Breathing Buildings was asked by a number of designers and contractors to provide a system which delivered the benefits of controlled mixing ventilation but which enabled single-sided ventilation. The challenge was further compounded by the request that the height to depth ratio of the rooms being considered precluded them from being purely naturally ventilated throughout the year; either spatial air quality variations or temperature variations would be experienced. The development of the hybrid NVHR was therefore progressed to provide a controlled natural ventilation damper arrangement which can be used on its own or in conjunction with opening windows, but combined with fan-assisted flow and mixing when necessary. The designers were eager for a system to be developed which was sufficiently slimline in vertical extent that it could either be easily incorporated in a ceiling void or else if exposed would not negatively impact the natural daylighting provision from typical window systems. Hence a 300mm deep unit was specified. Finally, in order to make the unit small enough to be easy to install and to fit in a typical window system, a 900mm dimension was selected for the width.
These slimline, easy-to-install, units are incredibly popular and have changed the world of low energy ventilation. A critical feature for optimal use of the Breathing Buildings hybrid ventilation strategy is that the climate should ideally be relatively temperate (winters not too severe) and internal heat gains should not be low. When these criteria are satisfied, the Breathing Buildings solution is more energy efficient than a mechanical ventilation heat recovery device because the fan power usage in the NVHR is very low, and heat recovery is not helpful until external temperatures during the occupied day fall to around zero degrees centigrade – which isn’t very often in temperate climates. Below this temperature MVHR is potentially more efficient but above this temperature NVHR is much more efficient, so that over the course of a whole winter the NVHR ends up being the most efficient system.
The mixing ventilation system allows single sided, natural ventilation in deep plan spaces whilst making the most of internal heat gains to deliver superb thermal comfort. It is designed to meet the Department for Education “Guidance on ventilation, thermal comfort and indoor air quality in schools” and the Facilities Output Specification (FOS) for the Priority Schools Building Programme (PSBP). The unit can be used with a grille or ducted arrangement to supply air.
· Can work individually or as multiples (typically a pair for a standard school classroom)
· System directs air to the back of the space to provide a quasi-cross-flow system
· Mounted horizontally
· Manual control with 4-position switch or fully automatic with override
· BMS interface option to report T/CO2 or allow set-point changes and enable / disable
· Automatic units switch between winter mixing, natural and summer boost modes and include night cooling
· Integrates with manual windows when it is above 16 ° C outside or can control actuators on windows or dampers
· In winter the units use internal gains to temper incoming ventilation air, rather than a dedicated pre-heater, which has a significant impact in reducing heater sizing and energy consumption
· In a modern, well insulated building be it a classroom or well-occupied office environment, there is an abundance of heat when ventilation demand is at its highest, and these units capitalise on this
· Can be used on noisy sites – a standard unit provides 26dB Dnew attenuation as independently verified by SRL
· Units can be exposed in a room or installed in a ceiling void
· Very low energy consumption - typical specific fan power 0.13W/l/s in winter mode for unit with grille
