Some issues of office temperature set-points
Brief
The amount of electricity consumed through a MVAC system is significantly influenced by the choice of desired environmental conditions. Maintaining satisfactory thermal conditions for the occupants by an adjustable indoor temperature set point of the air-conditioning system is one of the primary concerns in many air-conditioned office buildings.
In Hong Kong, the temperature set-point policy for acceptable thermal comfort in air-conditioned offices is recommended. According to the Environmental Protection Department, an ‘excellent’ office environment should be of air temperature 20-25.5°C, relative humidity (RH) 40-70% and air speed below 0.2 ms−1, while a ‘good’ one should be of air temperature, RH and air speed not exceeding 25.5°C, 70% and 0.3 ms−1 respectively [1]. An ‘excellent’ office controls the thermal environment within an acceptable range of thermal comfort; and a ‘good’ office biases on preventing an overheated thermal environment. It was noted that a tendency for people to over-dress because of over-cooling in a number of ‘good’ offices was reported. A warmer thermal environment is suggested for air-conditioned offices of Hong Kong and the maximum indoor temperature of 25.5°C has been specified in the guidelines on energy efficiency of air conditioning installations in Hong Kong [2].
Unfortunately, there is always a trade-off between energy saving and occupant comfort. An eco-efficient air-conditioning system conserves energy but not necessarily the thermal comfort needs of all occupants. For the context of air-conditioned offices of Hong Kong, my team members and I have investigated some related issues below
Occupant demand of thermal comfort
How much the thermal comfort contributes to occupant acceptance of overall indoor environmental quality (IEQ)? We need an IEQ model.
Thermal comfort model with consideration of adaptive clothing volume
Modelling for energy consumption and CO2 generation
Do temperature set-point dominate the thermal energy consumption in an occupied air-conditioned office? The answer is NO.
Bacteria/mould growth
Productivity loss
Special Issues: Up-tune the average air temperature at Hong Kong air-conditioned office buildings to 25.5oC
There are a number of enquiries from the local industry regarding to up-tune the air temperature from the current context (below 25.5oC) to 25.5oC. Some consider this is an effective response to the call for energy saving in air-conditioned offices. Suggestions such as increasing airspeed were made.
From our studies, we found some facts below regarding this issue,
Office occupants prefer an operative temperature of 22-23oC for thermal comfort in a range of RH [3]. Air speed increment was effective for warm feeling compensation only in a range of radiant temperatures. Relative humidity control cannot be ignored.
We might require a different dressing code for warmer environment. For this issue, we have conducted simulations to address the required dressing code [4].
Energy saving in up-turning room temperature is relatively limited as compared with the saving potential on precise demand-control of ventilation. We have estimates for entire office stock in Hong Kong [5].
Warmer environment promote additional bacteria and fungi growth [6]. We may require additional measures for cleanliness control.
Warm feeling lead to potential productivity loss for some office tasks [7].
Reference
[1] Environmental Education; Environmental Education and Awareness in Hong Kong. Hong Kong Environmental Protection Department, Government of the HKSAR, China; 2006.
[2] Electrical and Mechanical Services Department. Guidelines on Energy Efficiency of Air Conditioning Installations, HKSAR, China: 1998.
[3] Mui KW, Wong LT, 2007. Neutral temperature in subtropical climates - a field survey in air-conditioned offices, Building and Environment 42(2) 699-706.
[4] Mui KW, Wong LT, Hui PS, 2009. Indoor environmental quality benchmarks for air-conditioned offices in subtropics, Indoor and Built Environment 18(2) 123-129.
[5] Wong LT, Mui KW, Shi KL, 2008. Energy impact of indoor environmental policy for air-conditioned offices of Hong Kong, Energy Policy 36(2) 714-721.
[6] Wong LT, Mui KW, Hui PS, Chan WY, Law AKY, 2008. Thermal Environmental Interference with airborne bacteria and fungi levels in air-conditioned offices, Indoor and Built Environment 17(2) 122-127.
[7] Wong LT, Mui KW, 2009. Efficiency assessment of indoor environmental policy for air-conditioned offices in Hong Kong, Applied Energy 86(10) 1933-1938.