In order to understand the foundational principles of duct sizing, a thorough study of ACCA Manual D is a great place to start. This is an industry standard for duct design. In it, you’ll learn the principles behind accounting for indoor coil and air filter pressure drops and how to select the best duct fittings.
When it comes to sizing ducts, however, it’s evident that very few contractors have the free time to perform the needed steps or to reverse engineer the existing installation. Experience tells us that field installation conditions often don’t follow the perfection of a modeled design — this creates varying results and makes the design inaccurate.
To help you overcome this challenge, I would like to share three proven steps that many successful contractors use each day to resize existing duct systems.
The first step begins at the air-handling equipment. If you have a system equipped with a weak blower, restrictive coil, or undersized air filter, scratch trying to resize the ducts. These issues must be corrected first. Even a perfect duct system can’t overcome these obstacles.
The air-handling equipment you use should ideally have a maximum-rated total external static pressure (TESP) of .70 inches or greater. This is enough fan capacity to move the required airflow in most residential applications, but it’s good to note that a variable-speed blower is typically needed.
Many residential fans are only rated to operate at a maximum-rated TESP of .50 inches. This blower won’t be able to move the required fan airflow when static pressure exceeds the .50 inches rating. To verify this, you’ll need to look at nameplate data and the fan performance tables for the equipment.
Once you have a strong enough blower, refer to the specifications for the indoor coil. Look for one with a low pressure drop when wet. A wet coil pressure drop of .20 inches is ideal but can be hard to find in some sizes. If you can keep the pressure drop under .30 inches, you should be fine. In some instances, this is the best you’ll find.
Once a strong fan and low pressure drop coil are selected, the air filter is the next component to consider. Make sure the filter is large enough and has adequate surface area for the volume of air you need to move through it. Ideally, the filter should be sized for 300 feet per minute (fpm) face velocity or less. This keeps the pressure drop low and improves filtration effectiveness.
SIZE AND DESIGN THE DUCT SYSTEM
Once you have equipment in place that can move the required amount of airflow, you need to determine airflow for each room. In last month’s article, I discussed a method for estimating room airflow that you can use. A heat load calculation is another viable method to get your required room airflow numbers. You need this value to determine duct sizing needs and to compare against the available airflow capacity of the existing duct system.
Once you know the airflow values for each room, size your duct system by setting your duct calculator to .05 for flexible ducts and .07 for rectangular metal duct systems. To work, the installation must have straight, well-installed, and suspended ducts with balancing dampers. If you skip these details, results may suffer and leave you with no room for adjustments.
Start at the farthest supply register, and size each duct as you work your way back to the air-handling equipment. Determine the duct material you’re going to use. That will help you determine your friction rate setting. Then, line up how much air you need moving through the duct under the friction rate setting.
Once you have the friction rate and airflow settings aligned, determine the potential duct sizes you can use. If the required airflow falls in between a given duct size or is longer than 25 feet in length, increase to the next largest duct size. Continue to repeat this process for each duct run as you work back to the equipment until all have been accounted for. Once the supply is done, move to the return side of the system. It’s that easy.
If you don’t want your installation crews using a duct calculator, email me a request for the one-page NCI Duct Design Tables and Quick Duct Capacity Check that you can laminate to help make duct sizing faster and easier.
These design settings will feel way oversized to you at first and increase your duct materials cost by about 10 percent, but it’s worth it. Your customers will pay more for a duct system that performs better and provides the comfort they expect.
SET AND MEASURE AIRFLOW
You really didn’t think this stopped at duct design and sizing, did you? It doesn’t matter how you accomplish it: any design is ineffective without verification. Once the duct system is installed, you’ll need to assure it works as intended.
Begin by determining required fan airflow and set the blower to deliver this amount. Be sure to measure airflow at the equipment using static pressure and a fan chart or a traverse if you have an acceptable test site. Once fan airflow is set, measure supply-register and return-grille airflow from the redesigned system with a balancing hood. Measure and compare delivered airflow to design airflow.
While that’s the short story, this design method is simplified but will not work on systems that are excessively long or complex. However, it’s widely used in the industry today to produce duct systems that are verified to function as designed. Is there more to it? Sure there is. But this is enough for you to improve your duct designs immediately.
THE DESIGN CHALLENGE
If you believe your current duct design method works, I’m happy for you. However, I’m calling you out and inviting you to prove it. The only way to know how well your design works is found in step three. Once you measure airflow and compare design cfm to actual cfm delivered by the installed system, you’ll know if you hit the bullseye.
I encourage you to research and consider the ideas presented — don’t knock them until you’ve tried them. Some who read this article will discredit this method because it’s too simple and doesn’t include endless formulas and computer-simulated modeling. That’s OK: Everyone is entitled to their opinions. Remember, without measurements, you’re just another person with an opinion.
Thousands of HVAC contractors across the country use this approach as their preferred method for duct sizing, and they measure the results. I’ve personally installed, tested, and balanced systems using this approach for more than a decade. If you follow the steps, pay attention to details, and measure your systems, it works as intended. I invite you to take the challenge and see for yourself.
Publication date: 7/2/2018