Ductwork Installation Inspection

Per ACCA Manual D, design the duct system to deliver proper airflow to each room. Start with a Manual J load calculation to determine room-by-room heating and cooling loads in BTU/h, then convert to CFM (cubic feet per minute) requirements for each room. Select a duct layout type — trunk-and-branch is most common for homes, with a central trunk line from the air handler and smaller branch ducts feeding individual rooms. A radial layout runs individual ducts from a central plenum. Size all ducts using friction rate charts or duct calculators based on the required CFM.

1. Complete or obtain a Manual J load calculation for the home

2. Convert room loads to CFM requirements (typically 400 CFM per ton of cooling)

3. Select duct layout type: trunk-and-branch or radial

4. Size the main trunk to carry total system CFM

5. Size each branch duct for its individual room CFM requirement

6. Plan duct routing to minimize length and number of bends

7. Locate supply registers on exterior walls (under windows preferred for heating efficiency)

8. Plan return air locations — at least one return per floor, centrally located

- Undersized ducts — restricts airflow, increases static pressure, reduces system efficiency and comfort

- No Manual J calculation — guessing at duct sizes leads to rooms that are too hot or too cold

- Too many bends or long runs — each elbow adds equivalent length, reducing airflow significantly

- Insufficient return air — creates negative pressure in the home, causes drafts and backdrafting

Use galvanized sheet metal for main trunk lines and rigid branch runs for best airflow performance. Flexible duct is acceptable for final branch connections, limited to 25 feet maximum length with no more than 2 bends per run. All ducts in unconditioned spaces (attics, crawlspaces, garages) must be insulated to a minimum of R-8 per the International Energy Conservation Code (IECC). Use round duct where possible — it has better airflow characteristics than rectangular duct for the same cross-sectional area.

1. Select galvanized sheet metal for main trunk lines (typically 26 or 28 gauge)

2. Select rigid round metal duct for branch runs where possible

3. If using flex duct for branches, limit to 25 feet maximum per run

4. Verify duct insulation is R-8 minimum for unconditioned spaces (check local energy code — may be higher)

5. Size trunk line diameter for total system CFM using a duct calculator or friction chart

6. Size branch ducts: 6-inch for rooms up to 100 CFM, 7-inch for 100-150 CFM, 8-inch for 150-200 CFM

7. Purchase appropriate fittings: takeoffs, elbows, transitions, end caps

8. Purchase UL 181-rated mastic and metal foil tape for sealing

- Using flex duct for long trunk runs — excessive friction loss reduces airflow by 30-50 percent

- Undersized duct for the required CFM — creates whistling noise and insufficient airflow

- No insulation on ducts in unconditioned spaces — massive energy loss and condensation problems

- Using cloth-backed duct tape instead of UL 181 mastic — cloth tape fails within 1-2 years

Install the main supply and return trunks from the air handler plenum. Support trunk lines with metal hangers or straps every 10 feet maximum for metal duct, every 5 feet for flex duct. Seal all joints with UL 181 listed mastic sealant (not cloth duct tape). For sheet metal trunks, connect sections with drive cleats and S-clips, then seal with mastic. The supply trunk should be sized to reduce in cross-section as branch takeoffs reduce the required airflow — this maintains proper air velocity.

1. Connect the supply trunk to the air handler supply plenum with sheet metal screws and mastic

2. Hang trunk line sections using metal hangers or straps every 10 feet maximum

3. Connect trunk sections with drive cleats and S-clips

4. Seal all longitudinal seams and joints with UL 181 mastic

5. Install return trunk from the return plenum to the return grille location

6. Reduce supply trunk size after each major branch takeoff to maintain air velocity

7. Slope any horizontal trunk runs slightly toward the return for condensation drainage

8. Insulate all trunk lines in unconditioned spaces with R-8 insulation minimum

- Trunk not properly sealed — leaks conditioned air into unconditioned spaces, wasting 20-30 percent of energy

- Insufficient support — trunks sag and create low points where condensation collects

- No trunk reduction — air velocity drops as branches tap off, causing poor airflow to distant rooms

- Using cloth duct tape for sealing — dries out and falls off within 1-2 years, creating major leaks

Connect branch runs to the trunk line using proper takeoff fittings (round start collars for round branch ducts). Keep flex duct runs as short and straight as possible — every bend and extra foot of flex duct adds friction and reduces airflow. Support flex duct every 5 feet with wide strap hangers (not wire, which compresses the duct). Do not allow flex duct to sag more than 1/2 inch per foot of span. Avoid kinks and sharp bends — maintain a minimum bend radius of one duct diameter.

1. Install start collar takeoffs on the trunk at each branch location

2. Connect branch duct to the start collar and secure with mastic and screws (metal) or clamp and mastic (flex)

3. Route branch runs to register locations using the shortest, straightest path

4. Support flex duct every 5 feet with wide (1.5-inch minimum) strap hangers

5. Verify no kinks or sharp bends — minimum bend radius equals the duct diameter

6. Check that flex duct is fully extended (not bunched up) — compressed flex has much higher friction

7. Seal all connections with UL 181 mastic

8. Insulate all branch runs in unconditioned spaces to R-8 minimum

- Flex duct compressed or bunched — dramatically reduces airflow (bunched flex has 3-5x the friction)

- Kinks in flex duct at bends — completely blocks airflow to the room

- Wire hangers compressing flex duct — creates restriction points that reduce airflow

- Branch ducts too long — airflow is insufficient at the register, room does not heat or cool properly

Seal every joint, seam, and connection in the duct system with UL 181 listed mastic sealant or UL 181-rated metal foil tape. Do not use cloth-backed "duct tape" — it is not rated for duct sealing and fails within 1-2 years. Apply mastic with a brush or gloved hand to a minimum 2-inch overlap at every joint. Insulate all ductwork in unconditioned spaces (attics, crawlspaces, unheated basements, garages) to a minimum of R-8 with faced fiberglass or foam insulation. Seal insulation seams with foil tape.

1. Inspect every joint in the duct system — trunk connections, branch takeoffs, elbows, transitions

2. Apply UL 181 mastic to all joints with at least 2-inch overlap on each side

3. For large gaps, embed fiberglass mesh tape in the mastic for reinforcement

4. Verify no cloth duct tape is used anywhere in the system

5. Wrap all ducts in unconditioned spaces with R-8 faced insulation minimum

6. Seal insulation seams with foil tape to maintain the vapor barrier

7. Pay special attention to boot connections at registers — common leak points

8. Verify no exposed bare duct is visible in unconditioned spaces

- Using cloth duct tape instead of mastic — dries out, falls off, creates massive air leaks

- Missing seals at boot-to-register connections — leaks conditioned air into wall or floor cavities

- Insulation vapor barrier not sealed — moisture condenses on duct surface causing mold and dripping

- Mastic applied too thin — does not seal the joint, visible gaps remain

Install manual balancing dampers at each branch takeoff from the trunk line. Dampers allow adjustment of airflow to individual rooms for balanced comfort. Install supply registers sized for the room's CFM requirement — undersized registers create noise, oversized registers cause drafts. Supply registers on exterior walls should direct air toward the window. Install return grilles sized for system airflow — return grilles should be at least 1.5 times the area of the supply register to avoid air velocity noise.

1. Install balancing dampers at each branch takeoff point on the trunk

2. Verify damper handles are accessible for future adjustment

3. Cut register boot openings in floor, wall, or ceiling at planned locations

4. Install register boots and seal to ducts with mastic

5. Install supply registers — verify airflow direction points toward exterior walls/windows

6. Install return air grilles — verify size is adequate for system airflow (no whistling noise)

7. Label each damper handle with the room it serves

8. Leave all dampers fully open initially — balancing comes after system testing

- No balancing dampers installed — no way to adjust airflow to individual rooms, some rooms too hot or cold

- Return grille too small — creates a whistling noise and restricts system airflow

- Supply registers directing air into the room instead of toward the exterior wall — reduces comfort

- Register boots not sealed to the duct — air leaks into the wall or floor cavity

Measure the total external static pressure (TESP) of the system — it should not exceed 0.5 inches of water column (w.c.) for most residential systems. Excessive static pressure indicates restrictions, undersized ducts, or excessive leakage. Measure airflow at each supply register using a flow hood or anemometer. Adjust balancing dampers to direct more airflow to rooms that are underperforming. Conduct a duct leakage test if required by code — total leakage should be less than 4 CFM25 per 100 square feet of conditioned floor area.

1. Install a static pressure gauge at the supply and return plenums

2. Measure TESP with system running at full speed — should be below 0.5 inches w.c.

3. Measure airflow at each supply register with a flow hood or anemometer

4. Compare measured CFM to the design CFM for each room

5. Adjust balancing dampers — close dampers partially on rooms with excess airflow

6. Re-measure after adjustments to verify balanced distribution

7. Conduct a duct leakage test if required by code (duct blaster test)

8. Document all measurements for commissioning records and code compliance

- Static pressure exceeds 0.5 inches w.c. — ducts are too small, too many restrictions, or filter is too restrictive

- Some rooms receive too much air while others get too little — balancing dampers need adjustment

- Duct leakage exceeds code limits — must find and seal remaining leaks, then retest

- No airflow at a register — disconnected duct, closed damper, or kinked flex duct in the system