Replacing, sealing or adding air ducts at home
HVAC ductwork installation is worth considering when ducts are leaking, undersized, crushed, damp or missing. Sealing and insulating existing ducts is often enough, while full replacement makes sense when the duct system is damaged or cannot carry the airflow your equipment needs.
What does ductwork do in an HVAC system?
Ductwork is the delivery side of a forced-air HVAC system. The furnace or air handler pushes air into supply ducts that end at registers in each room, and return ducts bring air back to be heated or cooled again. A good duct system delivers the right amount of air to each room with little noise and little loss. Because ductwork often runs through attics, crawl spaces and exterior walls, any leak or missing insulation hands heat to space nobody lives in.
Ductwork also affects the HVAC equipment. Airflow that is too low strains the blower, can overheat the heat exchanger and cuts air conditioner performance. A new furnace on poor ductwork will not fix the comfort problem that made you replace the old one. The furnace installation guide notes why contractors check ducts at the first visit.
When should ducts be replaced rather than sealed?
Signs that ductwork needs attention include rooms that stay hot or cold, whistling or booming noises, dust that returns quickly, high energy bills for the size of the house, and ducts that are visibly disconnected, crushed or wet. Rooms over a garage or at the end of a long run are common problem spots, and a duct repair service can usually trace them.
Sealing is often the first repair. A technician seals joints with mastic or approved foil tape, which lasts far longer than the cloth duct tape most people picture, and adds insulation in unconditioned spaces. A duct leakage test shows how much air escapes, and a blower door test or a home energy audit can reveal where.
Replacement comes into play when the ducts are collapsed, contaminated by water or pests, made of materials past their service life, or simply too small. A duct system sized for a small furnace can choke an air conditioner added later. In that case adding or enlarging ducts on the main trunk may fix the problem more cheaply than living with it.
How is ductwork installed?
Installation starts with a design. A qualified contractor calculates room-by-room heating and cooling loads, then sizes the trunk, branches and registers to deliver the airflow each room needs. Guessing from the old layout repeats its mistakes. Where do the ducts go? In a new build they run between floor joists and in ceilings before drywall. In an existing house ductwork uses attics, basements, crawl spaces and sometimes chases built into closets, which is the hardest part of adding ducts to a home that never had them.
- Design the layout from the load calculation and the available routes.
- Fit the supply trunk and branches, using rigid metal where possible and short runs of flexible duct where needed.
- Seal every joint with mastic or approved tape, then insulate ducts outside heated space.
- Install registers and return grilles, with enough return air.
- Test airflow and static pressure, and check for leaks.
Permits may apply, and rules vary by province and municipality. Ask your contractor, who will know the local requirements. Our wider HVAC installation guide shows where ductwork fits in a whole HVAC system job.
Can you add ducts to a house that has none?
Sometimes. Homes heated by boilers, baseboards or electric heaters can be fitted with ductwork for central air or a heat pump, but routing ducts through finished walls and ceilings is disruptive and costly, and some layouts make it impractical. Ductless wall units are often the alternative for such a home; they use refrigerant lines, not ductwork. The challenges and options for older houses are covered in heat pump installation in older homes.
Consider the building envelope before you open walls. Better insulation and air sealing reduce the size of the equipment and the ducts you need, as the insulation upgrade priorities guide explains.
What does ductwork installation cost?
No national figure is dependable. The price depends on the length and complexity of the runs, how easy access is, the material, the amount of sealing and insulation, the house size and local labour rates. An HVAC ductwork job that reuses existing trunk lines and replaces a few branches costs far less than a new system in a finished house. Ask for itemised quotes that state the design basis, the materials, the sealing method and the testing. Check what the warranty covers. Programs may support duct sealing as part of an energy upgrade, so check the Government of Canada energy-efficiency pages, your provincial energy ministry and your utility for current terms.
Quick answers
How do I know if my ducts are leaking?
Warning signs include uneven temperatures, dust, high bills and ducts that are disconnected or damp. A duct leakage test by a technician gives a measurement.
How long do air ducts last?
Sheet metal ducts can last for decades if kept dry and sealed. Flexible ducts and tape-sealed joints tend to age sooner, so condition matters more than a fixed number.
Should I replace ducts when I replace my furnace?
Not automatically. Have the ducts checked for leakage, size and condition, then seal or replace only what is needed.
Does duct work improve indoor air quality?
Clean, sealed ducts with a good filter help keep dust and outdoor air from entering, but ducts alone do not clean air. Filtration and ventilation matter too.