While traditional mechanical heating, ventilation and air conditioning (HVAC) systems are still widely used, contemporary concerns for improving the environmental performance and “livability” of buildings has led to an increased interest in alternatives like natural ventilation and mixed-mode systems.
A major benefit of HVAC, particularly in deep plan and larger office buildings, is that they alleviate many of the problems associated with a naturally ventilated building such as noise, dust, insects, heavy winds, odors and uncomfortable temperatures.
However, there are some studies suggesting that a mixed-mode ventilation system of HVAC and natural ventilation, in combination with good passive design, can provide sufficient indoor air quality to keep occupants happy while reducing their reliance on mechanical air treatment.
Typically, a mixed-mode system operates in either of two modes and is best used for buildings’ perimeter spaces or for narrow planned buildings with good cross ventilation. The first is the natural ventilation mode that uses fresh external air from operable windows or dampers to cool the internal spaces and relies predominantly on the stack effect for hot air to be expelled through high-level exhausts. The second is the mechanical mode and sees the windows closed and the HVAC switched on and is used only when the external temperatures are too hot or too cold for natural ventilation, or when there is a heavy wind or rain.
Control of the operable windows can be manual or automatic and, in keeping with current trends towards automated buildings, we’re seeing more automated window systems come to the market that are compatible with Building Management Systems (BMS).
Some window manufacturers offer built-in automated systems that can be controlled by a BMS and that come with inputs for automated weather stations and sensors that detect smoke, rain, hail, temperature, wind, time, and smog. Others advertise that their products can be easily automated with add-on products like actuators and reed switches, and can also incorporate into a BMS.
These products render manual operation from building occupants—one of the most difficult-to-control influences on a building’s environmental impact—unnecessary, as the BMS detects when it is optimal to open and close windows and to operate the HVAC.
Some of the problems associated with the manual operation of a building’s HVAC and natural ventilation, like occupants leaving windows open with the HVAC on for example can, therefore, be avoided with automated mixed-mode air-conditioning.