📐 HVAC Duct Sizing Methods: Imperial & Metric Guide for Engineers

Learn how to size HVAC ducts using Equal Friction, Velocity Reduction, and Static Regain methods. Includes formulas, velocity benchmarks, and ASHRAE-compliant tips in both Imperial and Metric systems.

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🔍 What Is HVAC Duct Sizing?

Duct sizing determines the dimensions of air pathways that deliver conditioned air throughout a building. Proper sizing ensures:

• Adequate airflow (CFM or L/s)
• Acceptable velocity (FPM or m/s)
• Minimal pressure loss
• Energy-efficient fan operation

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📊 Common Duct Sizing Methods

1. Equal Friction Method

Used for: Residential and commercial systems
Goal: Maintain a constant friction rate across all duct sections

Steps:

1. Choose a friction rate:
   • Imperial: 0.1 in.wg per 100 ft
   • Metric: 0.8 Pa/m
2. Use Ductulator/Mcquay Duct Sizer or charts to find duct size for each airflow rate.
3. Apply the same friction rate throughout the system.

Formula:

• Imperial: Friction Loss = (Total Pressure Drop / Duct Length) * 100
• Metric: Friction Loss = (Total Pressure Drop / Duct Length) * 1

Pros: Simple and consistent
Cons: May oversize short branches

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2. Velocity Reduction Method

Used for: Noise-sensitive or high-speed systems
Goal: Control air velocity to reduce noise and turbulence

Steps:

1. Select target velocity:
   • Imperial: Main duct = 800 to 1200 FPM; Branch = 500 to 800 FPM
   • Metric: Main duct = 4 to 6 m/s; Branch = 2.5 to 4 m/s
2. Calculate duct area:
   • Imperial: Area (sq.ft) = CFM / Velocity (FPM)
   • Metric: Area (m²) = L/s / Velocity (m/s)
3. Convert area to diameter or rectangular dimensions.

Pros: Controls noise and turbulence
Cons: Requires velocity charts and pressure checks

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3. Static Regain Method

Used for: Large commercial systems
Goal: Recover static pressure lost due to velocity changes

Formula:

• Imperial: Static Regain = (V1 * V1 - V2 * V2) / (2 * g), where g = 32.174 ft/s²
• Metric: Static Regain = (V1 * V1 - V2 * V2) / (2 * g), where g = 9.81 m/s²

Example (Imperial):

• V1 = 1200 FPM, V2 = 800 FPM
• Static Regain = (1200 * 1200 - 800 * 800) / (2 * 32.174)
• Static Regain ≈ 12.43 in.wg

Example (Metric):

• V1 = 6 m/s, V2 = 4 m/s
• Static Regain = (6 * 6 - 4 * 4) / (2 * 9.81)
• Static Regain = (36 - 16) / 19.62 ≈ 1.02 Pa

Pros: Balances pressure across branches
Cons: Complex, best used with software

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📏 Duct Shape and Equivalent Diameter

Round ducts are more efficient than rectangular ones. Use this formula to convert rectangular ducts to equivalent round diameter:

Formula:

• Imperial: D_eq (in) = (1.3 * (W * H)^0.625) / (W + H)^0.25
• Metric: D_eq (mm) = (33 * (W * H)^0.625) / (W + H)^0.25

Where:

• W = width
• H = height

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📈 Velocity Benchmarks (ASHRAE Guidelines)

Duct Type	Comfort Systems (FPM / m/s)	Industrial Systems (FPM / m/s)
Main Ducts	800–1200 / 4–6	1500–2500 / 7.5–12.5
Branch Ducts	500–800 / 2.5–4	1000–1800 / 5–9
Terminal Grilles	300–500 / 1.5–2.5	800–1200 / 4–6