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BLD-HVAC07-06 · Lesson 10 · 14 clock hours · theory

Duct System Inputs from Load Data (Manual D Fundamentals)

Objective. Students use room-by-room load calculation results to determine required CFM for each room and identify trunk and branch duct sizes from Manual D friction rate charts, producing a duct sizing summary sheet for the residential case study.

Why Duct Design Starts with the Load Calculation

A duct system is not sized by guesswork or rule of thumb. It is sized to deliver exactly the right amount of conditioned air to each room so the equipment can maintain design indoor conditions. That connection — from Manual J load to Manual D duct size — is the foundation of this lesson.

ACCA Manual J gives you the sensible and latent heat load for every room. Once you have selected equipment (Manual S), you know the total system airflow in cubic feet per minute (CFM). Manual D then tells you how to distribute that airflow through a duct network so each room receives its proportional share.

Step 1 — Converting Room Load to Required CFM

The fraction of total system airflow each room needs equals the fraction of total sensible load that room represents. The formula is:

Room CFM = (Room Sensible BTU/h ÷ System Sensible BTU/h) × Total System CFM

For example, if the whole house has a design sensible cooling load of 24,000 BTU/h and the selected equipment delivers 1,000 CFM, a bedroom with a sensible load of 3,600 BTU/h needs:

(3,600 ÷ 24,000) × 1,000 = 150 CFM

Repeat this calculation for every room and record the results on your CFM summary sheet. The sum of all room CFMs must equal total system CFM. If it does not, recheck your arithmetic before proceeding.

Step 2 — Understanding the Manual D Friction Rate Method

Once you know how much air each room needs, you size the ducts to deliver it. Manual D uses the friction rate method, which works in two parts.

Available Static Pressure (ASP): Every air-handling unit has a rated external static pressure — the pressure it can overcome while moving air. From that rated value you subtract the pressure drops caused by coils, filters, grilles, registers, and any factory accessories. What remains is the pressure budget available to push air through the duct system itself.

Total Effective Length (TEL): The longest supply or return duct path from the air handler to the farthest terminal is measured. Fittings (elbows, tees, transitions) are converted to equivalent straight lengths using tables in Manual D and added to the actual straight length. This gives the TEL for the critical path.

Design Friction Rate (FR): Dividing the available static pressure by the TEL gives the friction rate in inches of water column per 100 feet of duct (in. w.c./100 ft). This single number is your sizing target for every duct in the system.

FR = Available Static Pressure (in. w.c.) ÷ (TEL ÷ 100)

With the friction rate and the required CFM for each duct segment, you enter the friction rate chart (also called a duct slide rule or Moody-based chart) and read off the duct diameter for round duct, or use the equivalent diameter tables to select rectangular dimensions.

Reading the Friction Rate Chart

A friction rate chart plots CFM on one axis and friction rate on another, with duct diameter as a family of curves. To size a duct segment:

1. Locate the design friction rate on the horizontal axis.

2. Move vertically to the CFM carried by that segment.

3. Read the nearest standard duct diameter. Always round up to the next standard size (e.g., from 5.8 in. to 6 in.) to stay within the pressure budget.

For rectangular duct, find a combination of width and height whose equivalent diameter matches the round size. Manual D provides tables for this conversion. Keep aspect ratios at or below 4:1 where possible to limit friction losses.

Trunk vs. Branch Ducts

A trunk duct (also called a main or plenum extension) carries air from the air handler toward the zones it serves. Its CFM equals the sum of all branch CFMs it feeds. Trunk CFM decreases at each branch takeoff, so a reducing trunk gets smaller as it moves away from the unit.

A branch duct serves a single room or terminal. Its CFM equals the room CFM calculated in Step 1.

Size each trunk segment using the CFM it carries at that point in the system, always at the design friction rate.

Recording Your Work — The Duct Sizing Summary Sheet

For each duct segment, record: segment label, CFM carried, friction rate used, calculated diameter (round), and selected standard size or rectangular equivalent. A simple single-line duct layout sketch — a schematic, not a to-scale drawing — shows how segments connect and makes it easy for an inspector or colleague to follow your logic.

Safety and Professional Notes

Duct sizing is a design activity, but duct installation involves real hazards. When you move from calculation to installation:

On the calculation side, always base your duct design on a completed Manual J report and a confirmed equipment selection. Sizing ducts for the wrong load or wrong airflow produces a system that will underperform, cause comfort complaints, and may contribute to moisture and indoor air quality problems.

Key terms9

Manual DACCA's residential duct design standard; it provides the friction rate method and tables for sizing supply and return duct systems based on Manual J load data.
Room CFMThe volume of conditioned air per minute required by a single room, calculated as the room's share of the total sensible load multiplied by total system airflow.
Available Static Pressure (ASP)The portion of the equipment's rated external static pressure that remains after subtracting pressure drops from coils, filters, and accessories; it is the pressure budget for the duct system.
Total Effective Length (TEL)The sum of actual straight duct length and the equivalent straight lengths of all fittings along the longest (critical) duct path; used to calculate the design friction rate.
Design Friction Rate (FR)Available static pressure divided by TEL per 100 feet, expressed in inches of water column per 100 feet; this value is used to enter the friction rate chart and select duct sizes.
Friction Rate ChartA chart (or duct slide rule) that relates duct diameter, airflow in CFM, and friction rate, allowing the designer to select the correct duct size for a given CFM and friction rate.
Trunk DuctA main supply or return duct that carries combined airflow from or toward the air handler; its CFM decreases at each branch takeoff.
Branch DuctA duct that serves a single room or terminal; its CFM equals the room's required CFM from the load calculation.
Equivalent DiameterThe diameter of a round duct that has the same friction loss per unit length as a given rectangular duct; used to convert between round and rectangular duct sizing.

Worked examples4

1. A living room has a sensible cooling load of 4,800 BTU/h. The whole house sensible load is 32,000 BTU/h and the selected equipment delivers 1,200 CFM. What is the required room CFM for the living room?
  1. Identify the values: Room Sensible = 4,800 BTU/h; System Sensible = 32,000 BTU/h; Total System CFM = 1,200 CFM.
  2. Apply the formula: Room CFM = (Room Sensible ÷ System Sensible) × Total System CFM.
  3. Room CFM = (4,800 ÷ 32,000) × 1,200 = 0.15 × 1,200 = 180 CFM.
Answer: The living room requires 180 CFM.
2. After subtracting losses for the coil, filter, and return grille from the equipment's rated external static pressure, the available static pressure for the duct system is 0.30 in. w.c. The total effective length of the critical supply path is 150 feet. What is the design friction rate?
  1. Identify the values: ASP = 0.30 in. w.c.; TEL = 150 ft.
  2. Apply the formula: FR = ASP ÷ (TEL ÷ 100).
  3. FR = 0.30 ÷ (150 ÷ 100) = 0.30 ÷ 1.50 = 0.20 in. w.c. per 100 ft.
Answer: The design friction rate is 0.20 in. w.c./100 ft.
3. Using a design friction rate of 0.20 in. w.c./100 ft, the friction rate chart shows that 180 CFM at that friction rate falls between the 7-inch and 8-inch diameter curves (closer to 7 in.). What standard duct diameter should be selected, and why?
  1. The chart reading falls between 7 in. and 8 in., closer to the 7-inch curve.
  2. Manual D practice requires rounding up to the next standard size to stay within the pressure budget and avoid undersizing.
  3. Select 8-inch diameter round duct to ensure adequate airflow without exceeding the available static pressure.
Answer: Select 8-inch diameter round duct.
4. A supply trunk segment feeds three branch ducts with room CFMs of 120, 150, and 180 CFM. What CFM must the trunk segment carry, and if a second downstream trunk segment feeds only the 150 and 180 CFM branches, what CFM does that segment carry?
  1. The first trunk segment (closest to the air handler) must carry all air it feeds: 120 + 150 + 180 = 450 CFM.
  2. After the 120 CFM branch takeoff, the trunk continues toward the remaining two branches: 150 + 180 = 330 CFM.
  3. The first trunk segment is sized for 450 CFM; the second (reducing) trunk segment is sized for 330 CFM, both at the design friction rate.
Answer: First trunk segment: 450 CFM; second trunk segment: 330 CFM.

The math in this lesson2

Learners apportion system CFM to rooms in proportion to their share of sensible load.
6.RP.37.NS.1
In class: Instructor explains the connection between load calculation and duct design: how room CFM is derived from room sensible load and equipment total airflow, and how Manual D friction rate method uses total effective length to set available static pressure for sizing. · Students calculate required CFM for each room of the residential case study using the formula (Room Sensible BTU/h ÷ System Sensible BTU/h) × Total System CFM and record on a room CFM summary sheet. · Using a Manual D friction rate chart provided by instructor, students size two supply trunk segments and three branch ducts for the case study, recording duct diameter or equivalent rectangular dimensions on a simple duct layout sketch.
Materials: ACCA Manual D (instructor-provided excerpts or summary charts), students' room-by-room Manual J reports, equipment selection memos (from Lesson 9), printed CFM summary worksheet, friction rate charts, pencils, rulers
Check: CFM summary sheet and duct sizing sketch submitted; pass = all room CFMs correctly calculated (within 5% of key) and at least five duct sizes correctly selected from friction rate chart.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

HVAC Excellence / NCCER HVAC — Load Calculations and Duct Design
1. A bedroom has a sensible cooling load of 2,400 BTU/h. The total house sensible load is 24,000 BTU/h and the system delivers 1,000 CFM. What is the required room CFM?
NATE — Air Distribution
2. Which ACCA manual provides the friction rate method for residential duct sizing?
NATE — Air Distribution
3. Available static pressure for a duct system is 0.25 in. w.c. and the total effective length of the critical path is 125 feet. What is the design friction rate?
HVAC Excellence — Duct Systems
4. When the friction rate chart reading for a branch duct falls between two standard duct sizes, Manual D practice requires the designer to:
NCCER HVAC — Duct Systems
5. Total Effective Length (TEL) for a duct path includes:
NATE — Air Distribution
6. A supply trunk feeds four branches with CFMs of 100, 120, 140, and 160 CFM respectively. At the point in the trunk just before the 100 CFM branch takeoff, what CFM is the trunk carrying?
NCCER HVAC — Safety
7. Which personal protective equipment is most important when handling sheet metal duct sections during installation?
NCCER HVAC — Safety / OSHA Compliance
8. Before performing any work inside an air-handling unit during duct installation — such as connecting ductwork at the supply plenum — the technician must:
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