Adaptive Comfort Temperatures
Recent standards (European Standard EN 152511) and guidance (CIBSE2, ASHRAE3) advise that comfort temperatures vary through the year as people adapt to changes in outside temperatures. Adaptation takes the form of changes in dress, opening windows etc.
As comfort temperatures vary, so heating and cooling set-points should be adjusted in harmony to maintain optimum comfort. This is in keeping with most peoples experience - a building at 24°C will feel cool in summer but feel hot during cooler periods of the year.
Varying set-points in sympathy with outside temperatures also has energy benefits. A higher set-point in summer will reduce cooling energy for air conditioned buildings. A lower set-point in winter will reduce heating energy. The variation in adaptive comfort temperature summer to winter is typically 6°C for a moderate climate giving heating4 and cooling5 energy savings of up to 20%.
Adaptive comfort temperatures are most appropriate to "free running" buildings where the occupant has control over themselves and their environment - ie they have adaptive opportunity6.
Where this is not the case, for example in air conditioned "close control" buildings which are sealed and operate a strict dress code, a smaller variation is appropriate.
Adaptive comfort temperatures for both building types can be accessed via the Data pages.
Adaptive opportunity6 enables occupants to adapt to maintain comfort over a wider range of temperatures. Examples include:
- Dress code
- Furniture type
- Consumption of hot / cold drinks
- Metabolic rate / posture
- Openable windows
- Operable blinds
- Local fans
- Spatial variations.
Spatial variations relates to the opportunity of occupants to for example move to get out of the sun, benefit from a cooling breeze, etc.
Free and Pre Cooling
In buildings with opening windows and similar ventilation devices, free and pre cooling can be used when outside air temperatures are below inside temperatures. In air conditioned buildings cooling energy savings can be made. In buildings with heating only, peak temperatures can be significantly reduced in summer.
On cool days free cooling can meet all the cooling load (top diagram). The space is ventilated to maintain a comfortable temperature.
On warm days there is less free cooling available, particularly later in the day (middle diagram). However, more than enough may be available earlier in the day due to lower outside temperatures, providing the opportunity for pre cooling. During the early part of the day the space is cooled slightly below comfort by increasing ventilation. This pre cooling is absorbed and stored by the building fabric (walls, ceilings, floors etc). As the space temperature increases later in the day this pre cooling is released, reducing the cooling load and/or restricting temperature rise.
On hot days there may only be limited free cooling available in the morning and no opportunity for pre cooling earlier in the day (bottom diagram). It may be possible to pre cool during the preceding night (often referred to as night cooling), though this will often be ruled out on practical grounds.
Pre cooling restricted to the morning occupied period will still have a major energy benefit during warm weather and can typically be expected to reduce cooling energy by 30%.
Predictive pre cooling strategies for the coming 24 hour period for various locations can be accessed via the Data pages.
Pre cooling at night
Pre cooling during the day would usually be undertaken by occupants opening windows. The occupants provide the control mechanism and a check mechanism to prevent overcooling (which could result in the heating coming on and wasting energy).
At night many buildings will be unoccupied so there are no occupants to provide control. There are also likely to be additional security concerns regarding opened windows in an unoccupied building at night. For these reasons it may be expected that it will only be practical to pre cool in most buildings during the morning occupied period.
Specialist automated systems are often installed to provide pre cooling during the night. These systems can typically be expected to reduce cooling energy by 50%7. Because they provide cooling on the hottest days they will also reduce peak cooling demands in air conditioned buildings and peak temperatures in heating only buildings (typically by 3°C7).