Heat Gains  Calculations  Page 1 2 3 4 5
Calculating Heat Gains
The load
on an airconditioning system can be divided into the following sections:
1. Sensible Transmission through glass.
2. Solar Gain through glass.
3.
Internal Heat gains
4. Heat gain through walls.
5. Heat gain through roof.
6. Ventilation and/ or infiltration gains.
The heat gain through the glass
windows is divided into two parts since there is a heat gain due to temperature difference between outside and inside
and another gain due to solar radiation
shining through windows.
The method adopted uses the CIBSE
guide A (2006) and CIBSE Guide J (2002)
.
The Tables that are referred to are CIBSE guide A (2006) Solar cooling loads in Tables 5.19 to 5.24.
CIBSE Guide J (2002) Air and Solair temperatures in Table 5.36 (
This
set of Tables is in Appendix A6 at the end of the guide. Table 5.36 (
If internal gains are to estimated then CIBSE Guide A (2006) Table 6.4 to 6.17 are also
required.
It would be helpful to have these
Tables close by, to complete the calculations.
An example of a heat gain
claculation is given in CIBSE Guide A (2006) section
5.8.2 example 5.3.
Heat gains through solid ground
floors are minimal and can be neglected.
1.0 Sensible Transmission Through Glass
This is the Solar Gain due to differences between inside
and outside temperatures. In very warm countries this can be quite significant.
This gain only applies to materials of negligible thermal
capacity i.e. glass.
Qg = Ag . Ug (t_{o} t_{r}) ........ eqn. 1
Where;
Qg = Sensible
heat gain through glass (W)
Ag = Surface area of glass (m2)
Ug = 'U' value for glass (W/m2 oC)
(see CIBSE guide A (2006)
Table 3.23 to 3.32).
to = outside air temperature (oC). Can be
obtained from CIBSE Guide J (2002)  Tables 5.36 to 5.38 for
various months and times in the day.
tr = room air temperature (oC)
2.0
Solar Gain Through Windows
This gain is when the sun shines
though windows.
The cooling loads per metre
squared window area have been tabulated in CIBSE
guide A (2006) Tables 5.19 to 5.24 for
various; locations, times, dates and orientations.
These figures are then multiplied
by correction factors for; shading and air node
correction factor.
Heat load is found from;
Q_{sg} = F_{c}
. F_{s} . q_{sg} . A_{g} ........
eqn. 2
where Qsg = Actual
cooling load (W)
qsg = Tabulated
cooling load from CIBSE Guide A (2006) Table 5.19 to 5.24 (W/m^{2})
F_{c} = Air
node correction factor from Table below.
F_{s} = Shading
factor from Table below.
A_{g
} = Area of glass (m2)
The Air point control factors (F_{c}) and Shading
factors (F_{s}) are given in the Table below for various types of
glass, building weights and for open and closed blinds.
Air
node correction factors (F_{c}) 


Building Weight 
Single Glazing 
Double glazing 

Horizontal blind 
Horizontal blind 

Light 
0.91 
0.91 

Heavy 
0.83 
0.90 

Shading
factors (F_{s}) 

Type of glass 
Building Weight 
Single Glazing 
Double glazing 


Open horizontal blind 
Closed horizontal blind 
Open horizontal blind 
Closed horizontal blind 

Clear 6mm 
Light 
1.00 
0.77 
0.95 
0.74 
Heavy 
0.97 
0.77 
0.94 
0.76 

Bronze tinted 6mm 
Light 
0.86 
0.77 
0.66 
0.55 
Heavy 
0.85 
0.77 
0.66 
0.57 

Bronze tinted 10mm 
Light 
0.78 
0.73 
0.54 
0.47 
Heavy 
0.77 
0.73 
0.53 
0.48 

Reflecting 
Light 
0.64 
0.57 
0.48 
0.41 
Heavy 
0.62 
0.57 
0.47 
0.41 
The CIBSE guide method of calculating solar gains through
glazing in Guide A (2006), section 5.8.1.1
uses a slightly different formula as follows;
Q_{sg} = S
. q_{sg} . A_{g}
where Qsg = Actual
cooling load (W)
qsg = Tabulated
cooling load from CIBSE Guide A (2006) Table 5.19 to 5.24 (W/m^{2})
S = Mean solar gain factor
at the environmental node or air node from CIBSE
Guide A (2006) Table 5.7.
A_{g
} = Area of glass (m2)
3.0 Internal Heat Gains  CIBSE Guide A
(2006)
Internal gains can account for most heat gain in buildings
in the
These gains are from occupants,
lights, equipment and machinery, as detailed below.
OCCUPANTS  Sensible and latent heat gains can be obtained
from CIBSE Guide A (2006)  Table 6.3.
Typical gains are shown below.
Conditions 
Typical building 
Sensible Heat Gain
( 
Latent Heat Gain ( 
Seated very
light work 
Offices,
hotels, apartments 
70 
45 
Moderate office
work 
Offices,
hotels, apartments 
75 
55 
Standing, light
work; walking 
Department
store, retail store 
75 
55 
Walking standing 
Bank 
75 
70 
Sedentary work 
Restaurant 
80 
80 
Light bench work 
Factory 
80 
140 
Athletics 
Gymnasium 
210 
315 
LIGHTING – Average power density from CIBSE Guide A (2006)  Tables 6.4.
ELECTRICAL
EQUIPMENT  PC’s and Monitors  CIBSE Guide A
(2006)  Tables 6.7 and 6.8.
Laser Printers and Photocopiers  CIBSE
Guide A (2006)  Tables 6.9 and 6.10
Electric Motors – CIBSE Guide A
(2006)  Table 6.13 and 6.14.
Lift Motors – CIBSE Guide A
(2006)  Table 6.15.
Cooking equipment – CIBSE Guide
A (2006)  Table 6.17.
Heat load is found from;
Q _{int.} = Heat from Occupants +
Heat from Lighting + Heat from Electrical Equipment + Heat
from Cooking
4.0 Heat Gain Through Walls
This is the unsteadystate heat flow through a wall due to
the varying intensity of solar radiation on the outer surface.
4.1 SolAir Temperature
In the calculation of this heat flow use is made of the
concept of solair
temperature,
which is defined as;
the value of the outside air temperature which would, in
the absence of all radiation exchanges, give the same rate of heat flow into
the outer surface of the wall as the actual combination of temperature
difference and radiation exchanges.
SOLAIR TEMP,
t_{eo} = t_{a} + ( ) ........ eqn. 4.1
where
teo = solair temperature (oC)
ta = outside air temperature (oC)
a = absorption
coefficient of surface
I = intensity of direct solar radiation on
a surface at right angles to the rays of the sun. (W/m^{2})
a = solar
altitude (degrees)
n = wallsolar
azimuth angle (degrees)
Is = intensity of scattered radiation
normal to a surface (W/m^{2})
hso = external surface heat transfer
coefficient (W/m^{2}oC)
The
Table 5.36
(
4.2 Thermal Capacity
The heat flow through a wall is complicated by the
presence of thermal capacity, so that some
of the heat passing through it is stored, being released at a later time.
Thick heavy walls with a high thermal capacity will
damp temperature swings considerably, whereas thin
light walls with a small thermal capacity will have little damping
effect, and fluctuations in outside surface temperature will be apparent almost
immediately.
The thermal capacity will not affect the daily mean solar
gain but will affect the solar gain at a particular time.
The particular time q of a solar gain is normally the time of the maximum gain.
The heat gain arrives at the inside of a thick wall some
time after the sun hits the outside surface of the wall.
This time lag is f.
The calculation is, therefore,
again split into two components.
1. Mean gain through wall,
Q_{q} = A . U ( t_{em}  t_{r}) ........
eqn. 4.2
where, Q_{q} = heat gain through wall at time q
A = area
of wall (m^{2})
U = overall thermal transmittance (W/m^{2} oC)
(See Thermal Transmission in Science section of the notes) or (see CIBSE
guide A (2006) Table 3.49 to 3.55) for
typical wall constructions.
tem = 24 hour mean solair temperature (oC) CIBSE Guide J
(2002)  Table 5.36 to 5.38.
tr = constant dry resultant temperature (oC). In
practice room dry bulb is used.
2. The variation from the mean solar
gain is subject to both a decrement factor and time lag.
Q_{f} = f ( t_{eo}  t_{em}) ........
eqn. 4.3
where Qf = Heat gain through wall at time (q  f)
f = time lag in hours (see CIBSE
guide A (2006) Table 3.49 to 3.55) for
typical wall constructions.
teo = solair temperature at time (q  f) (oC) CIBSE Guide J
(2002)  Table 5.36 to 5.38.
tem = 24 hour mean solair temperature (oC) CIBSE Guide J
(2002)  Table 5.36 to 5.38.
f = decrement factor (see CIBSE
guide A (2006) Table 3.49 to 3.55) for
typical wall constructions.
Therefore
the Solar Gain through a wall at time ( q  f) is;
Q_{q+f} = A . U [( t_{em}
 t_{r}) + f ( t_{eo}  t_{em})] ........ eqn. 4.4
where, Q_{q+}f = heat gain through wall at time q+f (
f = time lag in hours (see CIBSE
guide A (2006) Table 3.49 to 3.55) for
typical wall constructions.
A = area
of wall (m^{2})
U = overall thermal transmittance (W/m^{2} oC)
(see
CIBSE guide A (2006) Table 3.49 to 3.55) for
typical wall constructions.
tem = 24 hour mean solair temperature (oC) CIBSE Guide J
(2002)  Table 5.36 to 5.38.
tr = constant dry resultant temperature (oC) In
practice room dry bulb is used.
f = decrement factor (see CIBSE
guide A (2006) Table 3.49 to 3.55) for
typical wall constructions.
teo = Solair temperature at time (q  f) (oC) CIBSE Guide J
(2002)  Table 5.36 to 5.38
5.0 Heat Gain Through Roof
The heat
gain through a roof uses the same equation as for a wall as shown below.
Q q+f Roof = A
U [( tem  tr) + f (
teo  tem)] ........ eqn. 5
6.0 Ventilation and/or Infiltration Gains
Heat load is found from;
Qsi = n . V (t_{o}
t_{r}) / 3 ........ eqn. 6
where Qsi = Sensible
heat gain (W)
n = number of air changes per hour (h1) (see note
below)
V = volume of room (m3)
to = outside air temperature (oC) Can be
obtained from CIBSE Guide J (2002)  Tables 5.36 to 5.38 for
various months and times in the day.
tr = room air temperature (oC)
Infiltration gains should be added to the room heat gains.
Recommended infiltration rates are 1/2 air change per hour for most airconditioning cases or 1/4 air change per hour for double glazing or if
special measures have been taken to prevent infiltration.
Ventilation or fresh air supply loads can be added to
either the room or central plant loads but should only be
accounted for once.
Total
Room Load From Heat Gains
Q total
= Qg + Qsg +
Qint. + Q_{q}_{+}_{f}_{Wall} + Q_{ }_{q}_{+}_{f }_{Roof}
+ Qsi
Q total = Ag . Ug (to  tr) 1. Sensible Glass
+ Fc . Fs . qsg . Ag 2. Solar Glass.
+ Qint. 3. Internal
+ A.U [( tem  tr) + f (
teo  tem)] 4. Walls
+ A.U [( tem  tr) + f (
teo  tem)] 5. Roof
+ n . V (to  tr) / 3 6. Ventilation
........
eqn. 7
In the majority of cases, by far the greatest external fluctuating component is the solar heat
gain through the windows.
Therefore, it will be this gain which determines when the
total heat gain to the room is a maximum.
Heat
gains may be calculated and displayed in table form as shown below.
Heat Gain from 

% 
1. Sensible transmission
through glass 


2. Solar gain through glass 


3. Internal 


4. External walls 


5. Roof 


6. Ventilation 


Total 

100% 
Heat gain per m^{2} floor area
= 


Heat gain per m^{3} space
= 
Latent Gains
Latent heat gains are calculated so that the Total heat gain can be determined to complete a
psychrometric chart.
Total heat gain = Sensible heat gain + Latent heat gains
Also Latent heat gains
are required to size Chillers.
Latent heat gains are comprised of latent gain from occupants and from natural infiltration fresh
air.
Latent
heat gains from occupants can be obtained from CIBSE Guide A (2006)  Table 6.3 shown above.
The
following formula gives the infiltration latent heat gain.
Q_{li} = 0.8 .
n . V ( m_{so} –m_{sr} )
Where;
Q_{li}
= Infiltration
latent heat gain (W)
n = Number
of air changes per hour (h^{1})
V = Room
volume (m^{3})
m_{so}
= Moisture
content of outside air (g/kg d.a.) from psychrometric chart.
m_{sr} = Moisture
content of room air (g/kg d.a.) from psychrometric chart.