Ductwork Sizing Reference

Why this matters

Undersized ducts choke airflow → equipment short-cycles, comfort suffers, energy bills rise. Oversized ducts cost more + may cause stratification (cold air at bottom). The HVAC tech who can size ducts correctly delivers comfort + efficiency. This is the field card.

ACCA Manual D (residential ductwork design)

Industry-standard sizing for residential ducts. Requires:

• Manual J load calculation (heat gain / loss per room)
• Manual S equipment selection
• Manual D duct sizing (flow rate + pressure drop)

Software: Wrightsoft (Right-J / Right-S / Right-D), Elite RHVAC.

For service work: rough sizing tables suffice for replacement; full Manual D required for new builds + major modifications.

Airflow basics

CFM (cubic feet per minute): volume of air moved.

Friction loss / static pressure: resistance to airflow.

TESP (total external static pressure): sum of supply + return pressure drops. Most residential equipment rated 0.5" WC max.

Higher TESP = blower works harder + less airflow.

Goal: airflow at correct CFM per ton (typical 400 CFM/ton for cooling) + TESP < equipment maximum.

CFM per ton (rule of thumb)

Air conditioning: 350 - 450 CFM per ton (400 typical).

Heat pump heating: 350 - 400 CFM per ton.

Gas furnace heating: 120 - 200 CFM per 1,000 BTU (lower than cooling).

Example 3-ton AC: 1,200 CFM target.

Friction loss target

Goal: 0.08 - 0.10 inches WC per 100 ft of equivalent duct length.

Use ACCA "friction chart" OR sizing wheel for round duct sizing based on CFM + friction.

Round duct sizing (CFM at 0.08" WC / 100 ft)

4" 30 CFM; 5" 70; 6" 110; 7" 150; 8" 220; 9" 280; 10" 380; 12" 600; 14" 900; 16" 1,300; 18" 1,800; 20" 2,400.

Rectangular duct sizing

Typical residential trunk: 8 × 8 ~ 180 CFM; 8 × 10 ~ 240; 8 × 12 ~ 320; 8 × 14 ~ 400; 10 × 14 ~ 520; 10 × 20 ~ 850; 12 × 24 ~ 1,400.

Common branch + trunk sizing

Branch duct (room supply): small bedroom 6" round (100 - 150 CFM); master bedroom 7 - 8" (150 - 250); living room 8 - 9" (200 - 300); large open 9 - 10" (300 - 400). Trunk: 3-ton (1,200 CFM) 14 × 6 OR equivalent; 4-ton (1,600 CFM) 16 × 8; 5-ton (2,000 CFM) 20 × 8.

Return duct

Often undersized: leads to "starved" blower.

Should be 1.5 - 2x supply CFM in cross-section (return air must be moved at same volume + lower velocity).

Common mistake: single 14 × 24 return for 5-ton system; should be 14 × 30 OR multiple returns.

Filter slot sizing: filter face must allow rated CFM at low velocity (< 300 FPM).

Duct material

Galvanized sheet metal: standard residential trunk. Most-common in unconditioned spaces.

Flex duct (insulated): branch runs; install correctly (stretched, supported).

Duct board (fiberglass): sound-absorbing; specialty.

Aluminum (flexible): lighter than galvanized.

Spiral metal: commercial; round.

Rectangular metal: more turbulent flow than round; common trunk OR plenum.

Flex duct considerations

Smooth-bore vs spiral: smooth bore lower friction; preferred where pressure budget tight.

Don't stretch beyond manufacturer max: stretched > 4% adds friction + reduces lifespan.

Don't crush OR kink: catastrophic CFM loss.

Support spacing: per manufacturer (typical 4 ft).

Length limit: shorter is better; long flex runs add friction loss.

Connection: collar + clamp band + sealed inner liner; outer jacket secured.

Common ductwork mistakes

Undersized return: blower starved; short cycles; no comfort.

Crushed flex duct: dropping CFM 30%+ vs straight run.

Too many 90° bends: each adds 15 - 20 equivalent feet of duct length.

Oversized branch + undersized trunk: pressure drops in branch don't help.

Wrong takeoff angle (45° preferred over 90° to branch): improves flow.

Plenum-to-air handler gap: leaks reduce delivered CFM.

Missing balancing dampers: cannot adjust per room.

Filter slot leak: bypass air entering downstream of filter.

Equivalent length (for sizing)

Every fitting adds "equivalent length" to total flow path:

Total equivalent length = actual duct + sum of fittings × equiv. lengths.

Friction loss at 0.08" WC per 100 ft × total equiv. length / 100 = expected TESP contribution.

Static pressure budget

Equipment max TESP: 0.5" WC residential typical (varies; check nameplate).

Internal components consume:

• Filter: 0.05 - 0.30" WC
• Evaporator coil: 0.10 - 0.20" WC
• Heating element OR heat exchanger: 0.05 - 0.10" WC
• Internal duct connections: 0.05 - 0.10" WC

Available for external ductwork: 0.10 - 0.20" WC typical.

Stay within budget: too tight = blower starved + insufficient CFM.

Measuring + testing

Static pressure: manometer at supply + return ports; sum = TESP.

CFM at register: anemometer × area = CFM per register.

Total CFM: sum of register CFM should match equipment nameplate.

Balance: adjust dampers + register angle to equalize room-by-room comfort.

Common signs of bad ductwork

Hot/cold rooms (same system, different temps): unbalanced.

Noisy ducts: velocity too high; undersized.

Equipment cycling on/off (short cycle): starved airflow.

Whistling vents: oversized OR partially closed register.

Dust streaking on register face: high velocity + duct edge wear.

Insufficient CFM at register: leaks, undersized branch, crushed flex.

Customer education

Why ductwork matters: equipment can be perfect but duct delivery system limits performance.

Symptoms of duct issue: rooms different temps; noise; high bills.

Cost-benefit of duct work: improves comfort + efficiency; ROI in utility savings + comfort.

Truck stock

Manometer, anemometer, foil tape 181A, mastic + brush, mesh tape, balancing dampers, register boxes (4", 6", 8", 10"), flex duct (insulated R6), zip ties for flex jacket.

References

• ACCA Manual D (Residential Duct Systems)
• ACCA Manual S (Equipment Selection)
• ASHRAE Fundamentals Handbook
• SMACNA HVAC Duct Construction Standards
• Manufacturer technical data
• Manuall internal: Duct Sealing Methods, Ventilation Fan Ratings