Types of HVAC Duct Systems Explained
Overview of HVAC Duct System Types
Not all duct systems are designed the same way. The layout of your ductwork affects airflow distribution, installation cost, noise levels, and energy efficiency. Understanding the major duct system types helps you make informed decisions about new installations or renovations.
Extended Plenum System
The extended plenum (also called trunk and branch) is the most common duct layout in residential construction.
How it works: A single rectangular or round trunk line extends from the furnace or air handler through the building. Smaller branch ducts connect to the trunk at takeoff fittings and deliver air to individual rooms.
Advantages:
- Simple to design and install
- Works well for most home layouts
- Easy to add branches for new rooms
- Lower installation cost
Disadvantages:
- Air velocity and pressure decrease as you move farther from the air handler
- Rooms at the end of long runs may receive less air
- Requires careful sizing of the trunk to prevent starvation at distant branches
Best for: Single story homes, basements with accessible trunk space, standard residential construction.
Reducing Trunk System
A refinement of the extended plenum design. The trunk line gets smaller after each branch takeoff to maintain consistent velocity and pressure throughout the system.
How it works: The trunk starts at full size near the air handler. After each branch takes a portion of the airflow, the trunk reduces in size. This keeps the air velocity consistent instead of letting it drop off.
Advantages:
- More even air distribution than a standard extended plenum
- Saves material on the far end of the trunk
- Better pressure balance across the system
Disadvantages:
- More complex to design (requires sizing each trunk section)
- More transition fittings = more installation time
- Each section must be calculated individually
Best for: Longer homes where the trunk run exceeds 40 feet, any system where even distribution is critical.
Radial System
A radial system runs individual ducts directly from the air handler to each room with no trunk line.
How it works: The supply plenum connects directly to individual branch ducts, each going to a separate room. The layout resembles spokes on a wheel.
Advantages:
- Simple design with minimal fittings
- Equal pressure at each branch (all start at the plenum)
- Easy to size (each duct is independent)
Disadvantages:
- Requires more total duct material
- Works best with a central equipment location
- Not practical for large homes or multi story buildings
Best for: Slab on grade construction, small to medium homes, installations where the air handler is centrally located.
Perimeter Loop System
Designed specifically for slab on grade homes in cold climates where ducts are embedded in or beneath the concrete slab.
How it works: A continuous loop of ductwork runs around the perimeter of the building inside the slab. Feeder ducts connect from the air handler to the loop. Registers are located along the exterior walls at floor level.
Advantages:
- Delivers warm air at the coldest areas (exterior walls)
- Even heat distribution around the perimeter
- Works well with radiant effects from the warm slab
Disadvantages:
- Cannot be modified after construction
- Difficult to inspect or repair
- Only suitable for heating dominant climates
Best for: Cold climate homes built on concrete slabs.
Spider System
A variation of the radial system using a central distribution box (spider fitting) instead of a full plenum.
How it works: A large flexible duct or rigid trunk connects the air handler to a distribution box, typically in the attic. From the box, individual flex duct runs extend to each room.
Advantages:
- Quick installation in attic spaces
- Minimal trunk ductwork
- Works well for single story homes
Disadvantages:
- Relies heavily on flex duct (higher friction)
- Distribution box can be a noise source
- Flex duct runs must be kept short and straight
Best for: Single story homes in warm climates with attic installations.
Choosing the Right System
| System Type | Best For | Complexity | Cost |
|---|---|---|---|
| Extended plenum | Most residential | Low | Low |
| Reducing trunk | Long homes | Medium | Medium |
| Radial | Small homes, slab | Low | Medium |
| Perimeter loop | Cold climate slab | High | High |
| Spider | Attic installs | Low | Low |
Sizing Any System Type
Regardless of the duct system layout, every individual duct section must be properly sized for its airflow requirement. Use our HVAC Duct Calculator to size each branch and trunk section.
For fundamentals, read our how duct sizing works guide.