Air duct design explained

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If the AC is the heart of any forced air ventilation system, then air duct design is its bones. Once you've decided to install an air conditioner, air ducts will go from a subject you may never have thought of before to one of overriding prominence in your life.

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You probably know to comparison shop for the best air conditioner prices, to always get more than one air conditioner estimate, and to ensure there's a first-class warranty attached to the work. But often it's the smaller details you never thought of — like air ducts — that will get you in the end.

Duct size, air duct design specifications, connecting air ducts to air condition units, researching how often to clean air ducts and so on will take on a life of their own until the job is finished.

About air ducts

The first thing to learn about the ducting for an HVAC system is that it's the method by which warm and cool air are distributed throughout your home, carried in from either the furnace or the cooling element of your AC system.

Without an effective system of ducts in place to carry the air, no home cooling or heating system can be expected to do the job of equalizing the temperature of every room in the house.

Air duct design

A good design for air distribution will:

• Convey the air as directly as possible, following the shortest feasible path and with minimal changes in direction.
• Diverge only gradually, to prevent backpressure waves and concordant loss of power.
• Be as smooth as possible, as friction robs the moving air of forward momentum and creates noisy turbulence.

When you're choosing your system, you should know that your home's design will dictate the layout options available for your duct system. Ducts can be arranged to:

• Radiate outward from a central point.
• Branch organically away from each other like a system of roads or arteries.
• Or carry air directly away from a central trunk line, which carries the bulk of the system's air.

Your home will probably accommodate only one type of layout. Your contractor is the expert to ask to be sure.

Types of ducts

Now, you're left to decide on duct type. Basically, three types exist:

• Round metal pipe is the preferred option if you can afford it. This option offers excellent airflow with minimal friction and will — with proper cleaning and maintenance — last much longer than the alternatives. Unfortunately, round metal pipes have to be installed, which adds considerably to their expense.

• A low-cost alternative is insulated flexible duct, which you might know as the big silvery tubes lying around construction sites. These are pre-insulated. Their easy installation makes them by far the cheapest option, which explains their ubiquity in residential construction. Unfortunately, they'll wear out sooner than other types, including rectangular ducts.

• Rectangular ducts last longer than the flexible air duct design, but are cheaper to install than round metal pipes.

Your contractor should be familiar with all three types but will probably have a preference for installation, so asking in advance is a good idea.

Now that you have a basic idea of how air duct design works, you're ready to connect with contractors. Each should come by your home before providing an estimate. When reviewing your space, ask him to look at your air duct system and whether it needs to be upgraded to work with your new HVAC system.

Photo credit: Emily Lewis via Compfight CC.