Duct Sizing Basics
CFM Requirements
Start with required airflow for each space:
- Typical residential: 400 CFM per ton of cooling
- Room CFM based on heat load calculation (Manual J)
- Supply CFM must match return CFM
Friction Rate Method
Calculate available friction rate:
- Determine total available static pressure (typically 0.5" WC)
- Subtract pressure drops for components (filter, coil, etc.)
- Remaining pressure ÷ total effective duct length = friction rate
- Use duct calculator to find size for CFM at that friction rate
Layout Principles
Trunk and Branch Design
Most common residential design:
- Large trunk line from equipment
- Trunk reduces in size as branches tap off
- Branch runs to individual rooms
- Keep branch runs as short as possible
Best Practices
- Avoid sharp 90° turns (use two 45s or radius elbows)
- Minimize flex duct length (max 15 feet, fully stretched)
- Support ducts every 4-5 feet
- Seal all joints with mastic or approved tape
- Insulate ducts in unconditioned spaces
Static Pressure Management
High static pressure causes:
- Reduced airflow and comfort
- Increased energy consumption
- Equipment noise
- Premature component failure
Pressure Budget
| Component | Typical Pressure Drop |
|---|---|
| Filter (clean) | 0.05-0.10" WC |
| Evaporator coil | 0.15-0.25" WC |
| Supply ductwork | 0.10-0.20" WC |
| Return ductwork | 0.05-0.10" WC |
| Grilles/registers | 0.02-0.05" WC |
Common Design Mistakes
- Undersized return air (should be larger than supply)
- Too much flex duct or poor installation
- Inadequate supply to master bedroom (large load, long run)
- Not accounting for filter upgrades
- Ignoring building pressure effects
Ductwork Supplies
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