Cold bedrooms, a hot upstairs and weak vents often come down to air leaking out of the ductwork before it reaches your rooms. Measure the airflow at each supply vent and this calculator compares it with what your furnace is rated to move, using the same airflow procedure our technicians follow.
Loading the Duct Airflow Loss Calculator.
How to take the readings
- Run the furnace fan on high, with every supply vent open.
- Hold an anemometer 6 to 8 inches from each supply grille and note the air speed in feet per minute (LFM).
- Measure the clear opening of each grille and pick it from the list.
- Enter your furnace’s rated airflow in CFM from its label or manual.
Airflow at a vent is the air speed times the open area of the grille. Vents delivering 100 CFM or more are at standard flow, 90 to 99 CFM is acceptable, and under 90 CFM is low.
Clear grille areas
| Grille | Open area |
|---|---|
| 2″ x 10″ | 0.138 ft² |
| 2″ x 12″ | 0.166 ft² |
| 2.5″ x 10″ | 0.173 ft² |
| 2.5″ x 12″ | 0.208 ft² |
| 3″ x 10″ | 0.208 ft² |
| 3″ x 12″ | 0.25 ft² |
| 3.5″ x 10″ | 0.242 ft² |
| 3.5″ x 12″ | 0.291 ft² |
| 4″ x 10″ | 0.275 ft² |
| 4″ x 12″ | 0.333 ft² |
| 6″ x 10″ | 0.416 ft² |
| 6″ x 12″ | 0.5 ft² |
What the result means
If the vents together deliver 15% or more less air than the furnace is rated for, a large share of the air is escaping through gaps in the ducts. Sealing them usually restores even temperatures and lowers energy use.
See duct sealing and duct cleaning, or call 587-792-1456.
Frequently asked questions
How do I measure airflow at a vent?
Hold an anemometer 6 to 8 inches from the supply grille and read the air speed in feet per minute (LFM). Multiply it by the clear opening area of the grille in square feet to get the airflow in cubic feet per minute (CFM).
What counts as high duct leakage?
If the vents together deliver 15% or more less air than the furnace is rated to move, a large share of the air is escaping before it reaches your rooms. Leaking joints and seams in the ductwork are the usual cause.
What are the signs of leaky ducts?
Rooms that stay too hot or too cold, weak airflow at some vents, high energy bills and extra dust are common signs that conditioned air is leaking from the ducts into walls, floors or the attic.
How are leaking ducts fixed?
Accessible joints can be sealed by hand. Leaks inside walls and floors can be sealed from the inside with an aerosol duct sealing process, which Alberta Indoor Comfort offers.