Some Ductwork Help for Dummies

Air Filter Rack Sizing
I realize this will never happen if you use the furnace filter size; however, Air Filter Rack Sizing for efficient operation - Size Gross Return Air filter grille area for 200-sq. ins. per ton. For a 5-Ton system, that would mean Two filter racks 25X20's each, I would go with Two racks & 25X25's each.

Keep air velocities through the filter(s) as low as possible. It is a good idea to Size the Filter rack for a Free Air Area with a Velocity of 350-FPM or less.

A 5-Ton system requires two filter racks to achieve a low enough air velocity through those filters. Most 5-Ton systems nearly always have too much air velocity through initial clean filters, let alone when they begin loading.

ACCA Manual D specifies a maximum Return grille velocity of less than 500 ft per minute and a maximum Supply outlet of 700 ft per minute (fpm) or less.

All filter mfg'ers should print the free air area of the clean filter on the edge of the filter along with the pressure drop data. Divide the rated CFM the duct is carrying by the free area sq.ft. of the filter for airflow velocity in FPM. (Or use above forumula with the duct's sq.ft. area for duct airflow velocity)

Velocity in FPM = Known designed CFM to room divided / by Sq. feet of duct area.
I.E., 8" duct 8x8 = 64 x .7854 = 50.26-sq. In. area / 144 = 0.3490666-sq.feet | designed CFM to room is 173-CFM X's .3490666 = Velocity of 495.6-FPM, you can use a ductulator to get the actual Friction Rates (FR).

Formula for finding CFM Airflow and/or Velocity in FPM
If you can measure the air velocity coming from a duct, here is a rough ballpark formula to get the CFM:
CFM = (velocity in (FPM) Feet per Minute times the square footage of the duct area)
I.E., 16" Rd duct 201-sq.ins. / 144 = 1.3958333-sq.ft. X's Velocity of 800-fpm = 1116-CFM
Times 1000-FPM = 1395-CFM. Branch ducts: 7" Rd duct 38.48-sq. ins. = 0.2672222-sq.ft. X's 500-fpm=133-cfm
All filter mfg'ers should print the free air area of the clean filter on the edge of the filter along with the pressure drop data. Divide the rated CFM the duct is carrying by the free area sq.ft. of the filter for airflow velocity in FPM.

Remember:

When sizing ducts, the use of one Fiction Rate value throughout will usually guarantee incorrect duct size, velocities & CFMs for some duct run segments. If Supply Side is 0.05" per 100 ft of duct run, or less is used, some runouts of the system may be oversized, creating zones that are too cold in summer. Usually the furthest from the air handler will be undersized creating zones that are too warm in summer. Install dampers in all Supply Air ducts for some balancing.

That is an incorrect design. The modified equal friction method of Manual D requires that the Available Static Pressure (ASP) from the fan be "consumed" by the duct through its run from fan to outlet/inlet, with no shortage or excess at the end. Also, pressure drop per 100 ft is not an input--it is a calculated intermediate value.

A contractor or Tech who knows how to do the proper calculations to determine the available static pressure - ASP and correctly allocate it to supply and return, is a very rare Tech indeed. First, select the CFM & velocities you want, that may result in small Friction Rate variations on the various Branch Runs & run lengths.

Always use "Manual D" for proper FRs & duct sizing for required CFMs to each room.
I used figures & formulas below for the table, & wanted 450-Btuh wet coil per ton airflow
Converting square duct inches to round duct size, i.e., an 8" x 8" duct = 64 -sq.ins. x .7854 = 50.26 sq. ins. You round off to 50 sq. ins. for an 8" duct.
Round duct diameter to Sq. Ins., duct diameter 6"x6" = 36 X's .7854 = 28.27-sq. ins.
CFM = (velocity in (FPM) Feet per Minute times the square footage of the duct area)
Quick method: The heat gain and Btu/hr of cooling is done for each room.
Use 30-Btu/hr for each Cubic Foot per Minute (CFM) of Airflow.

Duct Sizing Chart for Approximately 425 to 450-CFM Wet Coil Per Ton on Main Runs
BTUH
CFM
Rd. DIA.
Sq"
Vel/FPM

1290
43
4" BR. Outlets
12.5
493





2031
68
5
19.6
499





2910
97
6"
28
489

10x8
12x7
9x9

3200
106
6" Br
28
540

14x2
7x4


4008
134
7" Br
38.48
502

10x4
8x6
7x6

5200
173
8" Br
50
496

7x7
10x6 8x7

6666
222
9" Br
64
505

8x8
10x7
12x6

9812
327
10" Br
78.5
600

14x6 12x8
16x5

18000
700
13"Metal SA|RA 14" 154-sq"
201 RA
RA 502
RA 14x11
12x12 18X8
16X9

24000
900
14" Metal SA/RA 16" 201 RA
RA 645
RA 14x14
18x10
20x8
14x12

32000 1125
16" metal SA/RA 20"
254 RA
RA 20" 516 RA 14x18
20x10 21x11


36000
1350
18" SA RA 22" 380"
254 SA
RA 22" 512 24X11
16X16
18x14


42000
1575 18" metal SA/RA 22"
380 RA
RA 597
14x22

16x16


48000
1800
20" SA/RA 24"
380 RA
RA 573
14x27
20X19



60000
2000 22" SA/RA 24" 452-sq.ins. 452 RA
RA 637 RA 14x32
16x30
22x21



Solving for Available Static Pressure (ASP) - When Designing or Redesigning Duct Systems, TEL, FR:
Find the Total Return and Supply Lengths by measuring duct, runouts, and adding transitions, turning elbows with or without turning vanes, trunk take-offs, and boots, diffuser pressure drops, filters, etc.

Once you have all the correct Pressure Drops (PDs) for those lengths & devices, use the manufacturer's nameplate pressure (IWC) or .5" ESP and subtract all airstream device pressure losses (filters, wet coil, etc. all available in manual D) from that given value. That will leave the "Available Static Pressure" - ASP for duct & blower design purposes.

You can figure the Total Equivalent Length (TEL) by using the Manual D length additives for the various fittings, then use your Duct Designer ductulator to properly size the duct system to meet the velocities and CFMs required in respect to the blower's Nameplate ESP and its performance graph data.
There are charts available to determine what the total pressure drop will be when you figure the "Total Equivalent Length" run of the longest Supply duct runs; ALL lengths of duct and ALL fittings for turning, reductions, etc., have a Total Effective Length (TEL) additive to the duct length - that must be added to the regular length of that duct run!

The best thing to do is figure your available static pressure. using .5” ESP is good, because most furnaces are designed for .5" ESP to get desired CFM, if needed, you can always use a lower blower speed. You subtract any external pressure drops to the furnace evap coil, filter, registers, dampers, anything outside of furnace cabinet.

Then get a total equivalent length of your ductwork most ductulators have this on the back of them. Friction Rate = Available Static Pressure times X’s 100; divided by the TEL, that is the friction rate per 100 ft of SA & RA duct.