After A Renovation
Drywall dust is the finest material a house ever generates and the duct system is the thing that distributes it everywhere.
Short answer
Drywall sanding, demolition and cutting produce very fine dust that gets into the return side and coats the whole system. Clean after the work is finished and the final sanding is done, not during. A new filter at the end, and the blower and coil included, because that is where the fine dust settles.
How it gets in
Open return grilles during demolition and sanding, which draw the dust directly into the system.
A return plenum in the work area, which is effectively an open duct in a dusty room.
The furnace running during the work, which distributes it to every room.
Open duct ends where a run has been modified, which is unavoidable during the work and should be sealed between visits.
Floor registers used as a convenient place to sweep debris, which happens more than anybody admits.
And a filter slot that was never sealed, drawing unfiltered air from a dusty basement.
Preventing most of it
Tape over every return grille and floor register in and near the work area for the duration. Seal any open duct end at the end of each day. Put a fresh filter in before the work and another after. Those four things cost a roll of tape and prevent the majority of it, and almost nobody does them.
When to clean
After the work is genuinely finished — after the final sanding, the final cut and the final sweep.
Not after the drywall and before the trim, which means doing it twice.
Before the final clean of the house, so dislodged dust is picked up.
With the system under negative pressure, so what is dislodged leaves the building.
With the blower, the cabinet and the coil included, because fine dust settles on the blower wheel and blinds the coil.
And with a new filter at the end, and another a month later, because fine dust keeps arriving for weeks.
The related items
The coil, which if it is blinded with drywall dust is an airflow and efficiency problem as well as a cleanliness one.
The humidifier, if there is one, because the pad and the tray collect it.
The HRV filters and core, if the house has one, which fill with construction dust fast.
Bathroom and kitchen exhaust fans, which will have been running during the work.
Any duct that was modified, which should be checked for proper reconnection and sealing — a disconnected branch in a joist space is a common renovation legacy.
And the balance, because a renovation that added or removed registers has changed the airflow distribution.
Common questions
When should I clean the ducts after a renovation?
After the work is genuinely finished, including the final sanding.
What stops it happening on the next job?
Tape over every return and register in the work area, and seal open duct ends daily.
Does the coil matter?
Yes. Drywall dust blinds it, which is an airflow problem as well as a dirt problem.