What to know
Start at the filter
A restricted filter reduces airflow everywhere at once, so it is worth eliminating before anything else. Judge it against a new filter of the same type rather than by holding it up to a light — a dense pleated filter blocks light when new, so that test is not the gauge people assume.
Fit a new one, run the system for a few hours, and see what changed before drawing any conclusions.
Open the paths
- Open every supply register, including in rooms nobody uses.
- Pull furniture and rugs off both supply registers and return grilles.
- Vacuum the register faces, which collect a surprising amount.
- Leave interior doors open where you can, especially bedrooms with no return of their own.
The return side is the half people forget. A system cannot push air into a room any faster than it can pull it back out, and a sofa against the return grille throttles the whole house.
Then look at the ductwork
If the filter is fresh and everything is open and airflow is still weak, the air is going somewhere between the furnace and the register.
- Flex runs that have kinked, sagged, or been crushed by something stored on them.
- A run that has come apart at a boot or a joint, common after remodelling.
- Build-up narrowing the run, particularly on the return side.
- Dampers left closed after somebody balanced the system years ago.
Most of that is in an attic, a crawl space, or behind a wall, which is why it survives so long undiagnosed.
What is worth doing yourself, and what is not
Changing a filter, opening registers, moving furniture, and vacuuming a grille are all sensible weekend jobs. Straightening a sagging flex run in an accessible attic is within reach for plenty of homeowners.
Cleaning the system is not. A shop vacuum at the opening moves what you can already see and leaves what you cannot, which is the part that was affecting the airflow.
When to have somebody look
If airflow has clearly dropped and the filter is not the reason, a look at the accessible ductwork will tell you more in half an hour than weeks of adjusting things from inside the room.
The inspection is free, and if the answer turns out to be a crushed run rather than a dirty one, you should hear that before anybody quotes you for cleaning.