Ducts carry every BTU your furnace, air conditioner or heat pump produces, and the equipment's efficiency rating means little if the ducts lose a large share on the way. Most duct problems come down to four things: the wrong size, leaks, missing insulation and poor return airflow.
ENERGY STAR says that in a typical house about 20 to 30% of the air moving through the ducts is lost to leaks, holes and poorly connected ducts. Sizing ducts to the airflow each room needs, sealing every joint with mastic or UL 181 foil tape, and insulating ducts in attics and crawl spaces fix most problems.
How a duct system works
The blower pushes conditioned air through supply ducts to registers in each room, and return ducts bring room air back to the equipment through the filter. A system needs both: if supply air can't find its way back, rooms with closed doors become pressurized and the system moves less air.
Duct materials
- Sheet metal: durable and smooth inside, with the least friction; the standard for trunks.
- Flexible duct: quick to install and usually pre-insulated, but needs to be pulled tight and supported; see flexible vs. rigid ductwork.
- Duct board: fiberglass panels formed into rectangular ducts, insulated by design.
The three compare at a glance on friction, insulation and where they fit a system:
| Material | Interior surface | Insulation | Typical use |
|---|---|---|---|
| Sheet metal | Smooth, lowest friction | None built in; wrapped separately | Trunks and long runs |
| Flexible duct | Ribbed, highest friction | Pre-insulated jacket | Short branch runs to registers |
| Duct board | Fibrous and rigid, rougher than metal | Insulated by design | Rectangular trunks and plenums |
The ribbed wall of flexible duct adds friction, so a flex run has to be one size larger than a smooth metal run carrying the same air. It also has to be pulled tight, because sags and tight bends choke airflow.
Layouts
- Trunk and branch: a main trunk with smaller branches to each room; the most common.
- Extended plenum: a long, constant-size trunk with branches along it.
- Radial: individual runs from a central plenum, common with equipment in the middle of the house.
Sizing
Each duct is sized from the airflow it carries and the friction rate the blower can afford, with room airflow coming from a room-by-room load calculation. ACCA's Manual D is the industry method for the full system. Our ductwork sizing calculator sizes round and rectangular ducts from airflow and friction rate.
A worked example
Picture a bedroom with an example cooling load of 6,000 BTU per hour, which is half of one ton at 12,000 BTU per hour. At an assumed design airflow of about 400 CFM per ton, that room needs 0.5 times 400, or 200 CFM.
Sizing 200 CFM at the calculator's default friction rate of 0.08 inches of water per 100 feet gives a 7.83-inch diameter, which rounds up to an 8-inch round duct. Air then moves at about 573 feet per minute, comfortably below the speeds at which a run starts to sound noisy in a home.
Return airflow
Returns are often undersized. Rooms without a return of their own need a path back, such as a transfer grille or a gap under the door; otherwise they get less supply air. See cold air return vents.
Leaks and sealing
ENERGY STAR says leaky ducts can reduce heating and cooling efficiency by as much as 20%, and the DOE puts duct air losses at about 30% of a cooling system's energy. Seal joints, seams and boots with mastic or foil tape listed to UL 181, not cloth duct tape.
For ducts hidden in walls, aerosol duct sealing seals from the inside. A duct leakage test measures the leakage before and after.
Insulation
Ducts in attics, crawl spaces and garages lose heat through their walls as well as their leaks. Insulate them after sealing, to the level your local energy code requires.
Cleaning
The EPA says duct cleaning has never been shown to prevent health problems, and isn't needed routinely. It's worth considering for visible mold growth, vermin, or enough dust and debris that it blows out of the registers. See is air duct cleaning worth it.
Repair, seal or replace
Most duct problems are fixed by sealing, not by tearing anything out; match the fix to what you actually find.
| What you find | The fix |
|---|---|
| Joints and seams leak, or a leakage test fails, but the duct is intact | Seal with mastic or UL 181 foil tape |
| One section is crushed, disconnected or torn, and the rest is sound | Repair or replace that section |
| Corrosion is widespread, or the system is badly undersized for the equipment | Replace the system |
Chronic hot or cold rooms, whistling registers and a system that runs without reaching temperature usually point to sizing or leakage, not worn-out duct. When replacing equipment, have the ducts checked at the same time. A new high-efficiency system on leaky, undersized ducts will not deliver its rated performance.
Frequently asked questions
How much air do leaky ducts lose?
About 20 to 30% of the air moving through them in a typical house, according to ENERGY STAR.
How do I know if my ducts are the wrong size?
Signs include rooms that never reach temperature, whistling registers, and a system that runs constantly. A Manual D design or our ductwork sizing calculator shows what each run needs.
Should I have my air ducts cleaned?
Only for visible mold, vermin or heavy debris, according to the EPA; routine cleaning hasn't been shown to improve health.
What's the best way to seal ductwork?
Mastic over joints and seams, or UL 181 foil tape on clean, smooth metal. Ordinary cloth duct tape fails.
Can I add a return air duct to a room?
Yes, and a room that stays warmer or colder than the rest with its door closed is often starved of return air. Where a dedicated return duct is impractical, a transfer grille or a gap under the door gives the supply air a path back to the system. See cold air return vents.
Is flexible or metal duct better?
Smooth sheet metal has the lowest friction and is the usual choice for trunks and long runs. Flexible duct installs faster and comes pre-insulated, but its ribbed wall adds friction, so it has to be sized one step larger and pulled tight to avoid sags that choke airflow. See flexible vs. rigid ductwork.
How long does ductwork last?
Sheet metal duct can last the life of the house when it stays sealed and dry, so ducts are usually replaced by condition rather than by age. Replace any run that is crushed, disconnected, torn or corroded, and have the whole system checked whenever you change the furnace, air conditioner or heat pump.
Sources & References
- ENERGY STAR: Duct sealing (20 to 30% of air lost in a typical house)
- ENERGY STAR: Duct sealing benefits (efficiency reduced by as much as 20%)
- U.S. DOE: Energy Saver 101, Home Cooling (PDF), duct losses about 30%
- EPA: Should you have the air ducts in your home cleaned?
- ACCA: Technical manuals (Manual D)
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