A recent study has for the first time directly compared the performance of automatic windows with manual windows in a natural ventilation system. Richard Arnott explains.
In the summer of 2009 Stadt Zurich, the municipal local authority for the city, commissioned a study at Untermoos School to compare the performance of manually opening and automatic opening windows in the context of a natural ventilation system. The study compared CO₂ levels and temperature in adjacent ground floor and first floor rooms both in winter and in summer. It also measured energy consumption in each of the areas.


One room on each floor had manually opening windows and one had automatic opening windows controlled by the WindowMaster NV Advance control system. The comparisons were designed to get a range of results to assess the advantages or otherwise of automatic opening windows.

Room five on the ground floor features automatic opening windows. For 80 per cent of the time the recorded concentration of CO₂ in the room was less than 1,350 parts per million (ppm).

Next door, room six features manually opening windows. For only 67 per cent of the time CO₂ was less than 1,350 ppm.

Upstairs, room seven on the first floor with automatic windows had concentrations of CO₂ at less than 1,350 ppm for 88 per cent of the time. The adjacent room with manual windows recorded concentrations of CO₂ under 1,350 ppm for just 44 per cent of the time. In fact, for 46 per cent of the time that the room was in use the CO₂ levels exceeded 1,650 ppm.

In summer, CO₂ levels in room five (automatic opening) were below 950 ppm for 67 per cent of the time. In the room with manual opening windows (room six) CO₂ levels were higher for nearly 80 per cent of the time, fluctuating between 950 ppm and 1350 ppm.

On the first floor the differences were greater. The room with automatic opening windows maintained CO₂ levels at below 950 ppm for 100 per cent of the time. Room eight with the manual windows, on the other hand, had levels of less than 950 ppm for less than 40 per cent of the time. For a further 40 per cent of the time the CO₂ levels recorded fluctuated between 950 ppm and 1,650 ppm. For the remaining time the levels of CO₂ in the room exceeded 1,650 ppm.

These readings indicate that CO₂ levels in rooms with manually opening windows are difficult to maintain at low levels. Automatic windows make controlling the levels of CO₂ in a room easier and can help keep levels below the limits set by regulators.

Temperature in the rooms was measured between 26 June and 10 July 2009. The daytime temperature peaked at 31°C, falling to around 16°C on the coolest nights. The median temperature throughout the period was 20°C.

During the period the ground floor room with automatic opening windows (room five) maintained a normal temperature of between 22°C and 25°C for 82 per cent of the time. For the remainder of the time the temperature fluctuated between 25°C and 26.5°C.

A normal temperature could only be maintained in the room with manual windows (room six) for 24 per cent of the time. The rest of the time the room was warm and for a short period - amounting to 15 per cent of the time - temperatures of over 26.5°C were recorded, which could be considered to be too warm for a comfortable indoor climate according to the Swiss guidelines.

Controlling the temperature on the first floor is more difficult because warm air rises. As a result, higher temperatures were recorded for longer periods of time in rooms seven and eight than in the rooms on the ground floor. However, the room with automatic windows remained significantly cooler than the room with manually opening windows.

Normal temperatures of between 22°C and 25°C were recorded in room seven for 30 per cent of the time. As the room warmed, the temperature climbed to between 25°C and 26.5°C for 49 per cent of the time so, in total, the temperature was acceptable for 79 per cent of the time. For the rest of the time the room was too warm, with temperatures at over 26.5°C.

Room eight (manual opening) on the other hand recorded temperatures of between 25°C and 26.5°C for 25 per cent of the time. For all the remaining 75 per cent of the time the temperatures in the room were unacceptably high at over 26.5°C.

The study shows that automatically controlled windows help to ensure that CO₂ levels are kept at their lowest possible levels and that temperatures are kept at comfortable levels for as long as possible. They achieve this more effectively than manually controlled windows.

With night time cooling available in the rooms with automatic windows the starting temperatures in the daytime were always lower than in the rooms with manually opening windows. Automatic windows can be opened at night for specific periods to purge the rooms thus ensuring an optimum internal air quality (IAQ) in the morning. This cannot be achieved with manual windows, which would remain closed throughout the night.

The study also found that automatic natural ventilation can help save energy. Findings showed that 15 per cent of the energy consumption within the rooms was saved.

Room five (automatic) expended 1,000 KWh while room six (manual) used 1,200 KWh. On the first floor room seven also used 1,200 KWh while 1,300 KWh were used in room eight (manual opening).

Directly comparing CO₂ concentrations, temperature and energy consumption in rooms with and without automatic opening windows in the same areas of a school building has highlighted a number of benefits to automatic natural ventilation. An automatic window control system enables precise control of the indoor climate to maintain the optimum levels of temperature and CO₂.

In the UK, the DfES Building Bulletin 101 states that 'ventilation should be provided to limit the concentration of carbon dioxide in all teaching and learning spaces. When measured at seated head height, during a continuous period between the start and finish of teaching on any day, the average concentration of carbon dioxide should not exceed 1,500 parts per million (ppm).' The CO₂ concentrations in the rooms with automatic windows were significantly lower than this benchmark.

During the summer, automatic control of the windows helps maintain lower indoor temperatures than in rooms with manual opening windows. Finally, heat/energy consumption is reduced where windows are controlled automatically.

In a 'head to head' comparison the study concluded that automatically controlled windows provide natural ventilation that maintains an optimum IAQ more efficiently than windows opened manually with no automatic control.