Ductwork Installation & Replacement

Why Ductwork Decides How Your System Performs

Ductwork — the network of air ducts carrying conditioned air from the furnace or air handler to each room — is the part of a heating and cooling system homeowners never see and contractors most often skip in a quote. It is also where much of the comfort is won or lost. A correctly sized, properly sealed duct system delivers the air the equipment produces; a poor one wastes it into an attic or crawlspace before it reaches a room.

This is why a new furnace or air conditioner sometimes disappoints. The equipment is doing its job, but the distribution system cannot carry the air. Replacing the unit without evaluating the ducts treats the visible half of the system and leaves the other half alone.

Signs Your Ductwork Needs Attention

• Rooms that never reach temperature while others overshoot
• Weak airflow at registers furthest from the air handler
• Whistling, rattling, or booming when the system starts up
• Visible dust streaking around supply vents
• Energy bills climbing without a change in how you use the system
• Ducts running through an unconditioned attic, garage, or crawlspace
• Flexible ducts that are crushed, kinked, or sagging between supports

Common Ductwork Services

• Full duct replacement — new trunk and branch runs throughout the home
• Partial replacement — replacing damaged, undersized, or unreachable sections
• New runs — extending the system to an addition, attic conversion, or finished basement
• Duct sealing — closing leaks at joints and seams with mastic rather than tape
• Duct design and load calculation — sizing the system to the house, not to what was there before
• Airflow balancing — adjusting dampers so each room gets its share after installation
• Zoning — adding dampers and controls so floors or wings can be conditioned independently

Air Duct Installation for Central Air Conditioning

Cooling asks more of a duct system than heating does. Central air conditioners and heat pumps are designed to move a set volume of air across the indoor coil. A rule of thumb of 400 cubic feet per minute per ton is often quoted, but ACCA, the trade body behind the Manual J, S and D standards, notes that it is not always the case: humidity, temperature, air density and elevation change the airflow a system needs, and the figure to work to is the one in the equipment manufacturer's tables (ACCA: 400 CFM per ton, or is it?). Many homes, particularly older ones built around a furnace, have ducts and return grilles sized for heating airflow alone. Adding or upsizing an AC on those ducts can leave the system short of air, which shows up as noisy registers, a coil that freezes, and rooms upstairs that never cool.

Where the ducts run matters too. Supply ducts in a hot summer attic gain heat before the air reaches a room, and uninsulated ducts carrying cold air can sweat and drip. A sound air conditioning duct installation sizes supply and return runs to the equipment, seals the joints, and insulates any run outside the conditioned space.

Homes with no ducts at all (houses heated by a boiler, radiators, or baseboard) have three routes to central cooling: a new duct system run through attics, closets, and chases; a small-duct high-velocity system that threads narrow tubing through existing walls; or ductless mini-splits, which skip ducts entirely. Which is cheapest depends on how easily ducts can be routed through your particular house, so it is worth asking a contractor to price more than one.

Sealing or Replacement — Which Do You Need?

Sealing is the cheaper intervention and often the correct one. If the duct layout is sound and the sizing is right, closing leaks at the joints can recover most of the lost airflow for a fraction of replacement cost. It is worth asking any contractor to price sealing as an option rather than quoting replacement by default.

Replacement earns its cost when the ducts are undersized for the equipment, physically damaged, contaminated, or laid out so badly that no amount of sealing will fix the airflow. A contractor who measures static pressure and inspects the runs can tell you which situation you are in. One who quotes replacement without looking cannot.

Duct Replacement and Installation: What the Job Should Include

Last reviewed October 26, 2026 · How we research

ENERGY STAR publishes a list of what a professional duct improvement project should cover, and it is a useful yardstick for any duct sealing or air duct replacement quote (ENERGY STAR: duct improvement projects):

• Inspect the whole duct system, including the attic and crawlspaces
• Check the balance between supply and return air; ENERGY STAR notes that many systems have return ducts that are too small
• Repair damaged and disconnected ducts, and straighten flexible ducts that are tangled or crushed
• Seal leaks and connections with mastic, metal tape or an aerosol-based sealant, and seal registers and grilles tightly to the ducts
• Insulate ducts in unconditioned areas such as attics, crawlspaces and garages with insulation of R-6 or higher
• Fit a new filter, and check airflow once the work is done
• In a home with gas or oil appliances, run a combustion safety test after the ducts are sealed, so that nothing backdrafts

When ducts are replaced or new runs are added, the sizing method to ask about is ACCA Manual D, the ANSI-recognized standard for residential duct design (ANSI/ACCA 1 Manual D – 2016), which sets one set of duct sizing principles and calculations for all duct materials (ACCA: Manual D). A duct design is only as good as the airflow it is sized for, which comes from the home's load calculation; our sizing guide explains how that is worked out. If a replacement quote names neither, ask how each run was sized.

Ductwork Installation Cost

It depends on the home and the scope of the work, so the reliable figure is a written quote for your home. Sealing an existing system usually costs less than replacing it.

The single biggest cost driver is access. Ducts in an open basement or a walkable attic are straightforward; ducts buried in finished ceilings or a tight crawlspace mean opening and repairing drywall, which adds labour. Material choice, the number of runs, and whether zoning controls are added move the figure as well. A contractor who has seen your house is the only source of a real number.

What a Good Duct Quote Includes

A proposal worth comparing says more than a price. Look for a load calculation rather than a like-for-like replacement, a stated plan for sealing and insulating runs in unconditioned space, post-installation balancing, and who is responsible for any drywall repair. If two quotes differ sharply, the cheaper one has usually left one of those out.