-use performance of whole-house mechanical ventilation heat recovery systems (MVHR) installed in 54 low energy dwellings in the UK, as part of a national research programme. The performance of the systems is assessed using monitored data on indoor air quality (temperature, relative humidity, CO2) and energy use, cross-related with actual experiences of operating these systems through resident surveys. Design/research team interviews were also used to ascertain the reasons for selecting MVHR as a ventilation strategy for a housing development. Cross-analysis of the quantitative and qualitative data helps to identify the key features of MVHR systems with respect to quality of design, installation and commissioning procedures.
Overall the study indicates that the rationale behind the use of MVHR systems is borne out – the rates of ventilation as evidenced very generally by CO2 levels are better, and the energy use overall is lower. However the study also highlights the prevalence of sub-optimal systems and the possible implications on both energy efficiency and indoor air quality. This would lead to houses being naturally ventilated, but relying entirely on opening windows where there is no provision for background ventilation. In some spaces where this is not possible (for example due to external factors such as noise or security), or where there is less adaptive behaviour (for example bedrooms overnight), very poor levels of ventilation are experienced.
In general the energy consumption in houses with MVHR systems was lower, but this needs to be contextualised – 77% of the MVHR dwellings with energy data were of Passivhaus construction, which in general have lower consumption within the domestic sample (albeit with MVHR as a key component). In some cases it was found that MVHR systems were selected to achieve compliance with the Code for Sustainable Homes, without much understanding of required air-tightness of the building envelope or the maintenance requirements of these systems. Key recommendations include better understanding of the design issues to ensure good airflow and avoid installation problems associated with ductwork; designing in maintenance requirements including unit location, filter cleaning and replacement; ensuring good communication of the design details with installers and commissioners in conjunction with better quality control onsite; along with improved handover processes and occupant guidance.