How to Test Your Furnace's Airflow

If a room in your house is too cold during the winer, it might be because your furnace isn't supplying enough warm air through the registers in that particular room. There's a simple way to test the airflow from your furnace registers to make sure they are working properly -- the garbage bag airflow test.

The test is a quick way to estimate airflow by determining how long it takes to fill a common plastic garbage bag. While it is not a precise measurement, it is better than no measurement at all and will give you a good ideas as to whether you need to call a technician to look at your ductwork.

To do the garbage bag airflow test:

  • Tape the mouth of the garbage bag to a coat hanger or piece of cardboard to keep it open.
  • Crush the bag flat and place it over the register or exhaust hood.
  • Count the number of seconds it takes for the bag to fully inflate.

If the inflation time is ...

2 seconds = 37 L/s (75 cfm)
4 seconds = 20 L/s (40 cfm)
10 seconds = 10 L/s (20 cfm)

If the measured airflow is less than 10 L/s, the furnace is delivering only a small amount of heat to a room and needs to be further inspected to determine why the heat is not going through.

The garbage bag airflow test is also useful if you have changed your heating or cooling systems or have made major renovations to your house.