Rigid, Flex, Or Both – What's Right For Your AC Retrofit?

Installing a new air conditioning system in an old home typically involves a substantial expense: ductwork installation. Unless your older home already includes forced-air heating, you'll need ductwork to carry conditioned air from your air handler assembler to the rooms in your house. The two most common types of ductwork in residential installations are rigid sheet metal and "flex" ducts.

Many HVAC installers work with flex ducts over sheet metal, often for the same reason that plumbers choose to work with PEX over copper. Like its plumbing counterpart, flex duct is relatively easy to install, forgiving, and, above all, cheaper. But is it the right choice for your new AC retrofit? Opinions on this topic can be heated, and the answer is anything but straightforward.

Is Sheet Metal Better Than Flexible Ductwork?

A common complaint with flexible ductwork is that the ridged interior can disrupt airflow, reducing the total throughput of the system and creating airflow restrictions that impact efficiency. These complaints are valid but tend to reflect the installation more than the product. Flexible ductwork must be tight to reduce interior airflow restrictions. Loose or sagging ductwork almost always creates problems.

In other words, flexible ducts aren't strictly worse than metal ducts for all use cases. Instead, they require more care during installation to ensure they provide adequate airflow. Since air conditioning systems rely on a specific amount of airflow to function at maximum efficiency, it's crucial to install flexible ductwork in a way that allows it to achieve its manufacturer-specified flow rate.

Likewise, tight kinks and bends can impact the interior diameter of flexible ductwork. As a result, these sections will reduce the airflow through the duct, creating a restriction that will impact the entire system. However, correctly installing flexible ductwork and avoiding tight bends can help avoid these issues and minimize the potential for restrictions.

Are There Any Disadvantages to Sheet Metal?

There's no question that sheet metal ductwork can provide better airflow, but it's also more costly to purchase and install. The differences are also likely minor, especially when working with smaller runs and comparing rigid ductwork to a correctly installed flex duct system. In other words, system design and installation tend to impact airflow more than the products used.

Leaks are another potential downside. In addition to its flexibility and ease of use, flex duct allows installers to use a single tubing piece where rigid installations require multiple joints and corners. As with plumbing, joints are the most vulnerable part of any air conditioning ductwork system. These areas can rust, break, and leak, reducing system efficiency and causing problems down the line.

Which Option Is Best for Your Home?

There's no single correct answer for a retrofit. Your installer must evaluate your home to determine the most efficient and cost-effective solution. In some cases, a combination of rigid and flex may be suitable. For example, well-supported flex ductwork can work well for long runs, with rigid ductwork at the plenum or sharp bends.

Ultimately, the best option is the one that your installer has the skills and experience to design. A well-designed air conditioning ductwork system will provide excellent performance, whether you use rigid ductwork, flex duct, or some combination of the two. 

For more information, contact a local AC company like B & H Heating & Air Conditioning.

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