Ductwork is an important part of many HVAC (heating, ventilation and air conditioning) systems. It is used to distribute conditioned air throughout a building. Proper installation and joining of ductwork is crucial to ensure the system functions efficiently. There are several methods and materials available to connect ductwork. The choice depends on the type of duct, its size, configuration, operating pressures and other factors. This article provides an overview of the most common options used to join ducts in HVAC systems.
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Ductwork Basics
Ductwork refers to the sheets of metal, fiberglass or other materials that are fabricated into tubing to route air throughout a building. The ducts contain airflow from heating and cooling equipment and deliver it to rooms via supply registers, grilles and diffusers. Ducts also extract air from rooms and return it to the HVAC system via return air grilles.
There are two main types of ductwork:
- Supply ducts – Carry conditioned air from the HVAC system to the rooms.
- Return ducts – Extract air from rooms back to the HVAC unit.
Common materials used to fabricate ductwork include:
- Galvanized steel – Most common. Durable and available in many thicknesses.
- Aluminum – Lightweight but more expensive than steel.
- Fiberglass – Insulated ductboard. Helps prevent condensation and reduces noise.
- Flexible duct – Used for short runs to registers/diffusers. Usually made of plastic and metal.
Ductwork comes in various shapes including round, rectangular, square and oval. Round ducts are very common because they have high airflow capacity for their cross-sectional area. Rectangular ducts can fit well into building cavities. Duct sizes typically range from 4 inches up to 8 feet in diameter for large commercial systems.
Duct Joining Methods
There are many techniques available for connecting HVAC ductwork. The optimal method depends on factors like:
- Duct material – galvanized steel, aluminum, etc.
- Duct dimensions
- Available space for connections
- Desired strength and leak-tightness
- Need for quick/easy duct modifications
- Cost considerations
Here are some of the most common duct joining methods:
Duct Crimpers
A duct crimper joins ducts mechanically by crimping a specialized metal clamp around the duct end connections. Variations of duct crimp joints include:
- Slip-joint crimpers – Used to join two plain duct ends by slipping them together and crimping a metal sleeve around them.
- Double-lipped crimpers – Have lips that interlock the duct ends and a wider crimping section.
- High-performance crimpers – Used for critical/high-pressure duct connections. Have deeper crimping sections.
Duct crimpers provide a very secure metal-to-metal joint. They require special crimping tools to apply the precise pressure needed. Crimp joints are best for large, straight duct runs rather than systems with many branches or elbows.
Duct Sealant
Applying duct sealant is a common way to seal duct connections. Sealant options include:
- Duct mastic – Fiber-reinforced mastic that stays flexible. Applied by brushing/spreading on joints.
- Foil tape – Aluminum tape with acrylic adhesive. Wrapped around joint and pressed firmly.
- Joint sealants – Brush-on or aerosol sealants that dry rigid. Used for small gaps.
Sealants provide an airtight, protective seal at duct joints. They prevent air leaks that reduce HVAC efficiency. However, sealants may eventually crack or pull away, requiring reapplication. They also lack the mechanical strength of metal joints.
Snap-Lock Duct Connection
Snap-lock duct fittings connect ducts through built-in tabs, slots and tension springs. To join:
- Slide duct over fitting.
- Press duct down so tabs engage slots.
- Embedded tension springs provide tight seal.
Snap-lock connections make a secure metal joint without needing tools. They allow quick, easy duct system modifications. However, they are not suitable for high pressures or temperatures.
Flanged Duct Connection
Flanged duct fittings have outward-protruding rims that fit between the ends of two ducts. They are connected using gasketing material and bolts/screws around the perimeter. Steps include:
- Apply gasket to flange edges.
- Insert flange between duct ends.
- Drill bolt holes through flanges and ducts.
- Secure bolts through all layers.
Flanged joints can seal high pressure ducts. Installation takes time but allows ducts to be disconnected. Gasket maintenance may be needed if leaks develop.
Welded Duct Joint
For the strongest seal, ducts can be welded together. The steps include:
- Clean and prepare the metal duct ends.
- Align ducts with a small gap between ends.
- Use fillet welding to fuse the metal and fill the gap.
Welding forms an extremely robust, permanent joint. It provides reliable sealing even at high temperatures and pressures. However, welding requires skilled technicians and safety precautions.
Choosing the Best Duct Joining Method
Selecting the optimal duct joining technique depends on many factors:
Duct Type and Dimensions
The duct material and size will affect connections. For example:
- Larger diameter spiral ducts are often crimped, while smaller rounds use sealant.
- Rectangular sheet metal ducts typically have flanged joints.
- Flexible ducts use screw clamps and sealant on fittings.
Application Criteria
Consider conditions like temperature, pressure, corrosiveness, location, etc. For instance:
- Outdoor ducts need weatherproof sealed/flanged joints.
- High-pressure systems may require welded or heavily crimped joints.
- Corrosive conditions warrant seal-welded stainless steel ducts.
Access and Modifications
Will ducts need to be accessed or changed later? If so:
- Avoid welded joints or deep crimps that are hard to detach.
- Use flanged, sealed or snap-lock connections to allow modifications.
Installation Time
Factor in installation/labor time. For example:
- Flanged connections are secure but time-consuming to install.
- Sealants are quick to apply but slower than snap-locks.
- Welding is very labor intensive.
Cost Considerations
Duct joining costs vary significantly. For example:
- Sealants have low material costs but may need frequent reapplication.
- Snap-locks cost more than sealants but allow faster installation.
- Welding and flanges have higher material/labor costs.
Best Practices for Duct Joints
Follow these recommendations for secure, efficient duct connections:
- Select connection methods approved for the duct type and application.
- Always use UL-rated and code-compliant duct fittings.
- Clean and deburr duct ends for optimal adhesion and sealing.
- Install ducts securely with minimal direction changes to avoid stressing joints.
- Seal all connections thoroughly with mastic, tape or gasketing.
- Support duct joints with straps or hangers to prevent stressing connections.
- Avoid kinking or collapsing flexible ducts near connections.
- Inspect regularly for any loosening, sagging or air leaks.
Conclusion
Proper joining techniques are critical to ductwork performing efficiently and avoiding costly air leaks. A variety of methods exist to connect the many types of HVAC ducts encountered. Considering key factors like the duct characteristics, operating conditions, installation constraints and costs will lead to the ideal connections.
Following best practices like proper prep, sealing and support for all joints provides reliable, long-lasting performance. Well-connected ductwork helps HVAC systems function optimally for many years.