Energy Efficient Buildings
Outdoor Air Control Homework
1) ASHRAE 62.1 - 2004 reduces the occupancy-based ventilation requirement from 15
cfm/person to 7.5 cfm/person. Determine the difference in CO2 concentration (ppm)
between outdoor and indoor air to meet this requirement for occupancy-based
ventilation.
2) Consider the VAV system with multiple CO2 sensors shown below. The peak sensible
cooling loads on the three zones are: 24,000 Btu/hr, 40,000 Btu/hr and 60,000 Btu/hr.
The peak number of occupants in the three zones are: 20, 30 and 30. The supply air
temperature is 55 F and the return air temperature is 72 F. Code requires 15 cfm of
outdoor air per person. Determine the design volume flow rate Vsa and outdoor air
flow rate Voa at these conditions for three zones, and the total fraction outdoor air, foa,
supplied by the air handler.
Zone 1
Design
Vsa (cfm)
VOA (cfm)
foa
Z1
Zone 2
Zone 3
Z2
Z3
Total
Next, consider an actual operating condition. The actual sensible cooling loads on the
three zones are: 22,000 Btu/hr, 30,000 Btu/hr and 15,000 Btu/hr. The actual number of
occupants in the three zones are: 18, 10 and 5. Determine the volume flow rate Vsa,
outdoor air flow rate Voa, and required foa for each zone at these conditions.
Determine the critical (maximum) foa. Determine the actual outdoor air supplied to
each zone, and the actual total outdoor air supplied by the air handler.
Design
Vsa (cfm)
VOA,REQUIRED (cfm)
foa
VOA,ACTUAL (cfm)
Z1
Z2
Z3
Total
3) Determine annual cooling energy use (mmBtu/year) for a HVAC system in Chicago, IL
with and without a temperature-based economizer. The volume air flow rate over the
cooling coil is 1,000 cfm and the air density is 0.075 lb/ft3. The supply air leaves the
cooling coil at 55 F with an enthalpy of 23.3 Btu/lb. The return air leaves the zone at 72
F with an enthalpy of 28.3 Btu/lba. The minimum required fraction outdoor air is 0.20.
To do so, create a temperature bin file from the Chicago IL TM3 file using WeaTran, and
open it into excel. Remove extraneous columns. It may be helpful to match the results
shown below for Dayton, Ohio, and then paste the Chicago IL bin data into the first 5
columns. Note that in Dayton, Ohio, the economizer cut annual cooling energy use in
half.
No Economizer
psa = 0.075 lb/ft3
StrTemp EndTemp T(F)
h(Btu/lba) hrs1-24
Tsa(F)
has(Btu/lba)Tra(F)
hra(Btu/lba)foa,min
foa
Tma(F)
hma(Btu/lba)
Vsa(cfm) Qcc (mmBtu/yr)
========================================================================================================================
105
109
107
-99
0
55
23.3
72
28.3
0.2
0.20
79.0
2.8
1000
0.00
100
104
102
-99
0
55
23.3
72
28.3
0.2
0.20
78.0
2.8
1000
0.00
95
99
97
-99
0
55
23.3
72
28.3
0.2
0.20
77.0
2.8
1000
0.00
90
94
91
38.8
5
55
23.3
72
28.3
0.2
0.20
75.8
30.4
1000
0.16
85
89
87.4
38
123
55
23.3
72
28.3
0.2
0.20
75.1
30.2
1000
3.84
80
84
82.2
34.9
451
55
23.3
72
28.3
0.2
0.20
74.0
29.6
1000
12.83
75
79
76.9
32.7
563
55
23.3
72
28.3
0.2
0.20
73.0
29.2
1000
14.90
70
74
72.4
31.2
706
55
23.3
72
28.3
0.2
0.20
72.1
28.9
1000
17.73
65
69
68.1
28.6
794
55
23.3
72
28.3
0.2
0.20
71.2
28.4
1000
18.08
60
64
62.4
24.2
863
55
23.3
72
28.3
0.2
0.20
70.1
27.5
1000
16.23
55
59
57
21.1
657
55
23.3
72
28.3
0.2
0.20
69.0
26.9
1000
10.53
50
54
51.8
18.7
670
55
23.3
72
28.3
0.2
0.20
68.0
26.4
1000
9.29
45
49
47.5
16.6
487
55
23.3
72
28.3
0.2
0.20
67.1
26.0
1000
5.83
40
44
43.2
14.8
683
55
23.3
72
28.3
0.2
0.20
66.2
25.6
1000
7.07
35
39
37.5
12.6
693
55
23.3
72
28.3
0.2
0.20
65.1
25.2
1000
5.80
30
34
32.3
10.8
641
55
23.3
72
28.3
0.2
0.20
64.1
24.8
1000
4.33
25
29
27.4
8.8
390
55
23.3
72
28.3
0.2
0.20
63.1
24.4
1000
1.93
20
24
23.1
7.5
371
55
23.3
72
28.3
0.2
0.20
62.2
24.1
1000
1.40
15
19
17.8
5.7
302
55
23.3
72
28.3
0.2
0.20
61.2
23.8
1000
0.65
10
14
12.1
4
152
55
23.3
72
28.3
0.2
0.20
60.0
23.4
1000
0.10
5
9
7.2
2.6
105
55
23.3
72
28.3
0.2
0.20
59.0
23.2
1000
0.00
0
4
2.5
1.3
53
55
23.3
72
28.3
0.2
0.20
58.1
22.9
1000
0.00
-5
-1
-2
0
40
55
23.3
72
28.3
0.2
0.20
57.2
22.6
1000
0.00
-10
-6
-6.4
-1.1
11
55
23.3
72
28.3
0.2
0.20
56.3
22.4
1000
0.00
-15
-11
-13
-99
0
55
23.3
72
28.3
0.2
0.20
55.0
2.8
1000
0.00
-20
-16
-18
-99
0
55
23.3
72
28.3
0.2
0.20
54.0
2.8
1000
0.00
-25
-21
-23
-99
0
55
23.3
72
28.3
0.2
0.20
53.0
2.8
1000
0.00
-30
-26
-28
-99
0
55
23.3
72
28.3
0.2
0.20
52.0
2.8
1000
0.00
Total
130.68
Temperature-based Economizer
psa = 0.075 lb/ft3
StrTemp EndTemp T(F)
h(Btu/lba) hrs1-24
Tsa(F)
has(Btu/lba)Tra(F)
hra(Btu/lba)foa,min
foa
Tma(F)
hma(Btu/lba)
Vsa(cfm) Qcc (mmBtu/yr)
========================================================================================================================
105
109
107
-99
0
55
23.3
72
28.3
0.2
0.20
79.0
2.8
1000
0.00
100
104
102
-99
0
55
23.3
72
28.3
0.2
0.20
78.0
2.8
1000
0.00
95
99
97
-99
0
55
23.3
72
28.3
0.2
0.20
77.0
2.8
1000
0.00
90
94
91
38.8
5
55
23.3
72
28.3
0.2
0.20
75.8
30.4
1000
0.16
85
89
87.4
38
123
55
23.3
72
28.3
0.2
0.20
75.1
30.2
1000
3.84
80
84
82.2
34.9
451
55
23.3
72
28.3
0.2
0.20
74.0
29.6
1000
12.83
75
79
76.9
32.7
563
55
23.3
72
28.3
0.2
0.20
73.0
29.2
1000
14.90
70
74
72.4
31.2
706
55
23.3
72
28.3
0.2
0.20
72.1
28.9
1000
17.73
65
69
68.1
28.6
794
55
23.3
72
28.3
0.2
1.00
68.1
28.6
1000
18.94
60
64
62.4
24.2
863
55
23.3
72
28.3
0.2
1.00
62.4
24.2
1000
3.50
55
59
57
21.1
657
55
23.3
72
28.3
0.2
1.00
57.0
21.1
1000
0.00
50
54
51.8
18.7
670
55
23.3
72
28.3
0.2
0.84
55.0
20.2
1000
0.00
45
49
47.5
16.6
487
55
23.3
72
28.3
0.2
0.69
55.0
20.2
1000
0.00
40
44
43.2
14.8
683
55
23.3
72
28.3
0.2
0.59
55.0
20.3
1000
0.00
35
39
37.5
12.6
693
55
23.3
72
28.3
0.2
0.49
55.0
20.6
1000
0.00
30
34
32.3
10.8
641
55
23.3
72
28.3
0.2
0.43
55.0
20.8
1000
0.00
25
29
27.4
8.8
390
55
23.3
72
28.3
0.2
0.38
55.0
20.9
1000
0.00
20
24
23.1
7.5
371
55
23.3
72
28.3
0.2
0.35
55.0
21.1
1000
0.00
15
19
17.8
5.7
302
55
23.3
72
28.3
0.2
0.31
55.0
21.2
1000
0.00
10
14
12.1
4
152
55
23.3
72
28.3
0.2
0.28
55.0
21.4
1000
0.00
5
9
7.2
2.6
105
55
23.3
72
28.3
0.2
0.26
55.0
21.6
1000
0.00
0
4
2.5
1.3
53
55
23.3
72
28.3
0.2
0.24
55.0
21.7
1000
0.00
-5
-1
-2
0
40
55
23.3
72
28.3
0.2
0.23
55.0
21.8
1000
0.00
-10
-6
-6.4
-1.1
11
55
23.3
72
28.3
0.2
0.22
55.0
21.9
1000
0.00
-15
-11
-13
-99
0
55
23.3
72
28.3
0.2
0.20
55.0
2.8
1000
0.00
-20
-16
-18
-99
0
55
23.3
72
28.3
0.2
0.20
54.0
2.8
1000
0.00
-25
-21
-23
-99
0
55
23.3
72
28.3
0.2
0.20
53.0
2.8
1000
0.00
-30
-26
-28
-99
0
55
23.3
72
28.3
0.2
0.20
52.0
2.8
1000
0.00
Total
71.88
4) Consider a building that uses 100% outdoor, is maintained at 72 F, and exhausts and
intakes 20,000 cfm of outside air. The supply air passes over a heating coil and then a
cooling coil before being transported to the zones in the building. The temperature of
the air after the cooling coil is maintained at 58 F. Using hourly bin data from WeaTran
for Chicago IL, determine the annual sensible heating energy use (mmBtu/year) and
sensible cooling energy use (mmBtu/year).
Next, consider the case where the building uses an air-to-air heat exchanger between
the intake and exhaust air streams with effectiveness = 0.80. Determine annual sensible
heating energy use (mmBtu/year) and sensible cooling energy use (mmBtu/year).
[Answers: Qhc(no heat recovery) = 2,531 mmBtu/yr, Qcc(no heat recovery) = 935
mmBtu/yr, Qhc(heat recovery) = 2 mmBtu/yr, Qcc(heat recovery) = 1,802 mmBtu/yr. To
receive credit, show your bin calculations.]
5G) Graduate students only. Determine annual cooling energy use (mmBtu/year) for a
HVAC system in Chicago, IL with an enthalpy-based economizer. The volume air flow
rate over the cooling coil is 1,000 cfm and the air density is 0.075 lb/ft3. The supply air
leaves the cooling coil at 55 F with an enthalpy of 23.3 Btu/lb. The return air leaves the
zone at 72 F with an enthalpy of 28.3 Btu/lba. The minimum required fraction outdoor
air is 0.20.