Planning a Gravity Warm-Air Heating System
Several trade and professional associations provide information for planning a gravity warm-air heating system. A very useful source of information is the Gravity Code and Manual for the Design and Installation of Gravity Warm Air Heating Systems (Manual 5) from the former National Warm-Air Heating and Air Conditioning Association. You should also check relevant publications from its successor association, the Air-Conditioning Contractors of America (ACCA).
The following recommendations for planning such a system are offered as a basic planning guide:
1. Calculate the heat loss from each room in the structure, and add these together for the total heat loss.
2. Plan the leaders (i. e., the horizontal ducts or pipes leading from the furnace) so that none are longer than 10 ft in length.
3. Keep the number of elbows in the leader to a minimum.
4. Locate the warm-air outlets (registers) and return air grilles as close to the floor in each room as possible.
5. Locate the warm-air outlets (registers) on the inside walls of each room and in that part of the room closest to the furnace.
6. Select warm-air and return air ducts according to their Btu carrying capacities.
7. Select a furnace capable of delivering in Btu/h a register delivery equal to the total heat loss calculated for the structure.
8. Locate the furnace in the lowest part of the structure and as near to the center as possible.