Ductwork repair fixes ducts that are physically damaged: a flex run that has slipped off its collar, a branch flattened under stored boxes, a metal section rusted through in a damp crawlspace, or insulation torn open by rodents. In Daly City homes, where ducts hang in the garage ceiling, cross a crawlspace under a hillside house or lie in an attic, this kind of damage is common. It is also usually local: one or two runs, not the whole system.
What you notice and what it usually means
| What you notice | Likely damage | What the repair usually takes |
|---|---|---|
| One register blows little or nothing | Run disconnected at the trunk or boot, or crushed | Reconnect with a new collar, or replace the section |
| Garage or crawlspace warm while the heat runs | Disconnected or torn supply run | Find and rebuild the open run |
| Rattle or whistle near a register | Loose fitting or split seam | Refasten and seal the joint |
| Musty smell when the system starts | Wet, torn insulation or a damaged return in a damp space | New insulation, return repaired |
| Shredded insulation, droppings near ducts | Rodent damage | Chewed section replaced, entry point closed |
| Dust streaks around a register | Boot pulled away from the floor or ceiling | Boot re-seated and sealed |
| Flex duct hanging in loops | Failed or missing straps | Re-hung straight, excess length cut out |
If several rooms have been uneven for years rather than one room changing suddenly, the cause is more likely the layout than damage; see uneven cooling and heating.
How ducts get damaged in local homes
Garage ducts take knocks. In a house built over its garage, the trunk and branches often run along the garage ceiling. Shelving, ladders and things stored on the joists dent ducts and pull branches loose at their takeoffs.
Crawlspaces stay damp. Under hillside split levels, ducts cross crawlspaces where the coastal air rarely dries out. Older galvanized ducts rust at the seams there, and once the outer vapor jacket on duct insulation tears, the insulation soaks up moisture, sags and drags on the joints.
Other trades disturb them. Roof, insulation, solar, plumbing and cable work in an attic or crawlspace often knocks a run loose or steps on it. A room that changed after a remodel or a roof job is a strong clue.
Rodents chew flex duct for nesting material and get in through gaps around vents and pipes.
Age. The outer jacket of old flex turns brittle, and cloth tape on metal joints dries out and lets go.
How we repair
- Reconnect properly. A run that fell off gets a new metal collar, a clamp on the inner liner, mastic on the joint and its insulation pulled back over and secured. Pushing it back on by hand is not a repair.
- Replace damaged sections. Crushed, torn or chewed flex is cut back to sound duct and replaced with insulated flex of the same diameter. Where a run passes through the garage, the replacement is sheet metal. Rusted metal is replaced, not covered over.
- Re-support. Sagging flex is re-hung on wide straps so it runs straight, without kinks or low loops. Extra length is cut out, because slack and sharp bends cost airflow.
- Re-insulate. Wet or torn insulation on metal ducts is replaced so the duct does not sweat or lose heat into a cold crawlspace.
- Check the result. We measure the airflow at the affected register before and after, so you can see the room is getting air again.
Before anyone opens old joints
Many homes here date from the 1940s to the 1960s. Duct joints and wrap from that era can contain asbestos, often as a white or grey, paper-like tape at the seams. It does no harm left alone, but it should be tested before anyone cuts, scrapes or sands it. If we find material like that, we stop and explain the options before anything is disturbed. The same care applies to rodent droppings: keep the area closed until the damaged section has been dealt with.
Repair or replace the ductwork?
Repair is the right call when the damage is in one area and the rest of the system is in fair shape. When much of the flex is brittle, insulation is falling off whole runs, or the ducts were badly laid out from the start, new ducts are worth discussing; see ductwork installation. If the ducts are intact but leak at the joints, duct sealing is the lighter fix. We tell you what we found and why we would do one or the other; the decision stays with you.
What affects the price
Most ductwork repairs fall between $200 and $1,000 and take 1 to 4 hours. The price depends on how easy the damage is to reach (an open garage ceiling versus a low crawlspace), how much duct is replaced, whether insulation is replaced with it, and how many separate runs are damaged. You get a written estimate before work starts, with no hidden fees.
Damaged return ducts in a garage or crawlspace can also pull dust and damp air into the house, so a repair sometimes improves more than one room. If the air has felt stale or dusty, our page on poor indoor air quality lists the other usual causes.