In the previous post, we looked at the importance of considering and understanding thermal comfort of occupants during the design stage of a building. We described the six variables that affect thermal comfort; namely humidity, air velocity, air temperature, radiant temperature, metabolic heat and clothing insulation. Now let us turn to how exactly our thermal environments have an impact on our comfort indoors.
The thermal environment is strongly influenced by the environmental services strategy: methods of temperature and humidity control, air distribution and their associated controls. Clearly there will be significant differences between the thermal environments in naturally ventilated and fully air conditioned buildings. The area and location of heated or cooled surfaces and the air movement induced by the supply air terminal devices (or windows) may also have a major impact.
A large percentage of energy consumed by buildings is dedicated to HVAC, water heating, lighting and appliances. The method by which air is distributed through a space has a significant impact on energy consumption, thermal environment and indoor air quality. For example, the distribution of air from a ceiling requires momentum to encourage heat exchange with the air below and the supply air temperature must be cooled to between 10 and 14ºC to deal with maximum heat gain. Delivering air with low momentum close to the floor on the other hand requires significantly lower fan energy and air may be supplied at between 19 and 20ºC to avoid draughts. This is known as displacement ventilation: the room air is displaced by the supply air, moves through the room by natural convection induced by the heat sources therein, and is extracted at high level. This encourages stratification in the room and a temperature gradient which is the limiting factor for design, since this must not exceed 3K (degree kelvins) between foot and head to avoid discomfort.
Displacement ventilation was developed originally in Scandinavia for industrial applications. The systems have generally been designed to provide 100% outdoor air, supplying significantly more air than is required to meet occupancy needs, but with efficient recuperators to recover heat from the exhaust air. Floor to ceiling heights need to be slightly greater than normal, preferably no less than 3m, to allow for stratification above head level. A system using a similar principle was developed in Germany employing outlets recessed into a suspended floor (sometimes referred to as underfloor air distribution (UFAD)).
A “mixed-mode” approach
Not only ventilation but also the method by which the room temperature is controlled affects both thermal comfort and energy consumed. For example, systems that incorporate large surface areas that are warmer or cooler than room air temperature (such as radiators or chilled ceilings and beams) rely in part on radiant heat exchange between the surface and occupants’ bodies to provide heating or cooling. Radiators can be used to offset the radiant cooling from bodies to cold window surfaces, for example, whilst chilled ceilings/beams can offset the temperature gradient associated with displacement ventilation. In applications where it has been appropriate, mixed-mode systems are being adopted, and preferred.
“Mixed-mode” refers to a hybrid approach to space conditioning that uses a combination of natural ventilation from operable windows (either manually or automatically controlled), and mechanical systems that include air distribution equipment and refrigeration/heating equipment for cooling/heating. A well-designed mixed-mode building begins with intelligent façade design to minimise cooling and heating loads. It then integrates the use of air-conditioning when and where it is necessary, with the use of natural ventilation whenever it is feasible or desirable, to maximise comfort while avoiding the significant energy use and operating costs of year-round air conditioning. This type of application requires either a high level of user engagement, or a complex building energy management system.
Ultimately, adopting a more targeted, comfort-centric approach to HVAC design could be a crucial step in creating better performing, more sustainable buildings. Displacement ventilation and mixed-mode conditioning are two essential tactics to improve indoor thermal comfort.