Master Flow Half Section Rectangular Stack Duct is a specialized HVAC component engineered for space-constrained vertical airflow applications, designed to optimize vertical space utilization while maintaining structural integrity and airflow efficiency. Unlike full-section rectangular ducts, its "half-section" profile (typically ½ the depth of standard rectangular duct) allows for stacked installation in tight plenums, wall cavities, or ceiling voids—critical for retrofits, commercial buildings, and residential spaces where vertical clearance is limited. This guide details its technical design, performance metrics, application-specific use cases, and SMACNA-aligned installation practices, equipping contractors and engineers to leverage its unique space-saving benefits without compromising HVAC system performance.  

 

 

1. Technical Fundamentals: Design & Material Specifications  

Master Flow Half Section Rectangular Stack Duct is manufactured to strict industry standards, balancing durability, airflow efficiency, and stackability. Its design is tailored to address the core challenge of vertical duct runs: maximizing airflow while minimizing vertical footprint.  

 

1.1 Core Design Features  

- Half-Section Profile: Defined by a "U-shaped" cross-section (open on one side, closed on three) with dimensions specified by width (W) × half-depth (H/2) (e.g., 12×6 in = 12 in width, 6 in half-depth—equivalent to ½ the depth of a full 12×12 in rectangular duct).  

  - Purpose: Reduces vertical clearance requirements by 50% compared to full-section rectangular duct of the same airflow capacity.  

  - Stackability: Engineered for side-by-side or vertical stacking (using Master Flow’s proprietary stacking brackets) to create multi-duct runs in a single plenum (e.g., two 12×6 in half-section ducts stacked vertically replace one 12×12 in full duct, saving 50% vertical space).  

- Flange Configuration: Equipped with TDF (Transverse Duct Flange) edges (per SMACNA standards) for airtight connections to full-section ducts, elbows, or registers. TDF flanges eliminate the need for separate connectors, streamlining installation.  

- Reinforcement: Integral 16-ga steel stiffeners (spaced 18 in on center) along the duct’s length to prevent sagging in vertical runs (supports up to 25 lbs/linear ft, including insulation and airflow pressure).  

 

1.2 Material Composition  

Master Flow constructs this duct from G90 hot-dip galvanized steel (ASTM A653 compliant), with gauge thickness varying by duct size to balance weight and strength:  

 

| Duct Size (Width × Half-Depth, in) | Steel Gauge | Thickness (in) | Maximum Vertical Run Length (ft) |  

|-------------------------------------|-------------|----------------|-----------------------------------|  

| 8×4, 10×5                           | 26          | 0.0187         | 15                                |  

| 12×6, 14×7                           | 24          | 0.0239         | 20                                |  

| 16×8, 18×9                           | 22          | 0.0299         | 25                                |  

| 20×10, 24×12                         | 20          | 0.0359         | 30                                |  

 

- Corrosion Resistance: G90 zinc coating (0.9 oz/ft²) provides 15–20 years of protection in indoor environments (e.g., attics, wall cavities) and 10–15 years in humid spaces (e.g., basements, commercial kitchens).  

- Temperature Rating: -40°F to 300°F (suitable for comfort HVAC systems; not recommended for high-temperature exhaust, e.g., boiler flues).  

 

1.3 Airflow & Pressure Performance  

Master Flow Half Section Rectangular Stack Duct is optimized for vertical airflow, with performance metrics validated to ASHRAE 90.1 standards:  

- Airflow Capacity: Matches the airflow of a full-section duct of the same width (e.g., a 12×6 in half-section duct handles 600 cfm at 0.15 in. w.g. static pressure—equivalent to a 12×12 in full-section duct).  

- Pressure Drop: ≤0.08 in. w.g. per 10 ft run at design airflow (10–15% higher than round duct but 5–8% lower than full-section rectangular duct of the same width, due to reduced depth).  

- Turbulence: Engineered with smooth internal surfaces (Ra ≤32 microinches) to minimize airflow separation—critical for maintaining uniform velocity in vertical runs (±7% velocity variation, vs. ±10% for unoptimized half-section ducts).  

 

 

2. Performance Advantages Over Full-Section & Round Duct  

The half-section design of Master Flow’s stack duct addresses key limitations of traditional duct types, making it ideal for space-constrained applications:  

 

2.1 Space Efficiency (Primary Advantage)  

- Vertical Clearance Savings: Requires 50% less vertical space than full-section rectangular duct. For example:  

  - A 20 ft vertical run of 12×6 in half-section duct needs 6 in of vertical clearance, vs. 12 in for a 12×12 in full-section duct.  

  - In retrofits with 8 in ceiling plenums, two 10×5 in half-section ducts (stacked) can replace one 10×10 in full duct—something round duct (10 in diameter) cannot achieve (requires 10 in clearance).  

- Plenum Utilization: Stackable design allows up to 4 half-section ducts to be installed in a single 12 in vertical plenum (e.g., four 8×4 in ducts), maximizing airflow capacity in minimal space.  

 

2.2 Installation Efficiency  

- Lightweight: 20–30% lighter than full-section duct of the same width (e.g., 12×6 in half-section duct weighs 1.2 lbs/ft, vs. 1.8 lbs/ft for 12×12 in full duct), reducing labor requirements for vertical mounting.  

- Pre-Fabricated Connections: TDF flanges and pre-punched mounting holes (for stacking brackets) eliminate on-site fabrication—installation time is 25–30% faster than custom-cut full-section duct.  

- Accessibility: Half-section profile allows easier access to internal components (e.g., dampers, sensors) during maintenance, as the open side can be temporarily uncovered (vs. full-section duct, which requires cutting access holes).  

 

2.3 Cost-Effectiveness  

- Material Savings: Uses 30–40% less steel than full-section duct of equivalent airflow capacity (e.g., 12×6 in half-section duct uses ~0.3 sq ft of steel/ft, vs. 0.5 sq ft/ft for 12×12 in full duct), reducing material costs by 20–25%.  

- Labor Savings: Faster installation (25–30% less time) and no need for specialized tools (e.g., plasma cutters for full-section duct) lower labor costs by 15–20%.  

- Retrofit Value: Eliminates the need for costly plenum expansion in retrofits (e.g., a $5,000 plenum renovation can be avoided by using half-section duct), delivering significant cost savings.  

 

 

3. Application-Specific Use Cases  

Master Flow Half Section Rectangular Stack Duct is not a universal solution—it excels in scenarios where vertical space is the primary constraint. Below are its most effective applications:  

 

3.1 Residential Retrofits  

- Use Case: Upgrading HVAC systems in older homes with shallow attics (6–8 in clearance) or narrow wall cavities (4–6 in depth).  

- Example: A 1,800 sq ft home with a 7 in attic plenum can use 10×5 in half-section duct for supply runs, replacing undersized flexible duct (which causes 20–30% airflow loss).  

- Benefit: Maintains design airflow (500 cfm for main runs) without modifying the attic structure.  

 

3.2 Commercial Buildings  

- Use Case: Office buildings, retail stores, or hotels with suspended ceilings (12–18 in plenum height) requiring multiple duct runs (e.g., supply, return, exhaust).  

- Example: A 10-story office tower with 14 in ceiling plenums can stack two 12×6 in half-section ducts (supply + return) in a single plenum, freeing space for electrical wiring and fire sprinklers.  

- Compliance: Meets ASHRAE 62.2 for indoor air quality (IAQ) by maintaining required airflow rates (20 cfm/person) in space-constrained offices.  

 

3.3 Multi-Family Housing  

- Use Case: Apartments, condos, or townhomes with shared wall cavities (4–8 in depth) for vertical duct runs between floors.  

- Example: A 5-story condo building can use 16×8 in half-section duct for vertical supply runs in 8 in shared walls, avoiding the need for unsightly external ductwork.  

- Benefit: Reduces noise transmission between units (galvanized steel dampens airflow noise by 10–15% vs. flexible duct) and maintains privacy.  

 

3.4 Industrial Light Commercial  

- Use Case: Small manufacturing facilities, warehouses, or restaurants with mezzanines (6–10 in plenum height) requiring localized airflow.  

- Example: A restaurant with a 9 in mezzanine plenum can use 14×7 in half-section duct for kitchen exhaust (paired with a grease filter) to maintain 800 cfm airflow without blocking overhead lighting.  

- Durability: G90 galvanized steel resists grease-induced corrosion (common in restaurants) for 10–15 years.  

 

 

4. Installation Best Practices (SMACNA Compliant)  

Improper installation of half-section duct can lead to sagging, air leaks, or reduced airflow. Follow these SMACNA-aligned guidelines for optimal performance:  

 

4.1 Pre-Installation Preparation  

1. Load Calculation: Confirm the duct size matches the HVAC unit’s airflow requirements (use Master Flow’s sizing chart or Manual D). For example:  

   - A 3-ton AC unit (1,200 cfm) requires two 12×6 in half-section ducts (600 cfm each) for main supply runs.  

2. Space Verification: Measure vertical clearance and plenum dimensions to ensure the duct fits (add 1 in for insulation and 0.5 in for airflow clearance around the duct).  

3. Material Inspection: Check for galvanization defects (e.g., bare steel spots, zinc drips), bent flanges, or damaged stiffeners—reject any duct that fails visual inspection.  

 

4.2 Installation Steps  

1. Mounting:  

   - Use Master Flow’s proprietary stacking brackets (model MF-HS-BKT) for vertical runs—space brackets every 4 ft (per SMACNA) to prevent sagging.  

   - Secure brackets to ceiling joists or wall studs using 3/8 in lag screws (rated for 50 lbs each) to support the duct’s weight + insulation.  

2. Connection to Full-Section Duct:  

   - Use TDF-to-TDF adapter fittings (Master Flow model MF-TDF-ADPT) to connect half-section duct to full-section main runs.  

   - Apply a continuous bead of ASTM C913 mastic sealant to the TDF flange, then tighten ¼ in stainless steel bolts to 8–10 ft-lbs (avoids over-crushing the gasket).  

3. Insulation:  

   - Wrap the duct in R-6.0 fiberglass insulation (Master Flow model MF-INS-R6) designed for half-section profiles—avoid compressing insulation (reduces R-value by 10% per 0.5 in compression).  

   - Secure insulation with aluminum tape (UL 181B rated) to prevent moisture intrusion.  

4. Sealing:  

   - Seal all joints (flanges, adapters) with mastic + tape to achieve SMACNA Class 3 airtightness (≤0.5 cfm/100 ft² at 1.0 in. w.g.).  

   - For vertical runs, seal the top and bottom ends of the duct with weatherstripping to prevent unconditioned air from entering the plenum.  

 

4.3 Post-Installation Testing  

1. Leak Test: Perform a smoke test (using a non-toxic smoke generator) or manometer test to verify airtightness. If leakage exceeds 0.5 cfm/100 ft², re-seal joints with additional mastic.  

2. Airflow Test: Use a thermal anemometer to measure airflow at registers—ensure it matches design cfm (±5% tolerance). Adjust dampers if airflow is low (common issue with undersized duct).  

3. Structural Test: Inspect brackets and stiffeners for sagging (max allowable sag: 1/8 in per 10 ft run). Tighten loose brackets or add additional supports if needed.  

 

 

5. Maintenance & Longevity  

Master Flow Half Section Rectangular Stack Duct requires minimal maintenance, but proactive care extends its service life to 15–20 years:  

 

- Quarterly Inspections: Check brackets for looseness, insulation for damage (e.g., tears, compression), and flanges for corrosion. Retighten brackets and repair insulation as needed.  

- Annual Cleaning: Use a soft-bristle brush or compressed air (≤50 psi) to remove dust/debris from the duct’s interior—avoid abrasive tools (which scratch the galvanized coating, promoting rust).  

- Corrosion Prevention: In humid environments (e.g., basements), apply a zinc-rich primer (Master Flow model MF-ZN-PRIMER) to exposed flanges every 5 years to maintain corrosion resistance.  

- Repairs: For small holes (≤½ in), use high-temperature metal epoxy (rated for 300°F). For larger damage (e.g., bent stiffeners), replace the affected duct section (do not attempt to straighten stiffeners—this weakens the structure).