A.O. Smith 120 trough 500 Service Handbook page 9

Daily Schedule
When the heater is installed in an interior room with no roof access for vertical ducts,
horizontal make-up air ducts should be installed. When using horizontal ducts, two
openings are required - one within 12 inches of the top of the enclosure and one within
twelve inches of the bottom of the enclosure. Each opening shall have a free area of not
less than 1 square inch per 2000 BTUH of the total input of all appliances within the
enclosure.
A.O. Smith Water Products
Irving, Texas ©2000
BTR TANK TYPE HEATERS
SERVICE HANDBOOK
MAKE-UP AIR
HORIZONTAL DUCTS
8
1 Square
Inch Per
2,000
BTUH
Service Handbook
Training Department