Lessons learned

Monodraught has a long history of working with academic partners to help develop its range of low-energy building services products. These include the natural ventilation system known as Windcatcher, which is used by over 1100 schools throughout the UK to deliver fresh air to a room from roof level whatever the wind’s direction, and without mechanical assistance. Monodraught and Brunel University have been investigating the performance of Windcatcher systems for the past four years by measuring key components of air quality (temperature and carbon dioxide concentration) in five schools and sixteen school classrooms ventilated by square Windcatchers. The data has been compared against current UK benchmarks, such as Building Bulletin 101 (BB101).

The adverse effects of poor indoor air quality (IAQ) on the health and productivity of building occupants are well known, and can be particularly detrimental to children who spend approximately 12% of their time inside school buildings. This is more than in any building type other than their homes. In school they are subject to a variety of pollutants from internal pollutants such as furnishings, IT equipment and bio-effluents, to external pollutants such as nitrogen dioxide and carbon monoxide. The potential for children to sustain long lasting damage is amplified because their tissue and organs are still growing, they breath more air relative to their body mass than adults, and ventilation rates in school classrooms are often found to be below those recommended by BB101.

In the UK, BB101 uses CO2 as an IAQ indicator for schools and prescribes a maximum concentration of 5000ppm and a mean occupied concentration of 1500ppm. In the absence of a combustion source, the rate at which CO2 is produced within a space depends on the number of occupants, as well as their size, activity, and metabolic rate.

Thus, one can link desired CO2 levels to ventilation rates and here BB101 specifies a minimum of 3l/s per person, and a daily mean of 5l/s per person. BB101 also specifies three internal temperature criteria for a classroom during the summer season, of which two must be met: no more than 120 hours above 28ºC, a maximum of 32ºC, and on average the internal temperature should be no more than 5ºC above the external temperature.


Automatic control

In the sixteen classrooms featured in the research, each Windcatcher is automatically controlled and opens according to room temperature. Dampers at the base of the units control the flow of air into and out of the supplied room, and when a prescribed temperature or set point is reached the dampers open 20% for every 1°C above the set point: in the winter the set point is 22°C and in the summer it is 16°C. In the summer the dampers also open fully from midnight until 6am to provide night cooling unless the internal temperature is at or below 15°C. Occupants may override the control settings at any time using a wall-mounted override switch that will fully open or close the dampers.

The summer and winter set points are designed to modify the performance of the natural ventilation units according to the heating season, so monitoring was conducted during both the summer and winter seasons to determine the effectiveness of the ventilation strategy. Measurements of temperature were made in sixteen classrooms, whereas measurements of CO2 were made in nine.

IAQ was calculated as a mean over the data measured for the occupied hours of 09.00 to 15.30, covering a working week (Monday to Friday) in both summer and winter and, accordingly, is an indicator of compliance. The temperature requirements of BB101 only apply to the summer time, and were met by all sixteen classrooms. The maximum limit of 32°C (referred to in BB101) was never reached, nor did the temperature in the classrooms exceed 28°C, so it is predicted that none of the classrooms would exceed 28°C for more than 120 hours in the summer season. The difference between mean internal and external temperatures, which to be compliant must be less than 5°C, was met in twelve classrooms.

The higher differential recorded in the other four classrooms could be attributed to their comparatively large, unshielded, glazing areas and southerly orientation. It should be noted that during the summer months the dampers of a Windcatcher will automatically open fully from midnight until 6am to provide night cooling and help deliver the recorded temperatures. Therefore, the ability of the natural ventilation units to automatically deliver night time cooling, without compromising security, further helps to meet IAQ requirements in classrooms.

The CO2 concentration did not exceed BB101’s maximum limit of 5000ppm in any classroom at any time. In the summer months, all classrooms met BB101’s mean CO2 limit of 1500ppm and the Windcatcher dampers were, on average, fully open for over 60% of the time, never fully closing. In the winter months, six of the nine monitored classrooms met the BB101 mean CO2 requirement and the dampers were, on average, closed for 92% of the time.

Accordingly, in winter the CO2 levels are largely controlled by background ventilation and this is the likely cause of the rise in CO2 levels seen in the winter months.


Meeting demand

These measurements show that a classroom containing a Windcatcher is capable of meeting BB101 criteria CO2 in the summer months indicating it is delivering a mean ventilation rate of 5l/s per person. However, the results suggest that, in a minority of cases, this ventilation rate is not always delivered during winter months. This may be attributed to the control strategy rather than the performance of the Windcatchers themselves. Ventilation rates provided by a Windcatcher are governed by the dampers, which in these classrooms are controlled according to the internal air temperature.

To improve the ventilation delivered by a Windcatcher Monodraught recommends using a control strategy based on the CO2 concentration found in a classroom to create demand controlled ventilation. This type of strategy would incur heat losses, although for a mean ventilation rate of 5l/s per person with a difference between the internal and external temperatures of 10°C, the energy lost by ventilating a classroom would be approximately 62W per person. It is possible that this could be generated by occupant heat gains; for example, it is common when designing a natural ventilation strategy for a classroom to assume combined sensible and latent heat gains of 120W per child. Of course, thermal comfort issues arise in the winter when ventilating using external air that is significantly colder than the internal temperature, particularly in the draught-risk zone close to a ventilation opening. However, the Monodraught units provide top-down mixing-ventilation that allows the incoming cold external air gradually to dilute the warm internal air, which is more acceptable than opening windows.

Whilst the research has taken time and investment, Monodraught considers the results positive. On balance it shows that, as a natural ventilation device, the Windcatcher is capable of ventilating a school classroom in accordance with UK government requirements, but also that a ventilation strategy can be improved by effective control of temperature and CO2 in all classrooms to ensure good IAQ and minimise heat losses: both lessons that are really worth learning.

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