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Improve Air Conditioning: Avoid These 5 Ductwork Design Mistakes

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Last Updated on March 4, 2020


HVAC technician implementing ductwork design plans

Renovating your commercial space and looking to improve your air conditioning in the process? When the work is done, do you want comfortable, consistent temperatures in your space? How about good air quality? Energy efficiency? If you do, then it pays to plan updates to your HVAC system and its ductwork design early in the renovation process to avoid mistakes.

You probably didn’t realize that your HVAC system and its air duct system is something you need to be thinking about even before the architect has completed the plans. Find out how to improve your air conditioning with better duct design, and why you need HVAC experts involved early in your renovation process.

Consequences of Poor Ductwork Design

According to the US Department of Energy, the average HVAC duct system is only about 60 percent efficient. That means air is not flowing through your space and your HVAC system as it should be, which leads to all kinds of undesirable consequences, including:

  • Hot and cold spots, drafts and stuffy air in your space due to impeded air flow.
  • Extra wear and tear on your air conditioner, since it needs to run longer and work harder to compensate for flaws in the ductwork design, leading to more breakdowns and shorter equipment life.
  • Poor air quality that exposes the occupants of your space to increased levels of dust, pollutants, fumes, and even mold growth from too much humidity.
  • Unbalanced air pressure that causes odors to linger, doors to slam by themselves and distracting levels of noise in your space.

Related article: Crippling Air Conditioning Problems Caused By Poor Air Flow.

5 Common Ductwork Design Mistakes

Proper ductwork design ensures the level of air flow that your HVAC system needs to operate efficiently and provide the comfort you want and expect in your renovated space. Here are some of the common ductwork design mistakes that impede the function of your air conditioning:


Contractors can make the mistake of failing to consider the type of air conditioning system you have, load requirements of different rooms, where ducts and equipment are located, and the materials used to construct them. All of these factors affect the proper sizing of your ducts, and getting it wrong often means your HVAC ductwork is undersized. (See below for tips on getting it right.)

DUCTWORK DESIGN MISTAKE #2: Runs that are too long

When the location of HVAC equipment and duct system and not optimized in the planning phase, the equipment may end up far away from the space to be cooled. That may require long runs of ductwork that make it hard for your HVAC system to move conditioned air to certain areas within the space.


Just like long runs impede air flow, bends in the ductwork that are too sharp or too numerous also decrease the amount of air that actually reaches the space to be cooled.


Air conditioning ductwork that is incorrectly sealed or supported can end up leaking cooled air into the walls where it won’t do any good to the occupants of your space.


To maintain balanced air pressure and air movement, your duct system needs return vents for air in the room to be pulled back into the HVAC system. Not providing enough returns is a common ductwork design flaw that leads to comfort complaints.

6 Tips for Good Ductwork Design

To make sure your ductwork is properly designed, start by involving a knowledgeable HVAC design professional early in your renovation design process. An experienced pro will work with the architect and contractor to do the following:

1. Choose the best location for HVAC equipment and ducts. With proper planning, the HVAC equipment should be centrally located in the space to allow for the shortest possible duct runs. Ducts should be located in internal walls and ceilings to minimize the loss of conditioned air. Avoid installing ducts in attics and unconditioned crawl spaces for maximum efficiency.

2. A detailed load calculation. Especially when your space has different rooms or areas with varying heating and cooling requirements, it’s important that the load calculations be done individually for each room, rather than only for the space as a whole.

Related article: 4 Reasons You Need HVAC Design For Your Air Conditioning Install.

3. Consider your equipment type and supporting systems. Certain types of air conditioning systems, like heat pumps, require larger ducts. If your system includes air purifiers with activated charcoal filters, these also affect air flow and may require larger ducts and/or additional air returns.

4. Use the right materials, fittings and supports. Ductwork materials can vary depending on the requirements and the budget, but make sure your installer uses the right materials for your needs. Flexible ducts (often called “flex”) made from reinforced plastic are easier and cheaper to install, but not as strong and durable as sheet metal.

For more information about the use of flexible ducts, read this ACHRNews article about ductwork design.

If quiet operation and energy efficiency are very important to you, you might want to go with duct board, made from pressed fiberglass, which is more expensive but very quiet and efficient.

5. Choose the right duct size and layout. Once all the system variables have been decided, your HVAC design professional can determine the most efficient ductwork design layout and calculate the correct duct size. To account for all the necessary factors, many professionals use a tool called ACCA Manual D.

6. Ensure proper ductwork sealing. Did you know that as much as 20 percent of your conditioned air can be lost when duct joints are not correctly sealed? The problem is compounded with high efficiency systems, which run longer at a lower capacity. Air is in the ducts for a longer period of time and so more can escape through leaky joints. Make sure your duct joints are sealed with mastic gum or metal-backed tape to prevent leaks.

After reading this, are you concerned that your HVAC installer might not have the expertise to handle your ductwork design? For something as important as a renovation, getting the results you want might require you to find a more knowledgeable and experienced provider.

Changing vendors can seem intimidating, but it doesn’t have to be. Find out how to do it without stress with our free guide: Contract Confidence: Transitioning to a New HVAC Service Provider.

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