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 25.5^oC

There are a number of enquiries from the local industry regarding to up-tune the air temperature from the current context (below 25.5^oC) to 25.5^oC. 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 22-23^oC 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.