Submittal Sheet
Page 3
TABLE 1
AIRFLOW CHART
NUMBER OF ADJUSTMENT WEIGHTS ADDED
BUILDING SQUARE FEET
APPROXIMATE CFM
OF MAKE-UP
SUPPLIED TO PLENUM
SMALL LARGE GRAMS
1000 67 19 1 88
1250 83 13 1 76
1500 100 8 1 66
1750 117 2 1 54
2000 133 20 - 40
2250 150 16 - 32
2500 167 11 - 22
2750 183 3 - 6
3000 200 - - -
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.
CALUCULATE CFM OF MAKE UP AIR
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.