Installing A Make-Up Air Louver - Saturn 125 Operator's Manual

Installing a Make-up Air Louver

Exhaust Fans and Make-up Air Louvers
When exhaust fans are operated in tight buildings, there is little or no source of fresh air to replace the
air removed from the building by the exhaust fan. This results in negative pressure (vacuum) in the
building which creates severe backdraft problems at the furnace.
Properly sized make-up air louvers are designed to allow adequate fresh air to enter the building during
operation of the exhaust fans. The louvers automatically open under the vacuum created by the exhaust
fans. The louvers open just enough to provide the correct CFM of fresh air. The louvers automatically
close when the exhaust fans are turned off.
Sizing the Make-up Air Louver
IMPORTANT NOTE: When using louvers and grills, and the actual free area of the louver or grill is not
known, it is understood that wooden louvers/grills will have a free area of 25%, while metal louvers/grills will
have a free area of 75%. ("Free area" translates into the actual space allowing air to flow through.)
Procedure 1: (a) Determine the CFM rating of the exhaust fan. This information should be stamped on
a label on the exhaust fan. Make sure to add up the total CFM if more than one
exhaust fan exists in the building.
(b) Select the correct size of make-up air louver to provide the required CFM of fresh air.
If you cannot determine the CFM rating of the exhaust fan, use the following procedure.
Procedure 2: (a) Count the total number of exhaust fans in the building.
(b) Close all of the doors and windows in the building except for one overhead garage door.
(c) Start the furnace and allow it to run for 15 minutes to bring it up to temperature.
(d) Set the draft-over-fire for -.02" to -.04" W.C.
(e) Turn ON all of the exhaust fans in the building.
(f) Now close the opened overhead garage door slowly, just until the draft gauge moves down
into a positive (+) draft reading; as soon as it reaches this point, STOP the door at that
position.
(g) Slowly open the door back up, just until the draft reading comes back up to the -.02"
to -.04 W.C.
(h) Measure the size of the door opening, rounding this measurement off to the next largest
even foot.
Example of Calculations for Sizing Make-up Air Louver
Measured door opening: 12' wide x 9" high
• Round off to 12' wide x 1' high
• Convert to square feet (in this case, equals 12 square feet)
• Multiply by 1.1 correction factor (12 sq. ft. x 1.1 = 13.20 sq. ft.)
• Multiply by free area factor (13.20 sq. ft. x 1.25 = 16.50 sq. ft.)
• Refer to sizing chart (following page) - required grill size would be (1) 54" square grill
Operator's Manual: Models Saturn™ 140 & Saturn™ 230
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