Very few modern buildings are purely naturally ventilated. From the humble kitchen or toilet extractor fan, to the grandest of air handling units, all buildings have some form of mechanical ventilation.
Hybrid ventilation is the term most commonly used to describe schemes where the ventilation is neither entirely natural, nor entirely mechanical. All well-designed buildings will have some form of natural, low-level supply ventilation combined with high-level extract, mechanical ventilation. In these buildings, the designer has taken a pragmatic approach and has opted, not for the purest or simplest strategy, but for the best solution to meet the ventilation needs of the space, whilst minimising the cost, maintenance and energy consumption implications.
In terms of occupant comfort, a hybrid ventilation strategy, sometimes called mixed-mode ventilation, is a very attractive concept. A scheme designed to use natural ventilation for the majority of occupied hours can be assisted or augmented by mechanical systems under peak heating or cooling conditions. In principle, this hybrid solution is able to accommodate greater extremes of internal heat gains and external temperature fluctuations than can be handled by a purely natural ventilation strategy. The result is that energy consumption is minimised under normal day-to-day conditions, whilst ensuring sufficient ventilation capacity under a wide range internal/external scenarios.
Last year, we worked with Breathing Buildings to design and implement a low-energy ventilation solution for a new teaching block at St Raphael’s Primary School in North West London. The building comprises a double height hall, a series of ground floor classrooms and further classrooms to the first floor. Due to structural composition, it wasn’t possible to adopt a ‘one size fits all’ approach and different hybrid ventilation products were used in each of these areas.
In the double height hall, we specified e-stack S1500 units, designed for rooms which have access to the exterior via the roof and occupancy levels of 30 or more. The ground floor classrooms, with limited facade and no access to roof space are ventilated with NVHR-FX units. Whereas the first floor classrooms with direct access to the roof feature e-stack R-series units.
In order to assess the performance of the installation, temperature and CO2 data was monitored at 15 minute intervals, 24 hours a day between 14th March and 18th July 2015. Internal temperature was logged in four locations internally and CO2 in one location. External temperature was recorded on a shaded, North-facing external wall.
During the assessment period, external temperatures went as low as -1°C in March and as high as +36°C in July. Yet the temperature inside the building remained at an average of 22-23°C on all days the school was occupied.
To effectively validate this data, the external temperature recorded at the school over the assessment period was compared with that recorded by the UK Met Office at their London Heathrow weather station, just 5 miles from the school, and was found to closely correspond.