Master Catalog>>General Information>>Page Q-9


Ducting

Furnace Selection and Extended Plenum Duct Sizing

The size of the extended plenum (use standard duct)is as shown when the furnace is located:

Heat
Load
(BTU)

Located
One End of
Building

Building Divided
1/3 and 2/3

Furnace
Middle Of
Building

0-56,000

14 x 8

10 x 8 & 10 x 8

(2) 10 x 8

57-80,000

16 x 8

10 x 8 & 12 x 8

(2) 10 x 8

81-108,000

22 x 8

10 x 8 & 14 x 8

(2) 12 x 8

109-132,000

26 x 8

10 x 8 & 16 x 8

(2) 16 x 8

133-160,000

30 x 8

12 x 8 & 22 x 8

(2) 16 x 8

NOTE: If air conditioning is to be used at the present or in the future, the cooling load must be computed and duct size selected on the basis of the larger size indicated, shown below.

Maximum Applicable Cooling Unit

The size of the extended plenum in inches is as shown when the cooling unit is located:

Cooling
Load

Blower
Capacity
(CFM)

Located
One End of
Building

Building Divided
1/3 and 2/3

Located
Middle Of
Building

0 - 24,000

2 Ton (720)

16 x 8

10 x 8 &
12 x 8

(2) 10 x 8

25-36,000

3 Ton (1080)

22 x 8

10 x 8 &
14 x 8

(2) 12 x 8

37-60,000

5 Ton (1800)

30 x 8

12 x 8 &
22 x 8

(2) 16 x 8

Return Grille and Return Duct Sizing

Single return. Use one of size shown.

Furnace
Output
(BTU)

Grill Size
in
Free Area

Duct
Size

to 56,000

216 sq. in.

16" x 8" or 1 joist space

to 80,000

270 sq. in.

16" x 8" or 2 joist spaces

to 108,000

405 sq. in.

26" x 8" & 10" x 8" or 2 joist spaces

to 132,000

450 sq. in.

30" x 8" or 2 joist spaces

to 160,000

540 sq. in.

(2) 20" x 8" or 3 joist spaces

Two returns. Use two of size shown.

Furnace
Output
(BTU)

Grill Size
in
Free Area

Duct
Size

to 56,000

108 sq. in.

16" x 8" or 1 joist space

to 80,000

162 sq. in.

16" x 8" or 2 joist spaces

to 108,000

216 sq. in.

26" x 8" or 2 joist spaces

to 132,000

244 sq. in.

30" x 8" or 2 joist spaces

to 160,000

285 sq. in.

(2) 20" x 8" or 3 joist spaces

NOTE: When multiple returns are used, a preferred return system provides for a return from each room (except that air usually is not returned from the kitchen or bathrooms).
In the return system maintain air volume equal to or greater than air volume of the supply duct system, and use same duct sizing as for single or two-return system.

Orifice Capacities for LP-Gas

An orifice chart can only be a guide because of varying temperatures and barometric pressures from day to day in various localities. From the below drill sizes, it may be necessary to make adjustments for the most satisfactory operating conditions. Rates based on 2500 BTU/ft.3, manifold pressure 10.5" water column.

High Altitude Corrections

The specifications shown on this chart for orifice sizes are pressures from sea level to approximately 3500 ft. above sea level. For altitudes from 3500 ft. to 5000 ft., use one drill # larger, From 5000 ft. to 6500 ft., use two drill #'s  larger.  Above 6500 ft., use two or three drill sizes larger.

Refer to NFPA 54 for a comprehensive chart.

 

Wire Gauge