A Thermal Comfort Levels Investigation of a Naturally

A Thermal Comfort Levels Investigation of a Naturally

Ventilated and Air-Conditioned Office

R. Daghigh, N.M. Adam

a Department of Mechanical and Manufacturing Engineering

University Putra Malaysia (UPM)

MALAYSIA

K.Sopian, A. Zaharim

Solar Energy Research Institute, University

Kebangsaan Malaysia (UKM)

MALAYSIA

B.B. Sahari

cInstitute of Advanced Technology

University Putra Malaysia (UPM)

MALAYSIA

Abstract:- The purpose of this study is to investigate thermal comfort levels of a naturally ventilated

and air-conditioner office. Field experiments conducted in an office room in Universiti Putra Malaysia

(UPM) used survey questionnaires and physical measurements. When air conditioner was working, it

was found that the office was slightly cool based on ISO 7730, but result of survey revealed that staff

found condition to be cool. In naturally ventilated condition objective result showed that this office is

uncomfortable but subjective study revealed that staffs found condition to be comfortable. Results of

over 40 survey responses to thermal comfort questions in study room at UPM are presented.

Responses from those staff suggest a wider acceptable temperature range for occupants.

Key-Words: Thermal Comfort, Neutral Temperature, Naturally Ventilated, Mechanically Ventilation,

Objective Study, Subjective Approach, PMV.

.

1 Introduction In recent years, Malaysia’s energy

consumption has increased. Malaysia has one

of the fastest growing building industries

worldwide, where the corresponding energy

demand for artificial cooling through the use

of air conditioning units in order to provide

comfort in building has been increased,

Therefore, methods of energy saving are very

important and urgently needed. Primary

concern should be given to making sure that

the people inside are happy and comfortable to

stay – they are not too hot or too cold

.Through the knowledge of thermal comfort

behavior of human and energy utilization

behavior of buildings, the best strategy can be

adopted [1].

Since Malaysia is in the tropical regions and

has high daytime temperatures of 29°C to

34°C [2] and relative humidity of 70 to 90%

throughout the year, Thermal condition in

offices has to be considered carefully mainly

because of the high occupant density in these

areas and because of the negative influences

that an unsatisfactory thermal environment has

on learning and performance. In tropical

regions, the hot and humid climate may have

an adverse impact on occupant comfort

indoors [3]. Both thermal comfort and air

quality can have important impacts on

productivity.

The negative effect of poor indoor climate

conditions on the performance of the occupant

is described in several earlier investigations

[4]. The significance of maintaining good

indoor climate is self-evident when one

considers that in every modern economy a

significant part of the Gross National Product

(GNP) is earned by people working in office

buildings. In view of the importance of

thermal comfort and indoor air quality, the

design challenge is to achieve acceptable

indoor environmental conditions for individual

building occupant [5].

The main objectives of this study are as

follows:

8th WSEAS International Conference on SIMULATION, MODELLING and OPTIMIZATION (SMO '08) Santander, Cantabria, Spain, September 23-25, 2008

ISSN: 1790-2769 189 ISBN: 978-960-474-007-9

• To evaluate the thermal comfort level

of a naturally and mechanically

ventilated office.

• To validate the level of acceptability

of the findings with AHRAE 55 and

ISO 7730[6, 7].

• To determine neutral temperature in

naturally ventilated and air-

conditioned office, and

• To investigate the staff's’ perception

of the degree of comfort and indoor air

quality in two conditions.

2 Methodology To determine the thermal comfort

requirements for office, experimental work

and survey study were carried out in an office

(Figure 1) at the eighth level of block A office

building faculty of engineering in University

Putra Malaysia (UPM).

Figure 1: View of study office room and BABUCA

Four environmental variables were measured

using a measuring physical quantities

instrument (BABUCA): air velocity, relative

humidity, dry bulb temperature and mean

radiant temperature. Physical measurements

were carried out at one point in the middle of

office for two conditions. In one case air

conditioner was switched off (Naturally

ventilated condition) and in the other condition

air conditioner was switched on (Mechanically

ventilated condition). The samples were

recorded every one minute interval. Having

measured the environmental parameters, the

two personal parameters, metabolic rate and

clothing insulation were estimated in

accordance with ISO 7730. In this study, the

metabolic rate is set to be 1.2 met [7] which is

sedentary activities (office, dwelling, school,

laboratory) whereas the Clo-value (thermal

resistance) is set to be 0.5 where the males

were wearing underpants, shirt with short

sleeves, light trousers, light socks and shoes.

The females were wearing ‘baju kurong’

which is cotton or silk with light cotton

undergarments and a lightweight scarf.

Measurements of thermal comfort parameters

conducted in the middle of October, 2006.

Assessment of thermal comfort in the office

was based on responses to a questionnaire

survey, which was administered

simultaneously with the physical

measurements in each condition. A total of 40

respondents participated in the survey; all of

them were staff. The dominant gender

distribution sampled was female (60%) The

total response rate was 100%. Prior to the

survey, the subjects would have been seated at

their chairs for approximately 30 min [8], with

mostly sedentary activities. Sufficient time for

body precondition in each survey was

necessary to maintain respondent’s metabolic

rate (M) at the same level throughout the study

which was estimated to be equal to 1.2 met.

3 Results and Discussions Evaluation of Thermal Comfort

From the thermal comfort parameters, PMV

and PPD for naturally and mechanically

ventilated office using InfoGap and Microsoft

Excel was calculated. The ranges of predicted

mean vote (PMV) for naturally ventilated

office (Air conditioner off) was between 1.3-

1.4 and percentage people dissatisfied (PPD)

are ranged between 40-45% as shown in

Figure 2. Based on ISO 7730, the comfort

range was taken to be the conditions when the

PMV has the values between –1 and +1. This

result shows that more than half of the

occupants felt thermally uncomfortable in the

office space during sedentary activities at that

period of time with an average temperature of

28.6ºC and relative humidity of 68.1%.


ISSN: 1790-2769 190 ISBN: 978-960-474-007-9

Figure 2: Naturally ventilated Office- Predicted

Percentage of Dissatisfied (PPD) as a Function of

Predicted Mean Vote (PMV). ( ISO 7730)

For the mechanically ventilated condition (Air

conditioner on) as shown in Figure 3, the

predicted mean vote (PMV) is in the range of

(-1)-(-0.5) and percentage people dissatisfied

(PPD) is 10-25 %. This analysis resulted from

the measurement in another day at the same

time from 9.00 a.m. to 17.00 p.m. when air-

conditioner was on all the time, and it shows

that the office is thermally comfortable in this

time with an average temperature of 23.5ºC

and relative humidity of 56.1%.

The ASHRAE Standard 55-1992 states that

the comfort zone for summer conditions, air

temperature to be between 23ºC to 26ºC and

relative humidity between 20% to 60 %.

Figure 4 showed that the office is not within

the comfort range during working hours in

naturally ventilated condition but it is within

the comfort range for mechanically ventilated

condition. It can be observed that the state of

temperature for naturally ventilated office is

far from the thermal comfort of 26°C from the

guideline given by Malaysian Energy Efficient

Guideline, thus hard to achieve thermal

comfort during office hours from natural

ventilation according to Malaysian Energy

Efficient Guideline. The Department of

Standards Malaysia [9] recommended indoor

design temperature range from 23ºC to 26ºC.

Figure 3: Air-conditioned Office-Predicted

Percentage of Dissatisfied (PPD) as a Function of

Predicted Mean Vote (PMV). ( ISO 7730)

Figure 4: Thermal Comfort Range Based on

Psychometric Chart – ASHRAE 55-92

Surveys of human thermal response in South

East Asia produce the following equation

(Auliciems' equation) for estimating thermal

neutrality in base on the mean monthly dry

bulb temperature Tm [10]:

T n = 17.6 + 0.31 T m

It applies to both naturally ventilated and air-

conditioned buildings.

0102030405060708090

100110

-3 -2 -1 0 1 2 3

PMV

PPD(%)

0102030405060708090

100110

-3 -2 -1 0 1 2 3

PMV

PPD(%)


ISSN: 1790-2769 191 ISBN: 978-960-474-007-9

Regarding this equation the thermal neutrality

for office were, 26.2°C and 27.2°C for

naturally and mechanically ventilated

conditions, respectevely. The neutrality

temperatures (24.6, 26.1ºC and 27.4ºC) for

Malaysia [11] and [12] found earlier

correspond well to 27.4ºC ET* obtained in

Thailand and 28.5ºC in Singapore [13].

The proposed neutral temperatures are higher

than 24.5ºC recommended by ASHRAE

Standard 55. Result of this study like previous

studies in Malaysia and south east of Asia

suggest a wider thermal comfort range for

Malaysian than that proposed by international

standards, i.e., ASHRAE Standard 55-92,

which indicates that Malaysian are

acclimatized to much higher environmental

temperatures.

Evaluation of questionnaire

Figure 5 shows the profile of Thermal Vote

cast on the ASHRAE scale for office room at

naturally and mechanically ventilated

conditions. From the relative frequency of

votes in each category can be seen that the

thermal vote for naturally ventilated condition

centered on 1 and for mechanically ventilated

condition -2.

Figure 5: Relative Frequency of ASHRAE Thermal

Votes

Human thermal comfort depends on six

thermal ‘‘quantitative” variables. However,

thermal comfort not only depends on those six

variables but also on some ‘‘non quantitative”

factors such as mental states, habits, education.

Many studies confirm that human comfort

preferences vary in different locations and

long term experience in any climate. This may

result in endurance to higher temperatures of

people in hot and humid climate when

compared to people in colder regions.

Figure 6 gives a comparison of the various

methods of assessing acceptability for two

conditions. By equating the central three

categories of the ASHRAE 55 scale with the

notion of acceptability, 70% and 80% of the

staff is assumed to be satisfied with the

thermal condition in their office in naturally

and mechanically ventilated conditions,

respectively. The direct vote of acceptability is

80% and 90%, respectively. In contrast, the

thermal preference scale appears to be only

40% and 50% of the respondents in naturally

ventilated condition and air-conditioned office,

relatively. Different results can obtain from

different method of measurements and it is

similar to other studies [3 and 14].

Figure 6: Comparisons of Various Methods of

Assessing Thermal Acceptability

Figure 7 shows the opinions of staff on the air

quality in office for two conditions. Staffs

were satisfied with the air quality, so air

quality in the office room was within tolerable

limits for staff. ASHRAE Standard 62 [15]

defines acceptable air quality as conditions in

which more than 80% of people do not express

dissatisfaction, the information obtained from

the measurement and questionnaire show that

office in two conditions has good air quality.

4 Conclusions • Objective measurement of the office

in naturally and mechanically

ventilated conditions showed that

mechanically ventilated office had

0

20

40

60

80

100

Votes in central 3

Categories of ASHRAE

Scale

Votes of Direct

Acceptability

Votes of Preference

Methodes

Pe

rce

nta

ge

of

Vo

tes

(%)

NV AC

0

20

40

60

80

-3 -2 -1 0 1 2 3

ASHRAE Thermal Sensation Scale

Pe

rce

nta

ge

of

Vo

tes

(%

)

NV AC


ISSN: 1790-2769 192 ISBN: 978-960-474-007-9

Figure 7: Distribution of Air Quality

thermal conditions falling within the

comfort zone of ASHRAE standard

55-1992 and ISO 7730, but naturally

ventilated study office was not within

comfort range based on these

standards.

• The neutrality temperature is higher

than the ASHRAE Standard 55 for

two conditions. Results suggest a

wider thermal comfort range for

Malaysian than that proposed by

international standards. Therefore

adopting the international Standards

for interior comfort conditions for the

Malaysian hot and humid tropical

climate may lead to overcooling and

energy waste.

• A comparison of the various methods

of assessing thermal comfort reveals

that they produce diverse results. A

comparison of votes on the ASHRAE

scale with those on the direct scale

showed that a large percentage of

people voting at the cooler categories

found their thermal state acceptable in

naturally ventilated office. This

affirms the postulation that people in

the tropics voting in the extreme

categories of the ASHRAE scale may

not necessarily be in discomfort.

• In this study revealed that based on

Standards office was comfortable, but

most of occupants found thermal

condition in their environment

uncomfortable (cool) for air-

conditioned condition.

• The information obtained from

questionnaires show that the naturally

ventilated office and air-conditioned

office has good air quality.

• The percentages of thermal sensation

vote under room temperature and

humidity for air conditioned condition

indicate that to achieve indoor thermal

comfort, air conditioned office which

possesses low indoor temperature can

set the temperature higher than the

current setting; hence energy

consumption will be minimized. In

Malaysia, most offices have

traditionally relied on a ceiling fans

for naturally ventilated condition and

combination of air-conditioner and

fans to achieve thermal comfort.

Malaysian people can have a tolerance

to higher air temperature, and also,

adapted to high air movement from

fans in naturally ventilation condition. The measured rooms were

naturally

ventilated without ceiling fans. Ceiling fans should be used and

installed in this office to help energy

saving by increasing the room air

temperature set point and air

movement to provide a comfortable

indoor climate, and also, it can reduce

energy consumption in naturally and

air-conditioned conditions. Fans save

energy when they complement a

regular air-conditioning system and

move air at an acceptable speed

without compromising thermal

comfort. They allow a higher set-

point temperature, thus reducing

A/C operating hours and saving

electrical energy.

References

[1] Ahmad, A. (2004). Case study:

Thermal comfort study demonstration

low energy office (LEO), MSc.

Thesis, University of UKM, Malaysia.

0 10 20 30 40 50 60 70 80

Very Good

Good

Satisfactory

Bad

(%)

Distribution of Air Quality

NV AC


ISSN: 1790-2769 193 ISBN: 978-960-474-007-9

[2] Abdul Rahman, S. and Kannan, K.S.

(1997). Air Flow and Thermal

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ISSN: 1790-2769 194 ISBN: 978-960-474-007-9

A Thermal Comfort Levels Investigation of a Naturally

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