Duct & airflow data

Air speeds, duct metal gauges, pressure drops, air density and leakage limits.

Maximum air speed in ducts (fpm)

Building	Main supply	Main return	Branch supply	Branch return
Homes and apartments	1000	800	600	600
Auditoriums and theatres	1300	1100	1000	800
Offices, banks, libraries, meeting rooms, restaurants, shops	2000	1500	1600	1200
Hospital and hotel rooms	1500	1300	1200	1000
Industrial	3000	1800	2200	1500

In a residential duct, over 700 fpm is often noisy. Over 500 fpm at a register is felt as a draft. A draft is also anything over about 15 to 20 fpm in an occupied space.

Typical pressure drops (in. WC)

Device	Drop
Filter	0.08 to 0.15
Evaporator coil	0.10 to 0.30
Air cleaner	about 0.20
Damper	about 0.03
Register, diffuser or grille	about 0.03

Available static pressure = blower external static pressure − device drops. A residential system should not exceed 0.5 in. WC total (supply static plus return static). Replace a filter when its drop is more than 25% of the blower's pressure rise.

Round duct metal thickness (gauge)

Diameter	Commercial	Residential
12 in or less	26	30
13 to 18 in	24	28
19 to 28 in	22	–
27 to 36 in	20	–
35 to 52 in	18	–

Rectangular duct metal thickness (by wide side)

Wide side	Commercial galvanized	Aluminium (in)	Residential galvanized
12 in or less	26	0.020	28
13 to 23 in	24	0.025	26
24 to 30 in	24	0.025	24
31 to 42 in	22	0.032	–
43 to 54 in	22	0.032	–
55 to 84 in	20	0.040	–
85 in and over	18	0.050	–

Aspect ratio

Class	Ratio	Wide side	Perimeter
1	1 : 1	6 to 18 in	24 to 72 in
2	2 : 1	12 to 24 in	36 to 72 in
3	3 : 1	26 to 40 in	70 to 106 in
4	4 : 1	24 to 88 in	60 to 220 in
5	5 : 1	48 to 90 in	116 to 216 in
6	6 : 1	90 to 144 in	210 to 336 in

A square duct needs the least metal. A 20 × 5 duct has the same area as a 10 × 10 but uses 25% more metal.

Air density

Temp (°F)	Dry air (lb/ft³)	Dry air (ft³/lb)
0	0.08635	11.58
50	0.07786	12.84
70	0.07492	13.35
100	0.0709	14.10
120	0.0685	14.60
150	0.0652	15.30
200	0.0600	16.70

Standard air: 0.075 lb/ft³. Heat in air: 0.24 BTU/lb·°F. Sensible heat BTU/h = 1.08 × cfm × temperature difference.

Rules of thumb

Cooling airflow: about 400 cfm per ton. Cooling needs 30 to 50% more air than heating, so a blower may need a different tap or pulley for each.
Branches go into the side of a plenum, never the end, because an end duct hogs the air.
Reducing transitions: 1 inch of change per 4 inches of length. Enlarging: 1 inch per 4 to 7 inches.
A return grille blocks about 30% of its face. Allow for it.
Seal ducts with mastic (1 inch each side of the joint, with mesh for gaps wider than 1/4 inch), never duct tape.
Return grilles belong in the stagnant zone: near the floor for heating, near the ceiling for cooling, and far from supplies.
Duct leakage: a common limit is 4 cfm at 25 Pa per 100 ft² of conditioned floor area. Check the code that applies to your job.
Blower-door limit often quoted: 3 ACH at 50 Pa. Check local code.
Pitot tube: velocity (fpm) = 4005 × √(velocity pressure in in. WC) for standard air. Measure at least 10 duct diameters downstream of any fitting and average a traverse.
Swinging-vane anemometers are calibrated for 68 °F. At other duct temperatures, corrected speed = reading × (460 + duct °F) ÷ 528.

Typical air changes per hour

Space	Heating	Cooling
Homes	3 to 6	6 to 9
Offices	5 to 8	6 to 12
Public assembly spaces	5 to 10	6 to 12

Minimum for a home in heating is about 3 ACH. Outdoor air per person: about 15 cfm. Time for one change (min) = volume ÷ cfm.

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Calculation aids and reference figures for licensed technicians. Always follow the manufacturer’s data plate and manual for the exact model, and local codes. Gas, electrical and refrigerant work must be done by a qualified, licensed person. Pressures are typical saturation values and change with load and airflow.