Adaptive Comfort Temperature graph

Equations (based on EN15251 and CIBSE)

Adaptive comfort temperatures are based on outside temperatures during the preceding few days. The weighting or influence given to the outside temperatures is higher for recent days, reducing with distance back in time as people "forget".

A weighted running mean of outdoor temperatures Trm is calculated as follows:

Trm= (1 - αrm) [Te(d-1) + αrm Te(d-2) + αrm2 Te(d-3) ...]

where αrm is a constant between 0 and 1 which defines the speed at which the running mean responds to outdoor temperature, Te(d-1) is the daily mean outdoor temperature (°C) for the previous day, Te(d-2) is the daily mean outdoor temperature (°C) for the day before that, and so on. The recommended value of αrm = 0.8 has been used giving:

Trm= 0.2Te(d-1) + 0.16Te(d-2) + 0.128Te(d-3) ...

The adaptive comfort temperature is then calculated from the running mean:

Tacf = 0.33Trm + 18.8 (within the range 21°C to 27°C)

where Tacf is the adaptive comfort temperature for free running buildings.

For close control buildings this is modified to:

Tacc = 0.09Trm + 22.6 (within the range 22°C to 25°C)

where Tacc is the adaptive comfort temperature for close control buildings.

Building types

The temperatures calculated are valid for buildings with mainly sedentary (sitting based) activities including offices, conference rooms, auditoria, cafes, restaurants, classrooms, dwellings etc. Different temperatures would apply to buildings with different activity levels - generally speaking the more activity the lower the temperature.

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 opportunity

Adaptive opportunity6 enables occupants to adapt to maintain comfort over a wider range of temperatures. Examples include:

Personal

  • Dress code
  • Furniture type
  • Consumption of hot / cold drinks
  • Metabolic rate / posture

Building

  • 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.

Pre Cooling Strategies Graph Pre Cooling Strategies Graph Pre Cooling Strategies Graph

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).

 

Notes

  1. "EN 15251:2007 Indoor environmental input parameters for design and assessment of energy performance of buildings addressing indoor air quality, thermal environment, lighting and acoustics".
  2. “CIBSE Guide A: Environmental Design: 2006”.
  3. “ASHRAE Standard 55-2004: Thermal Environmental Conditions for Human Occupancy”.
  4. “Carbon Trust website: Cut carbon & reduce costs: Heating: Housekeeping”.
  5. “BCO 24°C Study: Comfort, Productivity and Energy Consumption”.
  6. “Revival Technical Monograph 2: Adaptive thermal comfort and controls for building refurbishment”.
  7. “Institute of Refrigeration Paper: Hybrid Cooling Solutions: Night Cooling and Mechanical Refrigeration”.
  8. Values are for moderate climates similar to UK.
  9. Temperature set-points should be subject to other considerations including local regulations, standards and guidance, other requirement criteria such as for equipment or artefacts, maintaining a dead band between heating and cooling systems so as to avoid conflict and energy wastage, localized thermal effects such as radiant cooling / heating close to building facades, etc.
  10. Window opening or opening of similar ventilation devices should be subject to other considerations including security, safety, noise, pollution, adverse weather, local discomfort, overcooling bringing on heating, fresh air requirements, etc.
  11. For the purposes of this website the term "air conditioning" is considered to include mechanical cooling and comfort cooling.