If we are working on a system with a gas furnace, we can figure this out with a bit of math. I look at the system’s data plate and find the temperature rise range. Whatever the range is, you want to find the midpoint of the temperature rise.

Find the Midpoint Temperature

The midpoint temperature will be the target. Running a furnace at the midpoint of the temperature range will give you a buffer against issues like when the furnace filter loads up with dirt.

You find the temperature midpoint by simply averaging the larger and smaller numbers.

So let’s say your range is 20°F to 50°F. Take the larger number and add the smaller number together, then divide by 2.

In this case: 50°F + 20°F = 70°÷2 = 35°F.

That 35°F is your target heating temperature rise.

Finally, we can solve for our target airflow by slightly rearranging the sensible heat formula:

BTU Output ÷ (temp rise x 1.08) = Required heating airflow

Now that I have my required airflow for the customer’s system, I start looking at the ductwork near equipment components.

Air temperature in the ductwork matters
HVAC field technician installing ductwork.

Two numbers that I want you to remember are 700 feet per minute (FPM) and 900 FPM. These numbers are the maximum velocity limits that ACCA recommends on a residential duct system. The first number, 700 FPM, is for the return, and 900 FPM is for the supply.

We can analyze the return and supply trunks with these numbers in mind. If you have a Ductulator, then it is easy. Plug in the duct size you are referencing and look at the CFM you are moving and the corresponding velocity. If it is above the maximum
velocity limits, the system is oversized, the ductwork is undersized, or, worst case, all of the above.

Sizing and Other Potential Issues

Remember that just because the velocity in the duct is less than ACCA’s limits does not necessarily mean the duct is sized correctly. It means no glaring red flag is waving in front of your face.

Another obvious issue might be a small evaporator coil on a large system. This would cause a high-pressure drop across the evaporator coil.

I always look at the filter size as well. The magic velocity I use for 1-in. filters is 300 FPM. If your velocity is higher than 300 FPM, it is likely causing a high-pressure drop, increasing the static pressure.

Converting filter CFM to velocity: CFM ÷ Area (sq. ft.) = velocity

Let’s say our hypothetical system moves 1,000 CFM through a 16-in. x 20-in. filter.

Temperature isn't part of this calculation

In this example, you can see that a 16-in. x 20-in. filter is inadequate for the system.

Taking National Comfort Institute’s “Duct System Optimization” class was also crucial to my process. It taught me how to identify duct system deficiencies and make surgical changes to the ductwork that positively impact the overall system performance.

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