On furnaces with ECM motors, it is pretty easy to determine what CFM the blower is producing since there is an LED on the board that flashes 1 time for every 100 CFM that the motor is producing. But how can you determine the CFM of a blower when there is no LED to show the CFM?
This goes back to Basic Heating 101. In school we learned a simple formula called the SENSIBLE AIR EQUATION. That equation is:
CFM = BTU’S PER HOUR / 1.08 X TEMPERATURE RISE
Since the manufacturer calculates the BTU OUTPUT based on proper manifold pressure, with a fossil fuel furnace, determining the BTU’s Per Hour is as easy as reading the name plate OUTPUT on the furnace. So, the BTU output of the furnace = BTU’s Per Hour for our formula.
1.08 is a constant used in the formula and represents the specific weight of a cubic foot of air.
The main variable in the formula is the temperature rise across the heat exchanger. Of course, when taking a temperature rise you need to let the furnace run at least 15 minutes before taking any readings. The supply air temperature should be taken in a main trunk so it is not in direct line of sight of the heat exchanger or “A” coil to prevent radiant heat from affecting the reading. The return air temperature can be taken in the return duct.
So, let’s look at an example. If a furnace has a name plate rating of 80,000 BTU OUTPUT; a supply air temperature of 122 degrees; and a return air temperature of 70 degrees, then the CFM would be:
80,000 BTU Output / 1.08 X (122 – 70) = 80,000 BTU Output / 1.08 X 52 degree rise =
80,000 Btu Output / 56.16 = 1424 CFM
You can also verify the CFM for an air conditioner using the same formula, you just have to be sure that you take the temperature rise with the fan on the COOLING SPEED of the blower.
If you read my previous BLOG on Air Flow Effects on Air Conditioning back on7/25/11, you can see how this could help on some problem cooling jobs as well.