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## EXAMPLE EXERCISE CALCULATING HEAT LOSS

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Title: EXAMPLE EXERCISE CALCULATING HEAT LOSS

1
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2
• EXAMPLE EXERCISE CALCULATING HEAT LOSS HEAT
GAIN
• Several exhibits in the class packet are
necessary to understand the entries into the Heat
Loss / Heat Gain calculation sheet.
• The example floor plan will be used to make
calculations required for the selection of
mechanical equipment necessary to maintain
comfort heating and cooling.
• Certain criteria are given on the lower left
section of the floor plan to be used in the
calculation. These are criteria established in
the design of the building envelope, for the type
building for which it is intended.

3
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4
• Heat flow by conduction in Btu/h, through
low mass or thin surfaces, such as doors and
glass, is calculated by multiplying the area,
times the U factor, times the difference in
temperature from one side of the material to the
other.
• The temperature differential is the
difference between the recognized high (for
summer) or low (for winter), from climatilogical
data, and the temperature desired to be
maintained within the space.
• The quantity of heat by conduction that
passes through surfaces of greater mass, such as
walls and roofs is calculated by multiplying the
area, times the U factor, times the Equivalent
Temperature Difference.

5
• RADIANT HEAT FROM DIRECT SUN IN SUMMER
• Radiant heat that enters a space by shining
through a transparent or semi-transparent
surface, such as glass is calculated by
multiplying the area times the amount of Solar
Gain, reduced only by the effectiveness of a
• AS AN ILLUSTRATION, calculate the total heat
loss (winter) and heat gain (summer) for the plan
of the example building. The construction is
medium weight masonry, such as brick veneer over
concrete masonry units.
• In this area, the peak summer daytime
temperature occurs around 400 P.M. Consider
that the color of masonry is light, such as would
be the shade of the brick on the architecture
building.

6
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7
• U values were
selected from
reasonable allowance of materials.
• ETD and Solar Gain values were
• selected from the charts.
• from a reflective glass.

8
ETD values

21
26
31
31
36
48
9
• Another factor that relates to direct sunlight
is solar gain through glass. Direct sunlight
will pass through clear glass without an
appreciable effect on the glass itself, since
glass is light in mass compared to the building
envelope.
• Solar gain, referenced by Sg on the chart, is
the amount of heat gain in Btu per square foot,
as the result of sun radiation that penetrates
glass.
• The angle of the sun is taken into account as
the result of the tilt of the earth, as well as
the time of the day. In this area, 400 P.M. is
the peak summer temperature.
• June 21, the beginning of summer produces the
worst sun angle on the north and east side of a
building, while September 21, the beginning of
fall produces the worst sun angle for south and
north.

10
SOLAR GAIN
23
66
196

11
• In order to determine a reasonable
temperature difference between outside and
inside, consult with climatilogical data for the
area a building is located.
• The chart that follows is taken from the
appendix of the text, and gives the outside
design temperatures for winter and summer for
Lubbock, Texas, in terms of dry-bulb
temperatures. The wet-bulb temperature given in
the summer column is a measure of the relative
humidity.
• Inside temperature is set by the designer as
the maintained desired interior temperature.

12
CLIMATILOGICAL DATA PAGE 1630 OF TEXT
13
• In the chart, note that winter outside
design temperature is 15 degrees, while summer
design temperature is 96 degrees.
• For the purpose of the example problem, say
it is desired to maintain a temperature inside of
75 degrees, for both summer and winter.
• So, temperature difference for summer
conditions would be 96 75 21 degrees. And
for winter conditions the temperature difference
is 75 15 60 degrees.

14
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15
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16
339
.075
21
534
1,526
60

17
339
.075
21
534
1,526
534
.075
26
1,041
2,403
60

18
339
.075
21
534
1,526
534
.075
26
1,041
2,403
60
972
216
.075
31
502

19
339
.075
21
534
1,526
534
.075
26
1,041
2,403
60
972
216
.075
31
502
484
2,178
.075
36
1,307

20
339
.075
21
534
1,526
534
.075
26
1,041
2,403
60
972
216
.075
31
502
484
2,178
.075
36
1,307
21
243
693
21
.55
60

21
339
.075
21
534
1,526
534
.075
26
1,041
2,403
60
972
216
.075
31
502
484
2,178
.075
36
1,307
21
243
693
21
.55
60
2,200
.050
48
5,280
6,600

22
339
.075
21
534
1,526
534
.075
26
1,041
2,403
60
972
216
.075
31
502
484
2,178
.075
36
1,307
21
243
693
21
.55
60
2,200
.050
48
5,280
6,600
0
60
21

23
339
.075
21
534
1,526
534
.075
26
1,041
2,403
60
972
216
.075
31
502
484
2,178
.075
36
1,307
21
243
693
21
.55
60
2,200
.050
48
5,280
6,600
0
96
.60
1,210
3,456
60
21

24
339
.075
21
534
1,526
534
.075
26
1,041
2,403
60
972
216
.075
31
502
484
2,178
.075
36
1,307
21
243
693
21
.55
60
2,200
.050
48
5,280
6,600
0
.60
96
1,210
3,456
60
21
1,814
144
.60
5,184

25
339
.075
21
534
1,526
534
.075
26
1,041
2,403
60
972
216
.075
31
502
484
2,178
.075
36
1,307
21
243
693
21
.55
60
2,200
.050
48
5,280
6,600
0
.60
96
1,210
3,456
60
21
.60
1,814
144
5,184
128
.60
4,608
1,613

26
339
.075
21
534
1,526
534
.075
26
1,041
2,403
60
972
216
.075
31
502
484
2,178
.075
36
1,307
21
243
693
21
.55
60
2,200
.050
48
5,280
6,600
0
.60
96
1,210
3,456
60
21
.60
1,814
144
5,184
128
.60
4,608
1,613
0

27
339
.075
21
534
1,526
534
.075
26
1,041
2,403
60
972
216
.075
31
502
484
2,178
.075
36
1,307
21
243
693
21
.55
60
2,200
.050
48
5,280
6,600
0
.60
96
1,210
3,456
60
21
.60
1,814
144
5,184
128
.60
4,608
1,613
0

96
23
.75
1,656
28
339
.075
21
534
1,526
534
.075
26
1,041
2,403
60
972
216
.075
31
502
484
2,178
.075
36
1,307
21
243
693
21
.55
60
2,200
.050
48
5,280
6,600
0
.60
96
1,210
3,456
60
21
.60
1,814
144
5,184
128
.60
4,608
1,613
0

96
23
.75
1,656
144
66
.75
7,128
29
339
.075
21
534
1,526
534
.075
26
1,041
2,403
60
972
216
.075
31
502
484
2,178
.075
36
1,307
21
243
693
21
.55
60
2,200
.050
48
5,280
6,600
0
.60
96
1,210
3,456
60
21
.60
1,814
144
5,184
128
.60
4,608
1,613
0

96
23
.75
1,656
144
66
.75
7,128
128
196
.75
18,816
41,144
Sub Totals
27,620
30
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31
• The heat gain and heat loss calculated in the
top part of the chart involved the integrity of
the building envelope.
• An examination of the components would reveal
that the foremost consideration of this building
design should be an analysis of what can be done
about the windows. Realize the AREA of the
windows is not the main concern, but rather the
orientation of the building with regard to window
placement.
• Glass area on the south side (next to worse
solar gain) is 144 sq.ft. compared to 128 on the
west side, yet the solar heat gain on the west
side is more than 2 ½ times that on the south.
• A more judicious concern for plan arrangement
would result in better conservation of energy.
• The bottom half of the chart is concerned
mainly with internal conditions of the building
and how they affect the overall heat loss/gain.

32
• PERIMETER Heat loss only. Refers to the
perimeter of the building specifically at floor
level near the ground outside. During winter the
ground will remain cold, since soil does not
readily convert electromagnetic energy to heat
because of its relatively light density.
• The ground temperature at the building will
probably be cold at least 12 deep. So, the
temperature difference between the soil and
inside the building at the floor level will be
sufficient to cause a significant heat loss
around the perimeter of the building.
• On the page you have in the packet labeled
ETD, find the written material on the left
column of the page under the words PERIMETER HEAT
• Perimeter Insulation . . . Use a value of
.81 btu/h for each UNINSULATED foot of building
perimeter.
• Use a value of .55 btu/h for each INSULATED
foot of building perimeter.
• Assume the example problem is INSULATED.

33
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34
Building perimeter 68406840 216
35
41,144
27,620
Sub-totals from above . . .

216 x 0.55 x 60 7,128

7,128

60
21
36
• VENTILATION is the removal of unwanted
air from a space. Ventilation cannot happen
unless stale air is replaced by new air and the
only source for new air is outside the space.
• If outside air is brought into the space, it
must be thermally treated in order to blend with
comfort air. So, in summer, heat must be removed
from the air, and in winter, heat must be added.
• The measurement for handling air quantity is
CUBIC FEET PER MINUTE (cfm), so a transition
must be made to convert cubic feet per minute to
btu per hour.
• The ventilation requirement for the building is
400 cubic feet per minute 200 for toilets and
200 for the room on the southwest side of the
building.

37
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38
• So, heat gain, or heat loss from ventilation air
is calculated simply
• CFM ventilation x 60 x .018 x temp. diff.
btu/h
• But remember to use the temperature difference
that applies to summertime for heat gain, and the
temperature difference that applies to wintertime
for heat loss.

39
41,144
27,620
Sub-totals from above . . .

216 x 0.55 x 60 7128
7,128
400 x 60 x .018 x 21 9072
9,072
25,920
400 x 60 .018 x 60 25,920
60
21
40
• INFILTRATION is unwanted air that gets
into the space by infiltrating through cracks
around doors and windows and by other means by
which un-conditioned outside air can get inside.
• To calculate the amount of air, use a factor of
.50 for each linear foot of crack around doors
and windows. The number, .50 represents ½ CFM
per foot of crack.
• The amount of crack is the measurement of the
perimeter of each window sash that is movable,
and doors.
• So the heat loss or gain from infiltration
equals
• total length of crack x .5 x 60 x .018 x
temperature difference (summer or winter)

41
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42
41,144
27,620
Sub-totals from above . . .

216 x 0.55 x 60 7128
7,128
400 x 60 x .018 x 21 9072
9,072
25,920
400 x 60 .018 x 60 25,920
110 x .5 x 60 x .018 x 21 1247
60
1,247
3,564
21
110 x .5 x 60 x .018 x 60 3564
43
• PEOPLE means the number of people that
occupy the space and contribute heat. The amount
varies with the size of individuals and their
level of activity. For the purpose of this
calculation, assume that approximately ten people
will occupy the space, and their physical
activity is rather tranquil.
• So use 250 btu/h for each person
• 250 x 10 2500 Btu/h
• (the number of people that occupy public
buildings is also given in the building code)

44
41,144
27,620
Sub-totals from above . . .

216 x 0.55 x 60 7128
7,128
400 x 60 x .018 x 21 9072
9,072
25,920
400 x 60 .018 x 60 25,920
110 x .5 x 60 x .018 x 21 1247
60
1,247
3,564
21
110 x .5 x 60 x .018 x 60 3564
10 x 250 2,500
2,500
45
• ELECTRIC WATTS involves the amount of heat
generated by the consumption of electricity
within the space, such as the operation of
mechanical equipment and electric lights.
• Since all the answers as to the electrical
design is generally not known at the time these
calculations are done, use an allowance of two
watts per square foot of space.
• 2200 square feet x 2 4400 watts
• Since one electrical watt produces heat of 3.4
btu / h, the total number of btu equals,
• 4400 x 3.4 14,960 Btu/h

46
41,144
27,620
Sub-totals from above . . .

216 x 0.55 x 60 7128
7,128
400 x 60 x .018 x 21 9072
9,072
25,920
400 x 60 .018 x 60 25,920
110 x .5 x 60 x .018 x 21 1247
60
1,247
3,564
21
110 x .5 x 60 x .018 x 60 3564
10 x 250 2500
2,500
2200 x 2 x 3.4 14,960
14,960
47
• Note on the chart that PERIMETER is heat
loss only, since there is no place for values in
the heat gain column.
• VENTILATION and INFILTRATION both
contribute to heat gain and heat loss.
• PEOPLE and ELECTRIC WATTS produce heat gain
only.
• At this point the chart asks for a total of
the amount of heat gain, called SUB TOTAL 1.
This amount is the sum of SENSIBLE HEAT from heat
gain within the space.
• Here we also total the column under heat loss.

48
41,144
27,620
Sub-totals from above . . .

216 x 0.55 x 60 7128
7,128
400 x 60 x .018 x 21 9072
9,072
25,920
400 x 60 .018 x 60 25,920
110 x .5 x 60 x .018 x 21 1247
60
1,247
3,564
21
110 x .5 x 60 x .018 x 60 3564
10 x 250 2500
2,500
2200 x 2 x 3.4 14,960
14,960
Add heat gain / loss columns from top of chart
down through elec. Watts
64,232
68,923
49
• The next line of the chart is Latent Heat Load.
• Realize that moisture is present within the
space, that will condense to water when the dew
point temperature is reached. And remember that
an amount of heat is required to change the state
of a substance. (water vapor to water)
• Where moist air passes over the air conditioners
cooling coil, the air will be at room
temperature, but the cooling coil will be
slightly above freezing, so the dew point
temperature will occur somewhere in between
resulting in condensed water forming on the coil,
collected in a drain pan.
• So, latent heat is a COOLING LOAD because some of
the capacity of the air conditioning unit is
required to condense moisture into water.

50
• But how much energy is wasted on
condensation of moisture . . . ?
• Because of the mean wet bulb temperature in
Lubbock, Texas, (from the climatilogical chart,
pg.1630 of text) the ratio of Sensible Heat to
Latent Heat within a space such as an office
building is approximately a 70 / 30 ratio.
• So, multiply the sensible heat load (sub
total 1) times 0.30 to get the amount of latent
heat in btu/h. Realize that latent heat is heat
gain and involves cooling load only.
• Add this amount to sub total 1 to get SUB TOTAL
2.

51
CLIMATILOGICAL DATA PAGE 1630 OF TEXT
52
41,144
27,620
Sub-totals from above . . .

216 x 0.55 x 60 7128
7,128
400 x 60 x .018 x 21 9072
9,072
25,920
400 x 60 .018 x 60 25,920
110 x .5 x 60 x .018 x 21 1247
60
1,247
3,564
21
110 x .5 x 60 x .018 x 60 3564
10 x 250 2500
2,500
2200 x 2 x 3.4 14,960
14,960
Add heat gain / loss columns from top of chart
down through elec. Watts
64,232
68,923
30 of sensible load 68,275 x .30
20,697
89,620
53
• The last entry in the chart is a
calculation that the designer must make based on
the location of ductwork used to move
conditioning air from the mechanical equipment to
the spaces to be conditioned.
• If ductwork is installed in interstitial space
such as inside chase space or attic space between
floors of a multi-story building where there is
no temperature difference, there will be no duct
heat loss and gain.

54
• If ductwork is installed in a ventilated,
un-insulated attic space, the walls of the duct
will be subject to higher temperatures in summer
and lower temperatures in winter, so one must
compensate for heat flow in the form of heat gain
in summer and heat loss in winter.
• For purposes of this calculation, take ten
percent of sub total 2 as duct loss and add the
sum to sub total 2 for a grand total of heat gain
and heat loss.

55
41,144
27,620
Sub-totals from above . . .

216 x 0.55 x 60 7128
7,128
400 x 60 x .018 x 21 9072
9,072
25,920
400 x 60 .018 x 60 25,920
110 x .5 x 60 x .018 x 21 1247
60
1,247
3,564
21
110 x .5 x 60 x .018 x 60 3564
10 x 250 2500
2,500
2200 x 2 x 3.4 14,960
14,960
Add heat gain / loss columns from top of chart
down through elec. Watts
64,232
68,923
30 of sensible load 68,275 x .30
20,697
68,275 20,482
89,620
88,757 x .10 64,232 x .10
Only if duct is in Un-conditioned space
8,962
6,423
98,582
70,655
56
• COMPARISON
• Total Heat Gain 98,582 btu/h
• Which equals 98,582/ 2200 44.81 btu / h per
sq.ft.
• Total Heat Loss 70,655 btu/h
• Which equals 70,655 / 2200 32.12 but / h per
sq.ft.

57
• Which indicates that orientation and
consideration of sun affects on a building
envelope, coupled with the availability of
daylight as an illumination source is of major