Furnace Efficiency

Written by bsrep, llc.

Calculating Your Furnace CFM’s
Knowing your furnace's CFM will give you a better idea of how well your unit is functioning.  The calculated CFM is what the furnace requires to operate at maximum efficiency. The CFM measurement at your vents should be close to what your furnace is rated at. If not, it’s time to have a qualified HVAC professional take a look at your furnace.

Needed Information:

Furnace Types
•    Condensing Furnace - Flue made of PVC pipe. (Furnace Multiplier = 150)
•    Induced Draft Furnace - Fan located below the flue. (Furnace Multiplier = 130)
•    Natural Draft Furnace - Has neither of the above items. (Furnace Multiplier = 100)
Calculating the CFM Required:
1.) Find furnace BTU rating.
     a.) Found on the manufacturer's nameplate attached to the furnace.
2.) Divide the BTU rating number by 10,000.
     a.) For example, if you find that your furnace's BTU rating is 90,000, you would divide this number by 10,000 for a factor of 9.
3.) Multiply the result by the "Furnace Type Multiplier"
     a.) Ie.., if you have a natural draft furnace the CFM would be 100 x 9 = 900 CFM
4.) This is the number that should be coming out of the vents. Use a velocity meter to check.

Determine the diameter / dimensions of the ductwork to get the 2nd multiplier:

Circular Duct – (4” = 17.17), (6” = 38.65), (8” = 68.71), (10” = 107.36)

Retangular Duct – Multiple 13.67 for every square inch of duct opening
    Ie. If duct is 4” x 6” = 24 Sq. In., then 24 Sq. In x 13.67 = 328.08

The following calculation is based off a popular and inexpensive velocity meter.
This particular meter being made in another country does not have the option to calculate CFM's
You need to choose a different velocity setting and perform some manual calculations.

Velocity Meter

Calculating Actual CFM at Vents
1.) Set the Digital Anemometer to “VEL” & "M/S”
2.) Use meter to calculate Velocity @ M/S (Meters Per Second)
3.) Put the meter over a vent until the number doesn't get any higher.
4.) Multiply that number by the 2nd Multiplier to obtain CFM (Cubic Feet Per Minute)

The duct is 4” x 6” = 24 Sq In.
24 Sq. In. x 13.67 = (2nd Multiplier of 328.08)
The meter reads 3 M/S
Multiply 3 M/S by (2nd Multiplier) = 984.24 CFM

In the scenario of a Natural Draft Furnace that should be putting out 900 CFM
and is shown to actually be putting out 984 CFM shows that the efficiency of this furnace is about as expected.