AIR SUPPLY

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Surface Area Formulas

Round Duct — Circumference

C = π × D

π ≈ 3.14. Convert inches to feet: divide by 12. SA = C (ft) × Length (ft)

Rectangular Duct — Perimeter

P = 2 × (W + H)

Convert to feet first. SA = P (ft) × Length (ft)

Oval Duct — Perimeter

P = π × b + 2 × (a - b)

a = major axis, b = minor axis. Same as a rectangle with semicircle ends.

90° Elbow — Surface Area

SA = C × (π × r / 2)

r = centerline radius of the elbow. Same principle: perimeter × arc length.

TAB Core Formulas

CFM — Airflow

CFM = Area × Velocity

Area in ft², Velocity in FPM. Divide area in in² by 144 to get ft².

Velocity from VP

V = 4005 × √VP

VP = velocity pressure (in. w.g.). Result in FPM.

Percent of Design

% = (Actual ÷ Design) × 100

Acceptable range: ±10% of design CFM.

Fan Law 1 — CFM vs RPM

CFM₂ = CFM₁ × (RPM₂ ÷ RPM₁)

CFM changes linearly with RPM.

Fan Law 2 — SP vs RPM

SP₂ = SP₁ × (RPM₂ ÷ RPM₁)²

Static pressure changes as the square of RPM ratio.

Fan Law 3 — BHP vs RPM

BHP₂ = BHP₁ × (RPM₂ ÷ RPM₁)³

Brake horsepower changes as the cube of RPM ratio.

Sensible Heat

BTUH = 1.085 × CFM × ΔT

ΔT = temperature difference (°F). 1.085 = air constant at sea level.

Hydronic Heat

GPM = BTUH ÷ (500 × ΔT)

500 = water constant (8.33 lb/gal × 60 min/hr).

Leakage Rate Calculation

Allowable Leakage CFM

L = F × SA ÷ 100

F = leakage factor from chart (CFM/100 ft²), SA = total surface area in ft².

Leakage as % of System CFM

% = (Leakage CFM ÷ System CFM) × 100

Used to verify the system is within acceptable leakage limits.

TAB Calculators

Enter values and press Calculate. Each result shows the formula with your numbers substituted in.

1. Allowable Leakage CFM

Leakage Factor (F)
CFM per 100 ft²

Surface Area (SA)
sq ft

2. Register / Grille Airflow

Width
inches

Height
inches

Face Velocity
FPM

3. Velocity Pressure

Velocity Pressure (VP)
in. w.g.

4. Fan Laws

RPM1 (original)

RPM2 (new)

CFM1 (original)

5. VFD — Speed & Fan Laws

Slide to a frequency. Enter nameplate values to see how CFM, SP, and HP change under the fan laws.

0 Hz 60 Hz 60 Hz

Nameplate RPM
(at 60 Hz)

Design CFM
(at 60 Hz)

Design SP
(in-wg, at 60 Hz)

Motor HP
(at 60 Hz)

Speed

100%

of nameplate

Actual RPM

RPM

CFM

proportional to speed

Static Pressure

proportional to speed²

Horsepower

proportional to speed³

Energy Savings

vs full speed

6. Motor Voltage Imbalance

NEMA limit: 1% max imbalance. Above 2% requires derating. Enter all three line-to-line voltages.

L1‑L2 (V)

L2‑L3 (V)

L1‑L3 (V)

7. Motor Amp Load

How hard is the motor working? Compare actual amps to nameplate FLA.

Actual Amps

Nameplate FLA

8. Synchronous Speed

The theoretical no-load speed of a motor. Actual speed is slightly less due to slip.

Frequency (Hz)

Number of Poles

5. Duct Sizing

CFM

Design Velocity
FPM

F Chart Calculator

F = CL × P^0.65 — Pick a leakage class and test pressure, then enter duct area to find allowable leakage.

Leakage Class (CL)

Test Pressure (in-wg)

F (CFM/100 sq ft)

Select CL and pressure above

Duct Surface Area (sq ft)

Seal Classes

Most Restrictive

Seal ALL transverse joints AND longitudinal seams and all penetrations. Required on high-pressure systems and where specified.

Moderate

Seal ALL transverse joints. Seams do not need to be sealed unless specified. Most common for medium-pressure systems.

Least Restrictive

No sealing required unless otherwise specified. Used for low-pressure systems where leakage is acceptable.

Pressure Classes

Pressure Class	Inches w.g.	Pascals	Operating Pressure
1/2" wg	1/2"	125 Pa	Up to 1/2" wg
1" wg	1"	250 Pa	Over 1/2" to 1" wg
2" wg	2"	500 Pa	Over 1" to 2" wg
3" wg	3"	750 Pa	Over 2" to 3" wg
4" wg	4"	1000 Pa	Over 3" to 4" wg
6" wg	6"	1500 Pa	Over 4" to 6" wg
10" wg	10"	2500 Pa	Over 6" to 10" wg

Leakage Classification — Default Values

Duct Type	Default Leakage Class
Rectangular Duct	16
Round Duct	8
Oval Duct	12

Common Sealants

Type	Best For	Notes
Mastic	Longitudinal seams, joints, large gaps	Flexible, long-lasting. Do not apply over dirt or oil.
Foil Tape	Joints, connections	Must be pressure class rated. Not all foil tape is approved.
Liquid Sealant	Penetrations, connections	Check manufacturer specs for temperature limits.
Duct Wrap + Mastic	Exposed ductwork requiring appearance	Used in schools, airports, public spaces.

Basic Test Kit Components

Component	Purpose
Test Fan	Pressurizes the duct section to the specified test pressure
Orifice Tube	Measures airflow through the system; has orifice plate + two pressure taps
Two Manometers / Magnehelics	One measures SP in duct; one measures ΔP across orifice plate
Calibration Chart	Converts ΔP reading to actual CFM leakage rate for the specific orifice tube
Tubing	Connects pressure taps to manometers

Prints F reference grid, classification charts, and your unit notes.

Duct Leakage Testing — F Chart Reference

F = CL × P^0.65 | L (allowable CFM) = (SA / 100) × F

F = CL × P^0.65 L = (SA ÷ 100) × F

Duct Leakage Testing — Classification Quick Reference

Seal classes, pressure classes, default leakage class values

Seal Classes

Class	Name	Requirement
A	Most Restrictive	Seal ALL transverse joints, longitudinal seams, and penetrations
B	Moderate	Seal ALL transverse joints. Seams unsealed unless specified.
C	Least Restrictive	No sealing required unless specified. Low-pressure systems.

Default Leakage Class Values

Duct Type	Default CL
Rectangular	16
Round	8
Oval	12

Pressure Classes

Class	In. w.g.	Operating Pressure Range
1/2"	0.5	Up to 1/2" wg
1"	1.0	Over 1/2" to 1" wg
2"	2.0	Over 1" to 2" wg
3"	3.0	Over 2" to 3" wg
4"	4.0	Over 3" to 4" wg
6"	6.0	Over 4" to 6" wg
10"	10.0	Over 6" to 10" wg

Common Sealants

Type	Best For	Notes
Mastic	Seams, joints, gaps	Flexible. Clean surface — no dirt or oil.
Foil Tape	Joints, connections	Must be pressure class rated.
Liquid Sealant	Penetrations	Check temperature limits.

Study Notes

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Lessons

Air Supply

Surface Area Formulas

TAB Core Formulas

Leakage Rate Calculation

TAB Calculators

F Chart Calculator

Seal Classes

Pressure Classes

Leakage Classification — Default Values

Common Sealants

Basic Test Kit Components