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Properties Of Air and Flow Through A Furnace

Depending on which study you read, it’s been estimated that somewhere between 58 and 72 Percent of air conditioning systems are not functioning as designed due to problems with the air handling system. Regardless of whether or not we accept any given number presented by any given study at the time, we have to admit that it’s important that the air flow through any HVAC system be proper in order to accomplish the necessary heat exchange and achieve the desired comfort level. And when it comes to forced-air gas heating systems, technicians can accomplish the task of ‘wrapping their head around’ the concept of proper air flow by understanding the fundamentals of the properties of air….and two formulas.

As far as the properties concept goes, since we always have to start somewhere with a standard, when we consider the properties of air we understand that two of these standard points are:

1. Sea level (14.7 PSI….meaning 14.7 pounds per square inch of atmospheric pressure pressing down on the earth, referred to as 1 Atmosphere of Barometric Pressure). 

2. A temperature of 68-degrees Fahrenheit.

Of course, a change in temperature or pressure will bring about a change in the air itself, but at the standard conditions mentioned above, we can derive two factors, which are:

1. Air will have a Density of 0.075 lbs. per cubic foot.

2. Air will have a Specific Heat of 0.24 BTU’s.

Which brings us to our first formula that HVAC technicians need to understand relative to air flow. When we apply these two factors in a formula along with a factor of time (one hour….expressed as 60 minutes in the formula), we can calculate what is known as The Sensible Heat Factor of Air.

0.075 x 0.24 x 60 = 1.08

And, that number that we arrived at can now be applied within the second formula, which is:

CFM = Qs / 1.08 x TD

And, once we are reminded that the factors in the formula are:

CFM = Cubic Feet Per Minute

Qs = Sensible Heat In BTU’s Per Hour

1.08 = Sensible Heat Factor of Air

TD = Temperature Difference Between Return and Supply Air….

….We can undersand that a manufacturer designs a gas furnace, this formula, along with other factors, are used to determine the size of the squirrel-cage blower and the horsepower rating and speed of the motor that ultimately determines the amount of air flow through the furnace cabinet.

To illustrate the formula, we’ll consider a specific gas furnace and plug in the necessary numbers to make it work.

Our equipment will be an 80,000 BTU furnace that has an efficiency rating of 80%, which means that the actual heat output (Sensible Heat Factor in BTU’s Per Hour) can be calculated as follows:

80,000 x 0.80 = 64,000

And we’ll also determine that our return air temperature will be 70-degrees, and that our supply air temperature will be 130-degrees, which means we can calculate our TD as follows:

130 – 70 = 60

As our next step in explaining the process, we’ll again present the original formula as….

CFM = Qs / 1.08 x TD

 …Which means that with our numbers plugged in, it reads as:

CFM = 64,000 / 1.08 x 60

…Which works out to:

CFM = 64,000 / 64.8

…Which means that what we have to do is divide 64,000 by 64.8, in order to arrive at:

987.65432 Cubic Feet of Air Per Minute

All of which boils down to the fact that this particular furnace needs 988 CFM traveling through its cabinet, along with properly designed and installed duct system, in order to accomplish the proper temperature rise, which accomplishes the intended goal of proper operation and achievement of the desired comfort level in the building.