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Airflow Basics – Part 1

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Lets look at the three basic components of airflow:  volume, velocity and pressure. 


Standard air is air that is 70ºF with 50% RH, at sea level.  We measure air volume as a rate in cubic feet per minute (cfm). 

1 ft3 of standard air weighs .0741 pounds 

13.33 ft3 = 1 pound of air 

13.33 cubic feet of air is about the amount of air in a typical residential refrigerator. 

.244 is the Specific Heat of air.  Specific Heat is the heat necessary to raise the temperature of 1 pound of a substance 1°F.   

That makes air relatively efficient when compared to water.  You probably already know that the specific heat of water is 1.  If you don’t, then think of the definition of a British thermal unit:  the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.   

It only takes about a quarter of a Btu to raise the temperature of one pound of air one degree, but it takes a whole Btu to raise the temperature one pound of water (about a pint of water) one degree.  That makes air about four times more efficient than water.  

So, if we multiply the specific heat of air by the weight of a cubic foot of air by 60 minutes in an hour (remember, we’re going from Btuh to cfm), then, we have our air flow constant of 1.085 which is usually rounded up to 1.10 for convenience: 

.244 x .0741 x 60 = 1.085 (often rounded to 1.10) 

We’re going to use this airflow constant in the Sensible Heat Equation below: 

cfm = Sensible Btuh 

          1.1 x ΔT x ACF 

This is one way of converting cfm to Btuh.  Remember, if customers call you and say that they don’t have enough heating or they don’t have enough cooling, then they are actually saying that they don’t have enough airflow (cfm).  That’s how the heating and cooling Btu’s get to the rooms or areas to be heated or cooled, via the cfm. 

Now let’s define all the components of the Sensible Heat Equation.  You know that 1.1 is an airflow constant, and you know how it was derived.  That brings us to ΔT.  That triangle represents the Greek letter “d”, or delta.  Anytime you see Δ in our industry it means difference.  ΔP is a pressure differential across a pump or blower and ΔT means temperature difference.  In the case of airflow, the temperature difference referred to is the difference between the temperature of the air leaving the furnace or evaporator coil (LAT) and the thermostat set point.   

Lastly, ACF is the Altitude Correction Factor.  It is taken from Table 10A in Manual J, 8th Edition.  It is used to correct the 1.1 airflow constant (which was based at sea level) for altitudes greater than 1000 feet above sea level.  The higher you go, the more cfm you will need for the same Btuh. 


We must deliver the volume (cfm) at a proper velocity.  We measure this velocity in feet per minute (fpm).  Typically, we would want to keep our trunk lines between 700 and 900 fpm and our branch run-out velocities, ideally, between 400 and 600 fpm, although there is some variability here.  Proper register sizing can go a long way to improve low branch velocities.   

Velocities less than recommended can produce poor (low) air flow and velocities greater than this will always produce noise. 


We measure pressure within a duct in inches of water column, a.k.a., IWC, IWG, “w.c. or even in. H2O.  No wonder there is so much confusion regarding airflow.  We have at least four different designations for pressure in a duct system! 

There are two primary pressures in a duct system.  The first is velocity pressure.  The faster the air moves, the greater the velocity pressure.  The other pressure within an operating duct system is static pressure.  Static pressure is bursting pressure.  An inflated balloon contains 100% static pressure.  When you release that balloon, the static pressure converts to velocity pressure.  Total pressure is velocity pressure plus static pressure. 

psig = 27.72 IWC  

Most of our residential duct systems are designed to operate in a range from 0.5 to 1.0 IWC. 

The Duct Designer’s Dilemma 

We know that a cubic foot of standard air weighs .0741 pounds.  Therefore, a 3-ton system handles 90 pounds of air, a 4ton system handles 120 pounds of air and a 5ton system handles 150 pounds of air.   

Think about it!  That much air every 60 seconds (cfm) with pressure that wouldn’t properly blow your nose (between .50 to 1.0 IWC).  How is that even possible?   

We do it all day long, one cubic foot at a time.  That’s because we’re the HVAC industry and we know how to!  More to come… 

Jack Rise
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Posted In: ACCA Now, Technical Tips

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