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TABLE 1
BUILDING SQUARE FEET
1000
1250
1500
1750
2000
2250
2500
2750
3000
The cfm of make-up air was based on 1/2 air changes per hour in the building, an average ceiling height of 8
feet and a return air plenum pressure of .4 inches of water column.
C
CFM O
M
ALUCULATE
F
AKE
To adjust the metering air damper for different air changes per hour, building volumes or different return air pressures
refer to Table 1.
1. Calculate building volume: Building square feet x average ceiling height.
2. Estimate desired building air change per hour (ACPH).
Typical ACPH for buildings:
Tight construction -.35 to .5
Average construction -.5 to .75
Loose construction .75 to 1.00
3. Calculate required CFM of make up air:
(Building volume x ACPH) ÷ 60.
4. Measure return air plenum pressure. In inches of water column pressure.
5. Refer to chart (or see example)
EXAMPLE: 2,000 sq. ft. building with 8 foot high ceilings required air changes per hour = .5 ACPH. Furnace return air
plenum pressure .3 inch.
1. Building volume = 2,000 sq. ft. x 8 ft. = 16,000 cu. ft.
2. Required CFM = (16,000 cu. ft. x .5) ÷ 60 = 133 CFM. Entering the chart at 133 CFM draw a line horizontally until
it intersects the .3 pressure curve. Then draw a line vertically down to the weight axis. For this example the
amount of weight to add to the weight level assembly would be 20 grams or 10 small washers.
AIRFLOW CHART
APPROXIMATE CFM
OF MAKE-UP
SUPPLIED TO PLENUM
67
83
100
117
133
150
167
183
200
U
A
P
IR
NUMBER OF ADJUSTMENT WEIGHTS ADDED
SMALL
19
13
8
2
20
16
11
3
-
Page 3
LARGE
GRAMS
1
88
1
76
1
66
1
54
-
40
-
32
-
22
-
6
-
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