Ductwork is an integral part of many HVAC (heating, ventilation and air conditioning) systems. It is used to distribute conditioned air throughout a building. Ductwork is typically constructed from sheet metal and formed into tubular sections. These sections are connected together to create a continuous pathway for airflow. Seams are necessary where two sections of ductwork meet. A good seam is tightly sealed to prevent air leakage. Air leaks can reduce HVAC efficiency and performance. There are several different types of seams used in ductwork, and proper seam construction is an important factor in achieving optimal duct system operation.
What is a Seam?
A seam is any junction where two pieces of sheet metal ductwork meet. Multiple seams exist in a typical duct system. Seams are needed to connect straight duct sections, route ductwork around corners, branch runs off the main trunk line, and link ducts to other components like registers, diffusers, grilles, and plenums. Seams may be longitudinal seams that run along the length of ductwork or transverse seams that connect two duct ends. The goal of any ductwork seam is to create a continuous airway by joining and sealing the sections together.
Ductwork Seam Types
There are three main types of seams used to connect ductwork:
S-Slip Seam
The S-slip seam or drive slip seam is made by folding back the edges of two duct sections in opposite directions. The folded edges overlap and are secured using screws or rivets through pre-punched holes. S-slip connections have minimal airflow resistance and are among the strongest duct seams.
Standing Seam
A standing seam is formed by turning up the edges of two duct sections in the same direction. The rising edges are then crimped together with a specialized seaming tool to seal the joint. Standing seams offer a clean interior surface and are moderately strong.
Pittsburgh Seam
The Pittsburgh seam, also called a Pittsburgh lock or groove seam, involves rolling a rounded bead on one duct end and a corresponding channel on the mating end. The bead fits tightly into the channel when joined. Pittsburgh seams provide good leakage protection at medium airflow resistance.
Ductwork Seam Specifications
Proper construction is critical to achieving well-sealed, durable ductwork. Industry standards and building codes define requirements for duct seams. Key specifications include:
- Minimum overlap of adjoining duct sections should be at least 1 inch.
- Mechanically fastened seams must have fasteners spaced no more than 12 inches apart.
- Sealer or sealant tape should be applied to the inside of mechanically fastened seams.
- Standing seams should have at least 1⁄2 inch crimps spaced no more than 2 inches apart.
- Formed seams like the Pittsburgh seam should have a minimum engagement depth of 1⁄4 inch.
Following published seam specifications helps achieve proper seam integrity. Some additional best practices for quality seams include:
- Thoroughly clean and prepare surfaces before sealing.
- Apply duct sealant to both sides of a seam.
- Use squeegees or brushes to spread sealant evenly and work it into joints.
- Reinforce seams with sleeve joints for critical duct sections.
- Allow sealants to fully cure before pressurizing system.
Ductwork Seam Issues
Poorly constructed seams that do not meet specifications can cause problems including:
- Air Leaks: Gaps in seams allow conditioned air to escape and unconditioned air to enter ducts.
- Condensation: Leaks introduce warm, moist air that contacts cold duct walls and condenses.
- Noise: Turbulent airflow and vibration at loose seams increase noise.
- Reduced Velocity: Leakage lowers air pressure and velocity in ducts.
- Decreased Efficiency: More fan energy is required to compensate for air loss from leaky seams.
Signs of leaky or deficient seams include rust stains, dust accumulations, and condensation droplets. Poor seam integrity requires duct repairs to resolve air leaks, moisture, and reduced system performance.
Importance of Quality Ductwork Seams
High performance HVAC systems rely on ductwork to distribute air precisely according to design needs. The system’s ability to maintain desired airflow, temperature, and humidity depends on ducts being airtight. Leaky ducts make it impossible for the HVAC system to deliver the right amount of conditioned air to all areas. Poor seam construction also allows entry of dust and pollutants that degrade indoor air quality. Mold and moisture damage can occur from condensation at leak points. Achieving required energy efficiency necessitates keeping losses within tight tolerances, so missing or deficient seams that leak air are detrimental. Any duct repairs mean added costs. For critical environments like hospitals and laboratories, leaky ducts compromise the ability to maintain strict air parameters. High quality seams are essential to avoid these issues.
Ductwork Seam Testing
Constructing duct seams according to industry standards helps ensure suitable seam integrity. Additional ductwork testing provides quantitative data to identify deficiencies and verify seam performance. Two common methods of duct leakage testing are:
Aerosol Testing
This test uses aerosol smoke or theatrical fog to visualize leaks. Smoke is injected into pressurized ducts and observers watch for any smoke escaping at seams or joints. Locations and severity of leaks can be pinpointed. However, the smoke does not indicate how much air is escaping from each leak.
Pressure Testing
For pressure testing, the entire duct system is sealed and placed under negative or positive static pressure with a blower door fan. Instrumentation monitors the airflow rate into or out of the pressurized ducts. Initial test readings combined with precise duct dimensions allow total leakage to be quantified in cfm (cubic feet per minute). Changes to readings after sealing provide data on the effectiveness of leak repairs. Pressure testing confirms overall duct tightness, but does not show individual leak sites like smoke testing.
Thorough seam inspections and whole duct system leakage testing ensure all seams meet specifications. Testing verifies seam integrity and validates workmanship quality.
Ductwork Seam Repairs
Even minor flaws in seam construction can lead to energy losses and issues with HVAC system performance and indoor air quality. When duct inspections or testing reveal leaks at seams, repairs will be needed to resolve the deficiencies. Steps for properly repairing faulty seams include:
- Use aerosol smoke to pinpoint leaks for repair.
- Clean around seams and abrade any metal that needs resealing.
- Apply new duct sealant to fully cover the leaking seam.
- Clamp or fasten seams to compress sealant as it cures.
- Re-test repaired seams to confirm adequate sealing.
For severely defective seams, it may be necessary to disconnect and completely reseal the duct sections according to proper standards. Duct repairs should be performed by experienced technicians to avoid introducing additional leakage points. Careful seam repairs help restore ductwork to proper functioning.
Maintaining Good Ductwork Seams
Even well-constructed duct seams can deteriorate over time. To maintain ductwork integrity, building owners and maintenance personnel should:
- Establish routine duct inspection schedules to check seams.
- Perform regular duct leakage testing to identify new leaks.
- Repair any compromised seams immediately to prevent worse damage.
- Keep duct interiors clean to avoid soil buildup at seams.
- Protect exterior ducts from weather and physical damage.
Proactive seam maintenance ensures ducts continue meeting ventilation requirements and prevents major duct repairs from deferred upkeep.
Benefits of Proper Seam Construction
Investing in quality workmanship to seal duct seams correctly has many benefits including:
- Optimized HVAC energy efficiency.
- Lower heating and cooling costs.
- Improved indoor air quality.
- Reduced risk of condensation and mold issues.
- Minimal duct repairs needed.
- Extended HVAC system life.
- Enhanced comfort and air distribution.
The nominal cost increase to build ducts with good seams is recouped many times over through better system performance and reduced lifetime maintenance.
Conclusion
Ductwork seam integrity is vital for HVAC systems to perform properly. Leaks at seams disrupt designed airflow rates, reduce efficiency, and can cause moisture accumulation and indoor air quality issues. There are several different seam types including S-slip, standing, and Pittsburgh forms. Seams must be constructed according to established industry standards. Ducts should be leakage tested to verify seam tightness. Even minor construction flaws or seam degradation over time can lead to significant performance impacts and require duct repairs. Maintaining robust QA standards for seam workmanship and conducting periodic inspections and testing ensures ducts remain leak-free. Investing in quality duct seams provides energy, cost, comfort, and air quality rewards over the system lifetime. With diligent duct maintenance, optimal seam integrity can be sustained long-term.