Why renovation dust is different
Normal household dust is mostly fabric fibre and skin, it is relatively heavy, and most of it never reaches the duct. Drywall sanding produces gypsum dust that is fine, abundant, and easily pulled into a return.
If the HVAC system ran during the work — and it almost always does, because the crew wants heating or cooling — the return has been acting as a vacuum cleaner for weeks. The dust ends up in the trunk lines, on the blower wheel, and on the evaporator coil.
What it actually causes
The blower wheel is the part that suffers. Gypsum dust cakes onto the fins, the wheel goes out of balance and moves less air, and the system runs longer to reach the same temperature. Homeowners usually notice this as a slow decline in cooling performance over the following summer rather than as a dust problem.
On the coil, the same film acts as insulation across the surface designed to transfer heat. Same result: longer run times, higher bills.
What to ask the contractor for, before the work
Filter media taped over every return grille for the duration of the dusty phase. This is cheap, it takes minutes, and it prevents most of the problem.
The system switched off entirely during drywall sanding, not just set back.
Supply registers covered — not to protect the duct, but to stop dust falling in and being blown back out on first start-up.
What to do afterwards
Change the filter first, and look at it. If it is grey and loaded, that is a good sign — it means the filter did its job and less went past it.
Then have the blower compartment and coil inspected before you have the ducts cleaned. If the dust went past the filter, it reached those first, and cleaning the ducts while leaving a caked blower wheel fixes the part you cannot see and leaves the part that costs you money.
This is the one case where we will recommend the full service without hesitation, and the one case where the before-and-after camera footage is genuinely dramatic.