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Proceedings of 7th

Windsor Conference: The changing context of comfort in an

unpredictable world Cumberland Lodge, Windsor, UK, 12-15 April 2012. London:

Network for Comfort and Energy Use in Buildings, http://nceub.org.uk

1

Significance of air movement for thermal comfort in warm climates:

A discussion in Indian context

Madhavi Indraganti 1,*

, Ryozo Ooka1, Hom B Rijal

2

1Institute of Industrial Science, The University of Tokyo, Japan

2Tokyo City University, Japan

* Corresponding Author: [email protected]

Abstract

Thermal comfort research is yet to gain momentum in India. Indian designers

follow verbatim the ASHRAE standard when designing the indoor environments.

This only leads to over design/ energy wastage and in environments inappropriate to

the local climates and customs. Our earlier study in 2008 highlighted the wide gulf

between the actual comfort temperature recorded on field and that specified in the

National Building Code- 2005. Therefore, comfort studies are exigent in India.

Addressing this need, the authors are conducting a field study in warm-humid and

composite climates of India from January 2012. The current paper highlights the

significance of air movement for comfort at elevated temperatures as observed in our

Hyderabad study. Subjects using fans had higher comfort temperature than those

without. The occupants successfully achieved higher air velocities through the use of

various personal environmental controls in order to comfortably offset the

discomfort during the warm-humid months.

Keywords

Comfort temperature; Air movement; Humidity; Thermal Comfort research in

India; Griffiths Method

Introduction

Thermal comfort research is mainly concentrated in the west and in some parts of

Asia and Africa. This is perhaps one of the reasons why thermal comfort standards are

not defined yet in India. In the absence of comfort standards and very little first- hand

field work (Sharma and Ali, 1986, Indraganti, 2010a), the tendency is to follow the

comfort standards as described in ASHRAE –Std-55 (2005) verbatim. However, the

base cases used in the above standard are predominantly based on western climates

and do not necessarily represent the wide ranging tropical climates in India. This

inappropriate standard neither matches the social attitudes nor cultural variety found

in Indian indoor environments. It is thus leading to over specification of air

conditioning systems and an enormous energy misuse.

On the contrary, several studies in the warmer climates have shown that increased

air movement can comfortably offset thermal discomfort at high indoor temperatures

without compromising the overall acceptability of the environment (Mallick 1996,

Nicol 2004, Brager 2000). The present paper discusses the importance of air

movement in Indian context, referring to literature on this, our earlier (Indraganti,

2010a) and present studies in India.

2

Ergonomics of thermal comfort

Thermal comfort is a six- dimensional topological solid, having at least six

parameters that give dimensions to any unique thermal condition. Two of these,

activity and clothing are specific to an individual while the other four are the

properties of the environment itself – air temperature, humidity air velocity and

radiation. Under isothermal and steady state conditions, the heat balance of the body

can be defined by these vital six variables.

While there are several other minor parameters like health, light etc. which also

influence thermal comfort, Fanger (1972) clarifies that comfort can be achieved by

many different combinations of the above variables. The effect of any of these factors

should not be considered independently as the effect of each of them depends on the

level and conditions of the other factors, and also by the use of many fundamentally

different technical systems (both passive and active). The effect of humidity and air

velocity shall be discussed in detail as under.

Humidity

While conductive heat gain is affected by the air and skin temperature difference,

the rate of heat exchange depends on the air velocity and the clothing. Although

humidity of air does not directly affect the heat load operating on the body, it

determines the evaporative capacity of air and hence the cooling efficiency of

sweating. In extremely hot conditions the humidity level determines the limits of

endurance time by restricting the total evaporation (Givoni, 1969).

Markus and Morris (1980, pp 59) add to this bio physical phenomena that,

evaporation is critical to heat loss from the body at high temperatures (above skin

temperature), and high humidity at this level impedes the rate of evaporation.

Evaporative cooling was exploited in many different ways both indoors and outdoors

in several cultures, since antiquity, for ex. pools in open courts and wind towers with

water sprays or pots in warm climates. There is no evidence that extreme humidities

are undesirable, from thermal comfort stand point. However, it leads to unwanted side

effects, such as ‘wettedness’ sensation at high humidity (June and July months in

Hyderabad) and dehydration of mucous membranes at low humidity (April and May).

Maroof and Jones (2009) from their Malaysian Mosque study point that humidity

had an overarching influence on thermal comfort than temperature, and that 30°C was

tolerable, with even a minor increase in the humidity adversely effecting the comfort

than temperature. Literature indicates that, relative humidity as low as 9 % is judged

comfortable over long periods of time. However, an acceptable range extends from 30

% to 65 %, with the optimum at about 50 %. High relative humidity, together with

high air temperature, increases heat stress because the body cannot be cooled by

evaporation.

Air velocity

Air velocity (VA) has little effect on evaporation at low humidity, as it takes place

readily. However it is of great importance in hot- dry conditions in affecting the

convection transfer. Though its effect is limited at high humidity, it is vital, as the

atmosphere’s ability to absorb moisture is limited. At increased air velocities, most

people remain comfortable even at higher relative humidity.

Nicol (1993) reports that, the effect of air movement is generally considered to be

roughly proportional to the square root of the air velocity. He has identified in his

3

Baghdad and Roorkie studies (Nicol, 1974) that there is little difference in the comfort

vote with VA ½

in the range of 0.5 to 1.0 (m/s)½ and when VA½

exceeds 1.0, i.e., the

major effect on comfort is observed than when VA ½

exceeded 0.5. He has also noted

that air movement has reduced discomfort from heat at temperatures above 31 °C; at

temperature exceeding 40 °C discomfort from heat is experienced whatever the air

velocity. The ISO - 7730 recommends that the mean indoor air velocity should be less

than 0.25 m/s for moderate thermal environments with light, mainly sedentary activity

during cooling season, and in winter it should be less than 0.15 m/s.

It is worth noting that acclimatised populations in warm humid climates report

comfort even at air velocities above 1.5 m/s, when air temperature is well above the

skin temperature and humidity is high (Krishnan, 2001). Nicol (1974) has similar

findings to present; in the range of 32- 40 °C, moderate air movement (VA > 0.25 m/s) 1 reduced both thermal discomfort and skin moisture in hot climates

2. However, air

velocities above 2 m/s are undesirable for prolonged periods because of other

inconveniences, (papers flying off, window shutters fluttering etc.). Thus, comfort can

be achieved over a much wider range of conditions than present standards recognize,

necessitating the need for less conservative comfort standards.

Role of clothing in warm climates

“While buildings are our third skin, clothing forms the second skin.

Clothing and buildings are similar in that, both use passive devices to

control natural flows of heat, air, and moisture vapor for the increased

comfort of the wearer or occupant”- Olgyay.

As Olgyay (1963) says, clothing forms a barrier to the convective and radiant

heat transfer between the body and the environment, as it interferes with the process

of sweat evaporation. It reduces the body’s sensitivity to variations in air temperature

and velocity, forming a protective layer.

At air temperatures below 35 °C the effect of clothing is always to reduce the

‘dry’ heat loss from the body, producing a heating effect. At air temperature above 35

°C (the case in Hyderabad Summer) the effect of clothing is quite complicated.

Clothing reduces the “dry” heat gain from the environment on one hand; it increases

the humidity and reduces the air velocity over the skin on the other hand. It results in

a reduction of the cooling obtained from sweat evaporation. In this case the

evaporation takes place mostly from the clothing and not from the skin, reducing the

cooling efficiency of evaporation. The net resultant evaporative cooling depends on

the metabolic rate, humidity and air motion (Givoni, 1969).

Significance of Air Movement and Ventilation in warm environments

Areas with poor thermal comfort conditions are mainly due to the poor natural

ventilation (Wang and Wang, 2005). Both natural and mechanical ventilation serve

the dual purpose of eliminating indoor surplus heat and contaminants in time. In fact,

air movement is the only way to achieve physiological comfort at high temperatures,

as it affects both evaporative and convective heat losses from the human body. Air

exchange efficiency and ventilation efficiency can both reflect the capacity for

eliminating indoor contaminants of ventilation system (Su et al. 2009). Holm and

1 These can be easily achieved with the use of a ceiling fan.

2 The humidity is too low to affect the sensation of warmth.

4

Engelbrecht (2005) maintain that air movement at a temperature below 37 °C cools

the body, while heating it at an air temperature above 37 °C.

The frequency distribution of wind velocity in natural ventilation is skew, and

people enjoy the fluctuations and remain comfortable, while that of mechanical

ventilation system is normal and has an unfavourable effect on thermal comfort. It is

well known that the fluctuation of natural wind can make people more comfortable3

and closer to nature. Furthermore, the airflow of natural ventilation moves at a low

speed for long time which can reduce the feeling of tiredness. The larger turbulence

intensity of natural wind is also believed to enhance the feeling of comfort, for it

intensifies the heat convection between people and the environment. Therefore,

natural ventilation is better than mechanical ventilation on the whole (Su et al. 2009).

ASHRAE (Std-55, 2004) indicates that acceptable indoor air speed in warm

climates should range from 0.2 to 1.50 m/s; yet 0.2 m/s for air conditioned

environments. These ranges specified by ASHRAE do not explicitly address air

movement acceptability, but focus mainly on overall thermal sensation and comfort.

Zain et al. (2007)4 find that in warm humid climes an increase in air movement from

0.0 m/s to 0.7 m/s has substantially increased the PPS from 44% to 100% at

temperatures around 28.69 °C. Maarof and Jones (2009) point out that at high

temperature (>30 °C) and high humidity (>70%), continuous air moment is important,

rather than spasms of air drafts.

While investigating the relation between air movement acceptability and thermal

comfort inside buildings, Cândido et al., (2010)5 have found that, at operative

temperatures above 24 °C, building occupants preferred mean air speeds up to 1

m/s. It has also been observed that complaints of draft did not occur in significant

numbers until air speeds exceeded 1 m/s; and natural ventilation combined with solar

protection, is the most efficient building design strategy to achieve thermal comfort

without (Stein and Reynolds, 2000, p. 57) resorting to mechanical cooling in warm

humid climes.

Similarly, Cheng and Ng (2006) reported that, when airspeed was increased to

about 1.5 m/s, the upper comfort temperature limit further moved up by 1.5 degC to

about 33.5 °C and the predicted comfort range far above the maximum outdoor air

temperature. This study suggested that, indoor natural ventilation of airspeeds up to

1.0 - 1.5 m/s is likely to be satisfying the thermal comfort requirement of 80% of the

occupants in hot summer period. Interestingly, Heidari (2008) found increased air

movement above 37 °C to be counterproductive, resulting in heating sensation.

Brager et al. (2004), report from their field study of NV office buildings that

peoples’ preferences for higher air movement have increased with increase in thermal

sensation. They also show that people consciously recognize air movement as having

a direct impact on their thermal comfort, and that their air movement preferences are

for a change of air movement as needed to return to comfort quickly, especially in

warm humid climates (Feriadi et al., 2004).

3 It well accords with the 1/f rhythm of body also

4 Thermal comfort field study of residential environments in Malaysia

5 Studied the effects of natural ventilation on thermal comfort and air movement acceptability of a

large number of university students inside naturally ventilated buildings in Brazilian hot humid zone

5

Perception of Air Movement

It is based on several parameters such as air velocity, air velocity fluctuations, air

temperature, and personal factors such as overall thermal sensation, clothing

insulation and physical activity level (metabolic rate) (Toftum, 2004). Even for the

same individual, sensitivity to air movement may change from day to day as a result

of, e.g., different levels of fatigue.

Draught is defined as an unwanted, local cooling of the body caused by air

movement. Toftum (2004) reports that, at temperatures up to 22-23 °C, at sedentary

activity and with occupants feeling neutral or cooler, there is a risk of air movement

being perceived as unacceptable, even at low velocities. Supplementing this, he points

that, a cool overall thermal sensation negatively influences the subjective perception

of air movement. When occupants feel warmer than neutral, at temperatures above 23

°C or at raised activity levels, humans generally do not feel draught at air velocities

typical for indoor environments (up to around 0.4m/s). In the higher temperature

range, very high air velocities up to around 1.6m/s have been found to be acceptable

at air temperatures around 30°C. However, at such high air velocities, the pressure on

the skin and the general disturbance induced by the air movement may cause the air

movement to be undesirable (Toftum, 2004).

Cena and de Dear (1999) have observed in their Karlgoorlie study that 40% of the

occupants who preferred higher temperatures also asked for more air movement

regardless of the fact that this would make them feel even cooler. This may indicate

that field study respondents find it difficult to balance and express their thermal

preferences. Thus, in field settings, the interaction between draught and thermal

sensation is not as clear as in the laboratory studies, but a warmer thermal sensation

results in a preference for higher air velocities, as also found in other studies (Toftum

2004).

While Cena and de Dear (1999) point out the incongruence between draught and

thermal sensation in a field study, and recommend a climate chamber for better

results, Toftum (2002) finds that climate chamber studies for activity and air

movement interaction do not yield realistic results. He notes that subjects in real

environments are exposed to interrupted exposure to air movement and non-sedentary

tasks without any stabilization of metabolic rate. These occupants’ activity level is

higher than the level corresponding to stationary, seated work in a climate chamber.

Methods

The hot- humid climate and the composite climate (experienced in Chennai and

Hyderabad respectively) represent about 80% of the geographic area in India. The

authors are conducting a thermal comfort research in offices in these climates after

Indraganti (2010b) has identified the vital absence of thermal comfort field work in

India.

Indraganti’s (2010a) residential buildings field study was conducted in

Hyderabad (17°27’ N, 78° 28” E). This study involved over 100 occupants of

naturally ventilated (NV) apartment buildings in summer, monsoon and post-monsoon

period. Hand-held digital instruments were used to measure the environmental

variables while the questionnaires recorded clothing, activity, sensation, preference

and adaptation in both longitudinal and transverse surveys, yielding about 4000 data

sets. Table 1 presents the scales used in this study.

Methodology adapted for further studies in India

The hot humid climate and the composite climate (experienced in Chennai and



Indraganti (2010a) identified the vital absence of thermal comfort field work in India.

About 20 office buildings are selected for yearlong comfort studies in two Indian

cities (Chennai and Hyderabad) from January 2012

buildings have air conditioning, although its use is limited to the hot periods in

majority of them. The survey is planned in two levels: transverse and longitudinal.

While the former is done once a month in all the offices involving a large subject

sample, the latter is conducted throughout the year and is limited to about half of the

total buildings and a limited number of occupants in each of these buildings.

All the four environmental variables, (viz: temperature, humidity, air speed and

globe temperature) coupled with CO

adaptation are recorded through field measurements and data loggers following

standard Class –I protocols. We expect to analyse the change in comfort vote in

conjunction with the air/ globe temperature, humidity,

both environmental and behavioural.

Results

Outdoor mean temperature during the study ranged between 27.4

the survey period, with indoor temperature ranging between 26.6

relative humidity varied substantially (15

quite dry. Thermal sensation (TS)

comfort equation of TS =

presented in Indraganti (2010

velocity in achieving comfort at high temperatures.

Effect of Air Velocity

Air velocity is one of the very significant environmental variables contributing to

the thermal comfort of occupants in naturally

it varied significantly during the survey (0 ~ 4

more tolerant of high air speeds and are quite discerning about the high air speeds.

The requirement of air movement changed with the thermal sensation.

Figure 1: Linear regression of TS and PMV on indoor globe temperature (all data, p<0.001)

6

Methodology adapted for further studies in India




identified the vital absence of thermal comfort field work in India.


cities (Chennai and Hyderabad) from January 2012- January 2013. Most of these

onditioning, although its use is limited to the hot periods in



ucted throughout the year and is limited to about half of the



globe temperature) coupled with CO2 levels, clothing, metabolic activity, and


I protocols. We expect to analyse the change in comfort vote in

conjunction with the air/ globe temperature, humidity, air movement and adaptation,

both environmental and behavioural.

Outdoor mean temperature during the study ranged between 27.4-

the survey period, with indoor temperature ranging between 26.6 –

ty varied substantially (15 – 76%) as the summers in Hyderabad are

Thermal sensation (TS) varied with globe temperature (Tg

= 0.31Tg - 9.06 (r= 0.61, p<0.001) (Figure 1

Indraganti (2010b). The present discussion focuses on the role of air

velocity in achieving comfort at high temperatures.


ccupants in naturally ventilated spaces in hot climates, where

it varied significantly during the survey (0 ~ 4 m/s). It is noted that people are much


ir movement changed with the thermal sensation.

: Linear regression of TS and PMV on indoor globe temperature (all data, p<0.001)




identified the vital absence of thermal comfort field work in India.


January 2013. Most of these

onditioning, although its use is limited to the hot periods in



ucted throughout the year and is limited to about half of the



2 levels, clothing, metabolic activity, and


I protocols. We expect to analyse the change in comfort vote in

air movement and adaptation,

- 35.2 °C during

42 °C. Indoor

76%) as the summers in Hyderabad are

g) resulting in a

(r= 0.61, p<0.001) (Figure 1, Table 1) as

). The present discussion focuses on the role of air


spaces in hot climates, where

m/s). It is noted that people are much


ir movement changed with the thermal sensation.

: Linear regression of TS and PMV on indoor globe temperature (all data, p<0.001)

7

As seen in figure 2, in May, air velocity at neutrality (median VA = 0.27 m/s, n=

108) is low, with a slow increase in air velocity as the TS increases to 1 (median VA =

0.3 m/s n= 438), to 2 (median VA = 0.4 m/s n= 480) and 3 (median VA =0.41 m/s,

n=361), which is achieved by using fans most of the time.

This is equivalent to an increase in comfort temperature by about 2-3 °C (Nicol

and Roaf, 1996). It is also important to note that in May at thermal sensation of 2 and

above, higher air velocities have caused discomfort, as the humidity is very low and it

produced undesirable effects like dryness etc. A few votes in May could be recorded

when people sitting in front of a high efficiency air cooler had experienced cold

discomfort i.e. at thermal sensation -1 at slightly higher velocity (median VA = 0.3 m/s

n= 14) and at -2 (median VA =0.58 m/s n=4), i.e. air movement requirement increases

at TS 2 and 3 in June, in July higher air velocity at TS - 2 is not desired.

The requirement for higher air velocities significantly increased as the humidity

increased from May to June. This is due to the fact that mean indoor temperature is

around average skin temperature (32 – 34 °C) and humidity is also high (mean 55%).

The only way subjects can achieve physiological comfort under these conditions

is through increased ventilation. This is achieved by using the ceiling fans and a few

subjects have also used air coolers, in roof exposed flats where the radiant heat from

ceiling at midday caused most discomfort than high humidity. High air velocity is no

longer required as mean thermal sensation (and Tg) falls down in July, although the

humidity is still high (~72%).

As the room temperature is below the skin temperature in July, air velocity

beyond 0.2 m/s induces drafts and causes cold discomfort (see figure 2) at normal

activities . Conversely, at high metabolic rates (>1.7 met) and at TS (2 and 3) higher

air velocities are preferred for short periods to provide physiological comfort through

increased ventilation.

Air Movement Sensation (AMS) and Preference and Preference (AMP)

Air movement sensation and preference were evaluated based on the responses to

the questions “How do you find the air movement” and “How would you prefer to

have? (Refer Table 1). AMS correlated well with TS (r= 0.43) and with OC (r= -

0.42). It has correlated robustly with median of square root of air velocity (r= 0.87)

and with AMP (r= 0.71). People’s preferences for higher air movement increased with

increase in thermal sensation (Brager et al., 2004). Toftum (2002) found clear impact

of activity and overall thermal sensation on human sensitivity to air movement.

Figure 2: Relationship between thermal sensation and air velocity (May, June and July - all data)

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

-3 -2 -1 0 1 2 3Med

ian s

quar

e ro

ot

of

air

vel

oci

ty i

n m

/s

Thermal sensation (TS)

May

June

July

8

The subjects voted the air movement ‘low’ when voting ‘hot’ on TS scale (mean

AMS vote = -1.42). When the AMS vote was ‘1 and above’ the subjects preferred the

air movement to be lesser. Similarly the subjects voted in the comfort range on the

overall comfort (OC) scale (OC = 4, 5 and 6 on a scale 1 to 6) when the AMS was

between ‘0 and lower’ coinciding with ‘neither high nor low, high, very high’

sensations.

Air movement sensation correlated positively with the thermal effect on

productivity (r= 0.15). When the air movement sensation vote was low, the self

declared productivity vote was also low. The air movement sensation correlated

well with globe temperature (r= -0.35) and with relative humidity (r=0.23). June and

July recorded very high relative humidity coupled with moderate temperatures. This

allowed the occupants to adaptively open the windows, promoting cross ventilation.

Moreover, high humidity prompted the subjects to use the fans more, increasing

indoor air velocities. As a result, AMS vote moved up with the relative humidity.

Conversely, at high indoor temperature most of the natural cross ventilation ceased,

due to the adaptive closure of windows. This prompted most of the subjects to give a

low AMS vote, even when the percentage of subjects with ‘fan on’ was high. This

was due to the fact that, at high levels of thermal distress, the air movement

requirements of the subjects could not be satisfactorily met by the ceiling fans alone.

Figure 3 shows the distribution of AMS vote and its relationship with median of

square root of air velocity. Understandably, as air velocity reduced, AMS vote has

also reduced indicating a need for higher air movement. Conversely, 27% of the

subjects voted ‘-2 and -3’ (low and very low), on the AMS scale even when the

recorded (median) air velocities were between 0.4~ 0.7 m/s.

The highest percentage of subjects has felt the air movement to be ‘neither high

nor low’ (38%). Similarly, Ogbonna (2008) found a weak relationship between the

actual vote and the air-velocity in his Jos, Nigeria study.

It is essential to note that, the air movement sensation related strongly with

thermal discomfort. Only when a subject was under thermal distress, he/she gave a

‘very low air movement’ sensation vote (-3). At all other times, the majority (52%)

found the air movement ‘lower’, but voted in the central zone, with a skew towards

the right side of AMS scale. This was partially due to the fact that, the air movement

induced by the natural ventilation in summer was less dependent and was variable.

Table 1: Thermal comfort Scales employed (Transverse and longitudinal surveys)

Scale value Description of scale

ASHRAE 's Thermal

sensation (TS)

Air movement

sensation (AMS) Air movement preference (AMP)

3 Hot Very high

2 Warm High Much less air movement

1 Slightly Warm Slightly high A bit less air movement

0 Neutral Neither high nor low No change

-1 Slightly Cool Slightly low A bit more air movement

-2 Cool Low Much more air movement

-3 Cold Very low

9

Moreover, the efficacy of the ceiling fans also was non-uniform, resulting in

inconsistent levels of satisfaction (Indraganti, 2010c). Therefore, the subjects always

desired ‘cooler air movement’ in hot summer rather than just ‘increased air

movement’. The ceiling fans or natural wind drafts in hot weather could not provide

this. The ceiling fans re- circulated the hot air around, causing further discomfort to

the subjects.

Correlation between air movement preference, temperature and relative

humidity

Interestingly, people’s preferences for air movement (AMP) changed with the indoor

globe temperature (r = -0.25) and relative humidity (r = 0.17). The correlation figures

indicate that at higher temperature, the subjects desired increased air movement (refer

Figure 5: Influence of Fan on the comfort temperature

y = -0.0955x + 2.3848

(AMP) R² = 0.0618

y = -5.7881x + 230.03

(RH) R² = 0.57240

20

40

60

80

100

-3

-2

-1

0

1

2

3

25 30 35 40 45

Rel

ati

ve

Hu

mid

ity

(%

)

Air

Mo

vem

ent

Pre

fere

nce

(A

MP

)

Indoor Globe Temperature ( °C)

AMP

RH

Linear (AMP)

Linear (RH)

Figure 4: Distribution of AMS and its relationship with median of square root of air velocity

1 2

7

38

25

19

8

0

10

20

30

40

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Ver

y h

igh

Hig

h

Sli

ghtl

y h

igh

Nei

ther

hig

h n

or

low

Sli

ghtl

y l

ow

Lo

w

Ver

y l

ow

Fre

quen

cy

(%)

Med

ian o

f sq

uar

e ro

ot

of

air

vel

oci

ty

(m/s

)-1

Air movement sesnation (AMS)

Frequency (%)

Median of square root

of air velocity (m/s)-1

Trend line

10

Figure 4). On the contrary, increased humidity had little effect on the air movement

preference, which returned a low positive correlation value (r = 0.17).

The positive trend reflects a desire for reduced air movement during period of

moderate temperatures experienced by the subjects in the humid season. It is

imperative to note that higher temperature in Hyderabad is usually associated with

low humidity (hot-dry season), while humid periods have moderate to low

temperature.

The following observations prove the existing standards redundant: the subjects

voted neutral on much wider higro-thermal regime. When voting neutral, the indoor

globe temperature varied between 26.6 – 36.9 °C, while the relative humidity ranged

17% - 76% and the air velocity was in between 0.00 m/s to 2.8 m/s. This can be

attributed to very high levels of adaptation (through reduced clothing and metabolism)

and acclimatisation coupled with moderate expectations from the domestic

environments.

Effect of air movement (ceiling fan) on comfort temperature

“Comfort temperature is a simple consequence of people optimizing

comfort under the various constraints of fashion, fuel, costs, climate,

available technology, climate, cost of clothing, etc.” – Humphreys (2008)

In naturally ventilated buildings, comfortable indoor temperatures are shown to

follow the seasonal shifts in outdoor climate and often fall beyond the stipulated

comfort zones of ASHRAE Standard-55. Steady state comfort theory based on human

physiology cannot completely account for this. Griffiths (1990) method is suggested

as an alternate method in the literature to evaluate comfort temperature when the field

data constitutes a smaller sample (Nicol, 1995– p. 151, Rijal et al., 2008, 2010). A

detailed discussion on the Griffiths method can be found in (Rijal et al., 2008).

Equivalence between changes in comfort vote and the measured temperature are

assumed first. Nicol (1995) modifies this method further by applying it to the centroid

Figure 6: Influence of Fan on the comfort temperature

Fan on: y = 0.2303x + 23.543

R² = 0.5363

Fan off: y = 0.3309x + 20.345

R² = 0.59329.0

29.5

30.0

30.5

31.0

31.5

32.0

25 26 27 28 29 30 31 32 33 34 35 36 37

Gri

ffit

h's

co

mfo

rt t

em

per

atu

re °

C

Mean out door temperarure (binned data) °C

Fan on

Fan off

Linear (Fan on)

Linear (Fan off)

of the body of data instead of applying it to the individual votes. He uses a Griffiths

equivalence coefficient of 0.33 in his Pakistan studies, which is derived from Fanger

(1972), who through climate chamber experiments deduced that

comfort vote, a three degree rise in temperature is needed.

However, as we have obtained

study, we used the same in the Griffiths method.

temperature obtained in this method may not be valid if the mean thermal sensation is

beyond the comfort zone (i.e. bey

invalid (Nicol et al, 1995

analysis (for ex: cases when the TS

thermal sensation is close to zero,

results. This is due to the fact that the mean thermal sensation of

population hovers around the neutral point on the sensation scale

and Humphreys, 2002).

Brager et al. (2004) i

temperatures are not only influenced by season but by the degree of personal control

Subjects who have more control over thermal conditions of their workplace (in

particular, the operable window) ha

warmer than subjects with minimal control, even though they experienced the same

thermal environments and exhibited no differences in

Griffiths comfort temperature (

evaluated for all the mean outdoor temperature bins at one degree periodicity with

their corresponding thermal sensation votes. The data is grouped under two

categories: fan on and fan off. Figure

and the mean outdoor temperature (T

As the outdoor temperature increased, the comfort temperature

the cases. More importantly, the use of fan pushed the comfort temperature up,

without any significant increase in clothing etc, although the subjects are more

Figure 7: NBC (India) and the

11


ivalence coefficient of 0.33 in his Pakistan studies, which is derived from Fanger

(1972), who through climate chamber experiments deduced that, for a unit rise in

comfort vote, a three degree rise in temperature is needed.

, as we have obtained an equivalence coefficient of 0.31 in the present

study, we used the same in the Griffiths method. It is important to note that neutral


beyond the comfort zone (i.e. beyond -1.5 to +1.5) where Fanger’s prediction is

(Nicol et al, 1995- pp 152). Hence we have eliminated the outliers

analysis (for ex: cases when the TSmean was 1.5 and more). In addition, when the mean

thermal sensation is close to zero, Griffiths comfort temperature shows realistic

results. This is due to the fact that the mean thermal sensation of

population hovers around the neutral point on the sensation scale of ASHRAE

(2004) in their California study have noticed that ideal comfort

temperatures are not only influenced by season but by the degree of personal control


particular, the operable window) have a neutral temperature that is 1.5


thermal environments and exhibited no differences in clo or met.

Griffiths comfort temperature (TnG = T g_mean + (0 – TSmean/ R), with R = 0.31) is


ermal sensation votes. The data is grouped under two

categories: fan on and fan off. Figure 5 shows the variation in the comfort temperature

and the mean outdoor temperature (To_mean).

As the outdoor temperature increased, the comfort temperature is



NBC (India) and the comfort zone of Hyderabad juxtaposed on the ASHRAE

adaptive model


ivalence coefficient of 0.33 in his Pakistan studies, which is derived from Fanger

for a unit rise in

an equivalence coefficient of 0.31 in the present

It is important to note that neutral


1.5 to +1.5) where Fanger’s prediction is

outliers from the

was 1.5 and more). In addition, when the mean

Griffiths comfort temperature shows realistic

results. This is due to the fact that the mean thermal sensation of acclimatised

ASHRAE, (Nicol

n their California study have noticed that ideal comfort

temperatures are not only influenced by season but by the degree of personal control.


ve a neutral temperature that is 1.5 °C (2.7 °F)


, with R = 0.31) is


ermal sensation votes. The data is grouped under two

shows the variation in the comfort temperature

is increased in all



comfort zone of Hyderabad juxtaposed on the ASHRAE’s

12

sensitive to the changes in the temperature (slope = 0.23), than the subjects without

the use of fans (slope = 0.33). This clearly renders support to the argument that the

use of fans not only provides comfort at elevated temperatures but also without the

escalation of energy bills emanating from the use of air-conditioning.

The findings recorded by Sharma and Ali (1986) and Nicol (1974) in office

buildings of Roorkie in North India, are particularly interesting in this context. While

Nicol (1974) reported reduced skin moisture when the indoor temperature ranged

between 31-40 °C, Sharma and Ali observed a decrease of 1.4 to 3.2 degC in the

Tropical Summer Index (TSI)6, (a measure for indoor comfort) for an increase in air

velocity from 0.5 to 2.5 m/s, all else remaining unchanged in addition.

Rijal et al., (2008) analysed the Pakistan data (Nicol 1996) separately for cases

with fan on and fan off. They corroborated the earlier findings, in addition to defining

equations for estimating the indoor comfort temperature with ceiling fan in use. They

used the concept of running mean temperature for factoring in the outdoor conditions

in their equations. When the running mean temperature was 30 °C, the indoor comfort

temperature with fan on was 31 °C, which is 2.2 degC more than when the fan was off

(Rijal, 2012).

It is important to note that the comfort temperatures to the tune of 31.5 °C with

or without the use of fan were achieved in indoor environments in our Hyderabad

study, as against the stipulated standard comfort zone of (23-26 °C)(refer Figure 6).

This finding calls for a revision of standards.

Conclusions

This paper discusses in detail the relevance of increased air movement at elevated

indoor temperatures and humidity, usually encountered in warm climates in the

tropical subcontinent. A thermal comfort field study in residential buildings

conducted by the authors in India revealed the following:

(1) Higher comfort temperatures are successfully obtained through the use of

fans,

(2) The subjects are comfortable at much higher indoor temperatures than those

specified in the standards (NBC, 2005),

(3) They achieved higher indoor air velocities in the months when the humidity

was very high, through the use of many adaptive controls (fans, air-coolers, windows

and balcony doors and

(4) The residents displayed a proclivity for higher air movement indoors.

(5) The air movement preference varied with temperature, while humidity had a

little effect on the air movement preference.

These findings call for the development of thermal comfort standards custom-

made to in Indian subjects and climates. The authors are now conducting a thermal

comfort survey in the composite and warm-humid climates of India from Jan 2012 to

Jan 2013.

6 The tropical summer index is defined as the air/globe temperature of still air at 50% relative

humidity which produces the same overall thermal sensation as the environment under investigation.

13

Acknowledgements

We wish to profoundly thank Michael Humphreys and Fergus J Nicol, who

advised, e-mailed and generously sent papers and books during our Hyderabad survey

in 2008. The present thermal comfort research in Indian offices is funded by the

Japanese Society for Promotion of Science, Japan and The University of Tokyo,

Tokyo, Japan. The authors sincerely thank these two institutions.

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