Doing Ducts Right

Where and how you install your ductwork can make or break a heating or cooling system.

High-efficiency equipment by itself doesn’t guarantee comfort or health; the ductwork also needs to be properly designed and sealed. However, some ducts struggle to deliver enough airflow, while others are poorly insulated or have leaky joints that dump air into attics, crawlspaces or garages.

Here are a few things to keep in mind when approaching a duct installation.

Ensure good airflow. Many residential ducts have excessively high static pressures. The mechanical equipment has to run longer and start and stop more often, making it more prone to breakdown and premature failure.

Common culprits include undersized ducts and those with too many directional changes. The cause can also be flex duct that was installed with little attention to detail. To minimize airflow-restricting friction, the flex duct's internal liner should be pulled tight, while bends and sags should be minimized. Some installers suggest using rigid fittings for all directional changes, then installing taut runs of flex duct between them.

Keep them inside. It's not news to you that uninsulated ducts are best kept out of unconditioned spaces. Getting the builder and designer on board may require getting involved at the design stage. Options include placing ductwork in chases or soffits, or using plenum trusses: modified roof trusses that put the ductwork above the ceiling yet inside the conditioned space.

Insulate and seal. If you have no choice but to put ducts in an unconditioned space, then make sure they're well insulated and their joints properly sealed. Standard R-8 duct insulation won't stop energy losses during extreme hot or cold, so some high-performance builders bury the ducts in a thick layer of blown fiberglass attic insulation. This also makes the home more energy efficient.

The worst case is an unsealed duct in an unconditioned space. It's not unusual for a leaky supply duct to waste as much as 500 cubic feet of air per minute—more than a ton of heating or cooling capacity. A leaky return in an attic can suck 140°F air into the system during summer and 30°F air during winter. The furnace and air conditioner end up working hard to heat and cool the outdoors.

Unsealed attic or crawlspace ducts can also create a slight vacuum inside the home, which draws outside air into the house through gaps in the building shell. This incoming air can include mold and radon from a crawlspace, carbon monoxide or chemicals from the garage or dust and insulation fibers from the attic. In extreme cases the negative pressure can even cause open combustion appliances to backdraft.

Of course, if the ducts are inside the conditioned space, leaks won't exchange air with the outdoors or suck pollutants into the home.

Good design and detailing can add a little extra time and money to the budget, which is probably why so many ducts are leaky or don't deliver the air they should. Builders and homeowners who understand the consequences are more open to having the job done right, making it worth the effort to try and educate them.

This article only scratches the surface. To learn more about duct design, register for the Residential Duct Design course from Lennox’ own training department, HVAC Learning Solutions.

Don’t have time for a full seminar right now? Use this 21-minute online training, Checking and Sealing Ductwork, to brush up on your ductwork knowledge, anytime.

Register Now

Whether you’re designing a residential system or retro-fitting an old one, we’ve got the installation supplies you need.

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