Supply vs Return Ductwork Explained
The Two Halves of Your Duct System
Every forced air HVAC system has two distinct duct networks that work together in a continuous loop:
Supply ducts carry conditioned air FROM the air handler TO each room. Return ducts carry room air FROM the rooms BACK to the air handler.
Air flows in a circle: air handler → supply ducts → rooms → return ducts → air handler → and the cycle repeats. If either side is restricted, the entire system suffers.
Supply Ductwork
Purpose
Delivers heated or cooled air to each room at the right volume (CFM) and velocity to maintain comfort.
Characteristics
- Connects to supply registers (the vents you feel air blowing from)
- Typically uses smaller duct sizes because air moves at higher pressure
- Branches out from the main trunk to individual rooms
- Sized based on each room’s CFM requirement
Sizing Considerations
- Standard friction rate: 0.08 in/wg per 100 ft
- Target velocity: 600 to 900 FPM for residential
- Each branch is individually sized for its room’s CFM need
- Trunk size reduces as branches take airflow (reducing trunk design)
Return Ductwork
Purpose
Collects stale room air and brings it back to the air handler for filtering, heating or cooling, and recirculation.
Characteristics
- Connects to return grilles (the larger vents that pull air in)
- Typically uses larger duct sizes because air moves at lower velocity
- Often fewer return points than supply (common mistake)
- Must carry the full system CFM back to the air handler
Sizing Considerations
- Standard friction rate: 0.06 in/wg per 100 ft (lower than supply)
- Target velocity: 500 to 700 FPM for residential
- Total return capacity should match total supply capacity
- Undersized returns are the number one cause of high static pressure
Why Balanced Airflow Matters
When supply and return are not balanced:
| Imbalance | Effect |
|---|---|
| More supply than return | Positive pressure in rooms, air pushed out through gaps, doors hard to close |
| More return than supply | Negative pressure, unconditioned air drawn in through cracks, higher energy use |
| Balanced | Neutral pressure, efficient operation, even comfort |
The Closed Door Problem
When bedroom doors are closed, return air flow is blocked if the room has no dedicated return. This creates positive pressure in the bedroom and negative pressure in the hallway and common areas.
Solutions:
- Install a dedicated return in each bedroom
- Add transfer grilles above doors (connects room to hallway for air return)
- Install jump ducts (short duct from room ceiling to hallway ceiling)
- Undercut doors by 1 inch (least effective method)
Common Mistakes with Supply and Return Ducts
1. Only One Return for the Whole House
Many older homes have a single large central return. This forces all air to travel long distances back to the air handler, creating high resistance and uneven temperatures.
2. Undersized Returns
Return ducts should be larger than supply ducts to accommodate lower velocity requirements. Many systems use the same size for both, which restricts return airflow.
3. No Return Air Path from Closed Rooms
Without transfer grilles or jump ducts, closing a bedroom door blocks return airflow and creates pressure imbalances.
4. Returns in Kitchens and Bathrooms
Returns should never be placed in kitchens (draws cooking odors) or bathrooms (draws moisture). These rooms should rely on supply air overpressure to push air toward return locations.
Sizing Both Sides Correctly
Use our HVAC Duct Calculator to size both supply and return ducts. Remember:
- Supply ducts: Use 0.08 in/wg friction rate and 700 to 900 FPM target velocity
- Return ducts: Use 0.06 in/wg friction rate and 500 to 700 FPM target velocity
- Return area should be 1.25 to 1.5 times the supply area to ensure adequate low velocity return flow