Architectural Science III

( heating, cooling & ventilation) : Arch 4242

SEFINEW E.
 Chapter Two
Thermal comfort
Contents
• Thermal balance and comfort
• Factors of thermal comfort
• thermal adjustment mechanisms
• Comfort indices, comfort zone
 THERMAL COMFORT

Thermal comfort: It is defined as the state of

mind in humans that expresses satisfaction with the
surrounding environment (ANSI/ASHRAE Standard
55).
Those characteristics of environment which effects a
person heat lose.
In terms of bodily sensation thermal comfort is a
sensation of hot, warm, slightly warmer, neutral,
slightly cooler, cool and cold.
It’s the condition when someone is not feeling too
hot or too cold.
 THERMAL COMFORT

SIGNIFICANCE OF THERMAL COMFORT

Satisfaction with the thermal environment is important for its
own sake and because it influences productivity and health.
Office workers who are satisfied with their thermal
environment are more productive.
Thermal discomfort has also been known to lead to sick
building syndrome symptoms.
The combination of high temperature and high relative
humidity serves to reduce thermal comfort and indoor air
quality.
Although a single static temperature can be comfortable,
thermal delight, is usually caused by varying thermal
sensations.
 THERMAL COMFORT - THERMAL BALANCE AND
COMFORT

The human body continuously produces heat by its

metabolic processes. The heat output of an average
body is often taken as 100 W, but it can vary from
about 70 W (in sleep) to over 700 W in heavy work
or vigorous activity (e.g. playing squash).
This heat must be dissipated to the environment, or
else the body temperature will increase.
This deep-body temperature is normally about 37°C,
whilst the skin temperature can vary between 31 and
34°C
THERMAL COMFORT - THERMAL BALANCE AND
COMFORT

The body’s thermal balance can be expressed as:-

M± Rd ± Cv ± Cd - Ev =ΔS
where
M = metabolic heat production
Rd = net radiation exchange
Cv = convection (incl. respiration)
Cd = conduction
Ev = evaporation (incl. in
respiration)
ΔS = change in stored heat.
THERMAL COMFORT - THERMAL BALANCE AND
COMFORT

A condition of equilibrium is that the sum (i.e. the

ΔS) is zero and such equilibrium is a precondition of
thermal comfort. However, comfort is defined as ‘the
condition of mind that expresses satisfaction with
the thermal environment, it requires subjective
evaluation’. This clearly embraces factors beyond the
physical/physiological.
THERMAL COMFORT – FACTORS OF COMFORT

The variables that affect heat dissipation from the

body (and thus also thermal comfort) can be grouped
into three sets:
THERMAL COMFORT – FACTORS OF COMFORT

Air temperature
Air temperature is the
dominant environmental
factor, as it determines
convective heat dissipation.
Air temperature is the
temperature of the air
surrounding the body. It is
usually given in degrees
Celsius (°C)
THERMAL COMFORT – FACTORS OF COMFORT

Air temperature
The temperature in a building is based on the outside
temperature and sun loading plus whatever heating or
cooling is added by the HVAC or other heating and
cooling sources. Room occupants also add heat to the
room since the normal body temperature is much higher
than the room.
The recommended temperature range to optimize indoor
thermal comfort for most people is 19°C to 28°C. This
temperature range is appropriate for the sedentary or
near sedentary physical activity levels that are typical of
general office work. This recommendation assumes that
people dress appropriately to the external seasonal
demands.
THERMAL COMFORT – FACTORS OF COMFORT

Air movement
Air movement: accelerates convection, but it also changes the
skin and clothing surface heat transfer coefficient (reduces
surface resistance), as well as increases evaporation from the
skin, thus producing a physiological cooling effect.
Apart from the mean air velocity, also the frequency and
amplitude of the velocity variations influence the comfort.
At the same constant mean air velocity, airflows with high
level of turbulence are sensed uncomfortable.
In contrast, at high ambient air temperatures and humidity,
high air velocities lead to better thermal comfort (ceiling or
table fans in summer conditions).
THERMAL COMFORT – FACTORS OF COMFORT

Subjective reactions to air movement are:

<0.1 m/s stuffy
to 0.2 unnoticed
to 0.5 pleasant
to 1 awareness
to 1.5 draughty
>1.5 annoying.
but under overheated conditions air velocities up to
2 m/s may be welcome.
THERMAL COMFORT – FACTORS OF COMFORT
Humidity
Air will generally include moisture in the form of
water vapour. Absolute humidity is the mass of
water vapour in a volume of air divided by the
mass of dry air.
Relative humidity is the ratio between the actual
amount of water vapour in the air and the
maximum amount of water vapour that the air can
hold at that air temperature
RH = (actual water vapour density / saturation
water vapour density) x 100
THERMAL COMFORT – FACTORS OF COMFORT

Humidity
Humidity influences thermal comfort. Medium
humidities (RH 30% to 65%) do not have much effect,
The higher the relative humidity, the less heat a person
is able to lose heat through the evaporation of moisture
on the skin, and so the hotter they will feel.
Conversely, air that is too dry can cause problems such
as dry eyes, nose, ears and throat. Typically, a relative
humidity of 40 to 60% is appropriate in many
buildings.
Humidity also affects the performance of buildings,
causing slip hazards, damage to equipment and the
corrosion and decay of the building fabric as well as
poor performance of insulation
THERMAL COMFORT – FACTORS OF COMFORT

Radiant temperature
Thermal radiation is the heat that radiates from a
warm object. Radiant heat may be present if there
are heat sources in an environment.
Radiant temperature has a greater influence than air
temperature on how we lose or gain heat to the
environment.
Examples of radiant heat sources include: the sun,
fire, electric fires, ovens, kiln walls, cookers, dryers,
hot surfaces and machinery, molten metals etc.
 THERMAL COMFORT – FACTORS OF COMFORT

Radiant temperature
Radiation exchange depends on the temperature of
surrounding surfaces, measured by the MRT, or
mean radiant temperature.
MRT is the average temperature of the surrounding
surface elements,
Each weighted by the solid angle it subtends at the
measurement point.
THERMAL COMFORT – FACTORS OF COMFORT

Radiation
The MRT cannot be
measured directly, only by
a black globe
thermometer, which
responds to radiant inputs
as well as to air
temperature. This may be
a 150-mm diameter
copper ball, painted matt
black, with a thermometer
at its centre. Globe
thermometer(GT)
THERMAL COMFORT – FACTORS OF COMFORT

The effect of this MRT depends on clothing. In

warm climates (with light clothing)

in cooler climates (people with heavier clothing) it

has about the same influence as the DBT

At or near comfort levels the difference between DBT

and MRT should not be greater than about 3K.
THERMAL COMFORT – FACTORS OF COMFORT

Metabolism
Metabolic rate is a function of activity level. The
unit devised for this is the met, which corresponds
to 58.2 W/m2 area

For an average person this would be about 115 W.

With higher levels of met a cooler environment will
be preferred, to facilitate the heat dissipation.
THERMAL COMFORT – FACTORS OF COMFORT

Metabolism
When measuring metabolic rates, many factors
have to be taken into account. Each person has a
different metabolic rate, and these rates can
fluctuate when a person is performing certain
activities, or under certain environmental
conditions. Even people who are in the same room
can feel significant temperature differences due to
their metabolic rates, which makes it very hard to
find an optimal temperature for everyone in a
given location.
THERMAL COMFORT – FACTORS OF COMFORT
Clothing insulation
Clothing is thermal insulation of the body. It is measure in
units of clo. which means a U-value of 6.45 W/m2K (or a
resistance of 0.155 m2K/W) over the whole body surface. 1
clo corresponds to a 3-piece business suit, with cotton
underwear. Shorts and short sleeved shirts would give about
0.5 clo, an overcoat may add 1 or 2 clo units to a business
suit and the heaviest type of arctic clothing would be some
3.5 clo (see Section 1.2.4). If clothing can be freely chosen, it
is an important adjustment mechanism, but if it is
constrained (e.g. by social conventions or work safety) in a
warm environment, it should be compensated for by a
cooler air temperature. Acclimatization and habit (being
used to ...)is a strong influence, both physiologically and
psychologically.
THERMAL COMFORT – FACTORS OF COMFORT

Clothing insulation
During cold weather, layers of insulating clothing
can help keep a person warm. At the same time, if
the person is doing a large amount of physical
activity, lots of clothing layers can prevent heat loss
and possibly lead to overheating. Generally, the
thicker the garment is the greater insulating
abilities it has. Depending on the type of material
the clothing is made out of, air movement and
relative humidity can decrease the insulating
ability of the material.
THERMAL COMFORT – FACTORS OF COMFORT

Clothing insulation
The amount of clothing is measured against a
standard amount that is roughly equivalent to a
typical business suit, shirt, and undergarments.
Activity level is compared to being seated quietly,
such as in a classroom. This standard amount of
insulation required to keep a resting person warm
in a windless room at 70 °F (21.1°C) is equal to one
clo.
THERMAL COMFORT – FACTORS OF COMFORT

Contributing factors
Food and drink habits may have an influence on
metabolic rates, which indirectly influences
thermal preferences.
Body shape is another factor that affects thermal
comfort. Heat dissipation depends on body surface
area. A tall and skinny person has a larger surface-
to-volume ratio, can dissipate heat more easily,
and can tolerate higher temperatures than a more
rounded body shape.
Gender
Age
THERMAL COMFORT – FACTORS OF COMFORT

Thermal stress
The concept of thermal comfort is closely related to
thermal stress. This attempts to predict the impact
of solar radiation, air movement, and humidity for
military personnel undergoing training exercises or
athletes during competitive events. Values are
expressed as the Wet Bulb Globe Temperature or
Discomfort Index.
THERMAL COMFORT – FACTORS OF COMFORT

Thermal stress
Generally, humans do not perform well under
thermal stress. People’s performances under
thermal stress is about 11% lower than their
performance at normal thermal conditions. Also,
human performance in relation to thermal stress
varies greatly by the type of task you are
completing. Some of the physiological effects of
thermal heat stress include increased blood flow to
the skin, sweating, and increased ventilation.
THERMAL COMFORT - ADJUSTMENT MECHANISMS

The body has several thermal adjustment

mechanisms to survive in drastic temperature
environments.
In a cold environment the body utilizes
vasoconstriction; which reduces blood flow to
the skin, skin temperature and heat dissipation.
In a warm environment, vasodilation will
increase blood flow to the skin, heat transport, and
skin temperature and heat dissipation.
THERMAL COMFORT - ADJUSTMENT MECHANISMS

If there is an imbalance despite the vasomotor

adjustments listed above,
in a warm environment sweat production will
start and an evaporative cooling mechanism will be
provided. If this is insufficient, hyperthermia will
set in, body temperature may reach 40°C and heat
stroke may occur.
In a cold environment shivering will start,
involuntarily forcing the muscles to work and
increasing the heat production by up to a factor of
10. If equilibrium is not restored, hypothermia will
set in which, can be fatal.
THERMAL COMFORT - ADJUSTMENT MECHANISMS

longer-term adjustments
If there is long term exposure human body may involve
cardiovascular and endocrine adjustments.
In a hot climate this may consist of increased blood
volume, which improves the effectiveness of
vasodilation, enhanced performance of the sweat
mechanism, as well as the readjustment of thermal
preferences.
Under continued under heated conditions the
vasoconstriction may become permanent, with reduced
blood volume, whilst the body metabolic rate may
increase.
THERMAL COMFORT - ADJUSTMENT MECHANISMS

Models of thermal comfort

Adaptive model: states that there is an optimal
temperature for a given indoor environment
depending on the outdoor air temperature. It takes
into account that humans can adapt and tolerate
different temperatures during different times of the
year. The optimal temperature for a given time is
determined by looking at the mean outdoor
temperatures of each month of the year. Also, field
studies are performed in these areas to see what
the majority of people would prefer as their set
point temperature indoors at different times of the
year.
THERMAL COMFORT - ADJUSTMENT MECHANISMS

Based on studies seasonal adjustment of thermal

comfort expressed as ‘neutrality temperature’ (the
median of many peoples’ votes) which is:

Where Tn is neutrality temperature and

To.av is the mean temperature of the month
THERMAL COMFORT - ADJUSTMENT MECHANISMS

Models of thermal comfort

Static model: states that the indoor temperature
should not change as the seasons do. Rather, there
should be one set temperature year-round. This is
taking a more passive stand that humans do not
have to adapt to different temperatures since it will
always be constant.
THERMAL COMFORT - COMFORT INDICES,
COMFORT ZONE

Comfort zone: The range of acceptable comfort

conditions is generally referred to as the comfort
zone.
The temperature limits of such a comfort zone (for
90% acceptability) can be taken relative to the
above Tn (neutrality temperature) as from
(Tn - 2.5)°C to (Tn + 2.5)°
As thermal comfort is influenced by another three
environmental variables, attempts have been made
which would express the combined effect of all four
(or at least several) of these variables.
THERMAL COMFORT - COMFORT INDICES,
COMFORT ZONE

Bio-climatic chart
A bio-climatic chart is a graphical means of
depicting the human comfort region. It shows the
association between air speed, thermal energy, dry-
bulb temperature and relative humidity. It applies
for a person in a specific activity and wearing a
particular amount of clothes.
SET(standard effective temperature): the latest
comfort index now generally accepted. The ET
constructed for 0.57 clo and 1.25 met has been
found to be valid for the following conditions
THERMAL COMFORT - COMFORT INDICES,
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THERMAL COMFORT - COMFORT INDICES,
COMFORT ZONE