Basic Mechanical Engineering
[MECE2011]
B.Sc Civil Engineering
Credit hours – 1+0
Lecture – 5
Heating and Cooling Load Calculations
Department of Mechanical Engineering
Capital University of Science and Technology
Islamabad, Pakistan
Introduction
 Buildings are built to provide a safe and comfortable internal
environment despite variations in external conditions.
 The extent to which the desired interior conditions can be
economically maintained is one important measure of the success of
a building design.
 Although control of inside conditions is usually attribute to the active
heating and cooing system, the design of HVAC must start with the
examination of the thermal characteristics of the envelope.
 They influence both the equipment capacity and the energy required
for its operation.
 The primary objective of this paper is to examine procedures for
evaluating the impact of the thermal characteristics of the building
envelope on the design of HVAC systems used to maintain comfort.
Health and Comfort Criteria
 The human body is an amazingly adaptable organism.
 With long-term conditioning, the body can function under quite
extreme thermal conditions.
 Variations in outdoor temperature and humidity, however, often go
beyond the normal limits of adaptability.
 Therefore, It becomes necessary to provide modified conditions
indoors in order to maintain a healthy, comfortable environment.
Thermal Comfort
 The figure illustrates the factors
that influence thermal comfort.
 First, body heat is generated b
metabolic processes to maintain
body temperature.
 Metabolic
processes
are
influenced by such factors as age,
health and level of activity.
 For example, a given range of
environmental conditions might
be quite acceptable in a space
occupied by a healthy person but
unacceptable for one who is ill.
 When people are willing to adjust their dress habits with the
changing seasons, they find that they are comfortable over a broader
range of environment conditions than they would expect.
Thermal Comfort
 The
body
is
continuously
generating heat, which must be
dissipated to maintain a constant
body temperature.
 Temperature control can be
accomplished
by
various
mechanisms.
 For a person at rest of doing light
word in a conditioned space, the
body dissipates heat primarily by
convection (carried away by the
surrounding air) and radiation ( to
surrounding surfaces that are at
lower temperature than the body
surface.
Thermal Comfort
 Each of these components faces
of heat dissipation accounts for
approximately 30 percent of the
heat loss.
 Evaporation,
from
both
respiration and perspiration,
accounts for the remaining 40
percent.
 As environmental conditions or
levels of activity change, these
percentages will vary.
 For example, if a person is doing
strenuous work, the primary heat
dissipation mechanism will be
evaporation.
Thermal Comfort
 Four
environmental
factors
influence the body’s ability to
dissipate heat
 Air temperature,
 The temperature of the
surrounding surfaces,
 Humidity,
 Air velocity.
 The amount and type of clothing
and the activity levels of the
occupants interact with these
factors.
 In designing an air conditioning
system we turn our attention to
the control of these four factors.
Thermal Comfort
 If a person is wearing appropriate
clothing, the following ranges
should usually be acceptable.
 Operative temperature :
20 to 26 oC.
 Humidity. A dew-point
temperature of 2 to 17 oC .
 Average air velocity: Up to 0.25
m/s.
Air Quality
 Air quality must also be maintained to provide a healthy, comfortable
indoor environment.
 Sources of pollution exist in both the internal and external
environment.
 Indoor air quality is controlled by removal of the contaminant or by
dilution.
 Ventilation plays an important role in both processes.
 Ventilation is defined as supplying air by natural or mechanical means
to a space.
 Normally, ventilation air is made up of outdoor air and recirculated
air.
 The outdoor air is provided for dilution.
 In most cases odor and irritation of the upper respiratory tract or
eyes are the reason for ventilation rather than the presence of health
threatening contaminants.
Air Quality
 If the level of contaminants in outdoor air exceeds that for minimum
air quality standards, extraordinary measures must be used.
 For the present discussion, it will be presumed that outdoor air
quality is satisfactory for dilution purposes.
 Following table presents outdoor air requirements for ventilation for
three occupancy types listed in standard.
 As noted in table, much larger quantities of air are required for
dilution in areas where smoking is permitted.
Air Quality
 Ventilation imposes a significant load on heating and cooling
equipment and thus is a major contribution to energy use.
 Space occupancies and the choice of ventilation rates should be
considered carefully.
 For example, if smoking is permitted in part of a building but
restricted in another part of the building, ventilation rates for
smoking should not be assumed uniformly.
 Also, the prospect of filtering and cleaning air for recirculation must
be examined carefully.
 The use of recirculated air will conserve energy whenever the
outdoor-air temperature is extremely high or low.
Air Quality
 The ASHRAE standard provided the following procedure for
determining the allowable rate for recirculation
𝑽. = 𝑽.𝒓 + 𝑽.𝒎
 Where,
 𝑉 . = rate of supply air for ventilation purposes, L/s
 𝑉𝑟. = recirculation air rate, L/s
 𝑉𝑚. = minimum outdoor air rate for specified occupancy, for example
the non-smoking value from Table 4-1, but never less that 2.5 L/s per
person
 Also,
. − 𝑽.
𝑽
𝒐
𝒎
𝑽.𝒓 =
𝑬
 𝑉𝑜. = outdoor air rate from Table 4-1 for specified occupancy (smoking
or non-smoking, as appropriate). L/s
 E = Efficiency of contaminant removal by air cleaning device.
Air Quality
 E = Efficiency of contaminant removal by air cleaning device. The
efficiency must be determined relative to the contaminant to be
removed.
 Table 4-2 provides values appropriate for removal of 1 μm particles.
Example 4-1
 Determine the ventilation rate, outdoor-air rate, and recirculated-air
rate for an office building meeting room if smoking is permitted. Air
air-cleaning device with E = 60 percent for removal of tobacco smoke
is available?
 Solution:
.
.
𝑽
−
𝑽
𝒐
𝒎
𝑽.𝒓 =
𝑬
 𝑽.𝒐 = outdoor air rate from Table 4-1 for specified occupancy (smoking
or non-smoking, as appropriate). L/s; From Table 4-1, outdoor air
requirements rate per person for meeting and waiting spaces with
smoking permitted = 17.5 L/s = 𝑽.𝒐
 𝑽.𝒎 = minimum outdoor air rate for specified occupancy; From Table
4-1; required outdoor-air rate for non-smoking spaces = 3.5 L/s = 𝑽.𝒎


.
𝑉𝑜. −𝑉𝑚
17.5−3.5
Ventilation rate;
=
=
= 23.3 L/s = 𝑽.𝒓
𝐸
60/100
Recirculated air rate = 𝑽. = 𝑉𝑟. + 𝑉𝑚. = 23.3 + 3.5 = 26.8
𝑽.𝒓
L/s per person
Estimating Heat Loss and Heat Gain
 Heat transfer through a building envelope is influenced by the
materials used; by geometric factors such as size, shape and
orientation; by the existence of internal heat sources; and by climate
factors.
 System design requires each of these factors to be examined and the
impact of their interactions to be carefully evaluated.
 The primary function of heat loss and heat gain calculations is to
estimate the capacity that will be required for the various heating
and air conditioning components necessary to maintain comfort
within a space.
 These calculations are therefore based on peak-load conditions for
hearing and cooling and correspond to environmental conditions
which are neat the extremes normally encountered.
 Standard outside design values of temperature, humidity and solar
intensities are usually available from handbooks.
Estimating Heat Loss and Heat Gain

1.
2.
3.
4.
Loads are generally divided into the following four categories.
Transmission
Solar
Infiltration
Internal
1. Transmission: Heat loss or heat gain due to a temperature difference
across a building element.
2. Solar: Heat gain due to transmission of solar energy through a
transparent building component or absorption by an opaque
building component.
Estimating Heat Loss and Heat Gain

1.
2.
3.
4.
Loads are generally divided into the following four categories.
Transmission
Solar
Infiltration
Internal
3. Infiltration: Heat loss or heat gain due to the infiltration of outside
air into a conditioned space.
4. Internal: Heat gain due to the release of energy within a space
(lights, people, equipment, etc)
In response to these loads, the temperature in the space will change or
the heating or cooling equipment will operate to maintain a desired
temperature.
Design Conditions
 The design conditions usually specified for estimating heating loads
are the inside and outside dry bulb temperatures.
 For heating operation, an indoor temperature of 20 to 22 oC is
generally assumed and for cooling operation, 24 to 26 oC is typical.
 A minimum relative humidity of 30 % in the winter and a maximum of
60 % in the summer is also assumed.
 For heating operation, the 97.5 percent value of the outside
temperature is usually chosen.
 This means that on a long-term basis of the outside dry bulb
temperature equals or exceeds this value for 97.5 percent of the
hours during the coldest months of the year.
 At the 97.5 percent outdoor temperature, the air is assumed to be
saturated.
Design Conditions
 The set of conditions specified for the cooling load estimates is more
complex and includes dry-bulb temperature, humidity and solar
intensity.
 Peak-load conditions during the cooling season usually correspond to
the maximum solar conditions rather than to the peak outdoor-air
temperature.
 Thus, it is often necessary to make several calculations at different
times of the day or times of the year to fix the appropriate maximum
cooling capacity requirements.
 When the cooling load calculation is made, will depend on the
geographic location and on the orientation of the space being
considered.
 For example, peak solar loading on an east facing room may occur at
8 AM, while for a west room the maximum load may occur at 4 PM.
Design Conditions
 Peak solar loads for south-facing rooms will occur during the winter
rather than the summer.
 Of course, when a cooling system serves spaces with different
orientations, the peak system load may occur at a time other than
the peak for any of the several spaces.
 Fortunately, after making a number of such calculations, one begins
to recognize likely choices for times when the peak load may occur.
 Table 4-3 provides outdoor design temperature data for a number of
locations.
 The table provides the 97.5 percent dry-bulb temperature for winter
and the 2.5 percent dry bulb and coincident wet bulb temperature
for summer.
 The 2.5 percent dry bulb temperature is the temperature exceeded
by 2.5 percent of the hours during June to September.
 The mean coincident wet-bulb temperature is the mean wet-bulb
temperature occurring at that 2.5 percent dry bulb temperature.
Design Conditions
Example 4-2
 Select outside and inside design temperatures for a building to be
constructed in Denver, Colarado.
 Solution:
 From Table 4-3, the summer design conditions are given as:
 Summer design dry bulb temperature = 33 oC
 Coincident wet-bulb temperature = 15oC
 Assuming that no special requirements exist, an inside design
temperature of 25oC and 60 % relative humidity is chosen.
 The winter design outside temperature is -17oC from Table 4-3, and if no
special requirement exist, an inside design temperature of 20oC and 30%
relative humidity is chosen.
 It should be noted that the inside design temperature only limits the
conditions that can be maintained in extreme weather. During heating
operation, when the outside temperature is above the outside design
value, an inside temperature greater than 20oC can be maintained if
desired.