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Assessment of MVHR systems and air quality in zero carbon homes (NF52)


MVHR (mechanical ventilation with heat recovery) systems provide ventilation while recovering heat from exhaust air. It has been widely adopted as an energy efficient design option and in 2013 it was estimated that a quarter of new homes were fitted with MVHR. In practice, however, deficiencies in the design, installation, commissioning and/or operation of MVHR have raised doubts over performance and whether adequate indoor air quality is always achieved. This report presents the findings from a programme of monitoring work on 10 new homes equipped with MVHR at the SSE development at Greenwatt Way in Slough. The work highlights a number of key considerations in the delivery of effective MVHR systems and in particular the importance of developing a detailed design for the system and the need for expert installation and commissioning. Importantly the work gauges the use of these systems by occupants and their feedback on the air quality achieved.

Summary of content

This primary research report on MHVR systems presents the findings from a two-year research project carried out by BRE entailing assessment and monitoring of 10 zero carbon Code for Sustainable Homes Code (CSH) Level 6 homes at Scottish and Southern Energy’s (SSE’s) Greenwatt Way development at Chalvey, near Slough, Berkshire.

Thanks to the cooperation and level of engagement from SSE it was possible to study the homes to some extent during construction and then to monitor them for a period of almost two years post-occupancy.  As well as inspection, testing and monitoring work it was possible to obtain occupant feedback and gauge perceptions of living in the zero carbon homes by use of questionnaires, walkthrough interviews and focus groups.  In addition to continuous monitoring of temperature, humidity and power consumption by the mechanical ventilation and heat recovery (MVHR) systems, periodic testing of indoor air quality and airtightness was carried out.  Towards the end of the project a study was undertaken in one of the homes in which air quality was monitored during gas and electric cooking, with the ventilation system on and off.

The project was conceived in response to concerns highlighted through Indoor Air Quality in Highly Energy Efficient Homes – A Review regarding the possible adverse consequences of increased airtightness in energy efficient homes on the quality of the indoor environment. In the case of homes built to high levels of the CSH, where energy reduction requirements are onerous, it is widely held that MVHR systems will be used to an increasing extent.

For the reasons described above, a substantial part of the research carried out in the homes at Greenwatt Way involved assessment and evaluation of MVHR systems, taking in design, procurement, installation, commissioning, performance, maintenance and occupant perceptions.  After approximately one year of occupation, nine of the MVHR fan units were recommissioned and changes made to room inlet air valves and air filters. In one home the MVHR fan unit was replaced and changes were made to sections of ductwork and its insulation.  As a result of pre- and post-monitoring these interventions provided more insights into operation of MVHR systems in airtight homes.