METERING AIR SYSTEM
Model: MAS-1
The Field Metering Air System consists of an intake air hood which is installed through the wall to
allow outside air in. The hood is connected via flexible 6” duct or standard duct pipe (not included) to
our metering air device mounted on the return/supply air plenum. This device effectively meters the
air inflow for proper air circulation, combustion and balance.
ITEMS INCLUDED
1) Metering air damper
1) Mounting template
1) 6" intake hood
20) Small washers (2 grams each)
3) Large washers (50 grams each)
English ...... Page 1
Français..... Page 3
Espanõl ..... Page 4
INSTALLATION AND ADJUSTMENTS
1. Apply mounting template onto the side of the return air plenum of the
furnace. By wetting the back side and applying to any vertical surface of
the return air plenum.
CAUTION: Unit must be mounted on a vertical surface. Level the template
horizontally by marking a line onto the plenum using a level. Then applying the
edge of the template on this line. (See Figure 1)
2. After allowing template to dry, drill (4) 1/8 inch diameter holes at the
location indicated on the template. Also, drill 4 or 5 holes along the cutting
line of the 6 1/8 inch diameter hole indicated on the template to allow for a
starting hole for cutting. Then cut hole with metal shears.
3. Mount the metering air damper using supplied sheet metal screw. (See
Figure 2)
INSTALLATION OF INTAKE HOOD
1. Locate intake hood at least 10 feet away from any device that
exhaust flue gases.
2. Install the intake hood at least 12 inches above grade.
3. Cut a 6 1/8 inch diameter hole through the wall. Then mount the
intake hood to the wall using suitable fastener. Then seal along the
mounting plate with silicone or equivalent sealant.
Figure 1
4. Connect 6" flex duct or standard 6" duct piping from the intake hood
to the metering air damper. Fasten and support piping, so no
straining of the metering air damper exist. (See Figure 3)
NOTE: Some areas require insulation wrapped around the duct. This will be
required if sweating occurs on the outside of the duct. Seal all duct joints and
seams with duct tape or equivalent.
ADJUSTING METERING AIR DAMPER
1. Determine building square footage. Then refer to the table on the next
page for recommended weighing of air damper.
2. Attach weights to the pendent on weight lever assembly using the supplied
#5 screw and nut. (See Figure 4)
Figure 2
Figure 3
Figure 4
Page 2
TABLE 1
AIRFLOW CHART
NUMBER OF ADJUSTMENT WEIGHTS ADDED
SMALL LARGE GRAMS
BUILDING SQUARE FEET
APPROXIMATE CFM
OF MAKE-UP
SUPPLIED TO PLENUM
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.
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