How to tell which house vents are returns?
Supply vents blow conditioned air into a room while return vents pull air back into the HVAC system. Because new systems or unlabelled grilles lack obvious airflow, homeowners struggle to tell them apart using simple tests like holding up paper towels, which stops them from properly balancing airflow or changing filters.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
- 1testing vents with a paper towel to check for suction
- 2checking if registers have adjustable louvers or dampers
- 3going up into the attic to follow the ductwork
- 4Turning appliances or utilities on one at a time to see if they exhaust air through the vent
- 5Installing a grate over the unknown vent opening
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“i tried testing which vents are return vents by using paper towel to see if it sucks it up (google said so) none of them seem to though itsa new hvac too at least the hvac itself im not sure if vents were touched up when previous owners got a new one”source ↗