Understanding Air Flow | Ask This Old House

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In this video, This Old House plumbing and heating expert Richard Trethewey and home technology expert Ross Trethewey explain everything you need to know about balancing airflow within a home.

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Kevin O’Connor meets plumbing and heating expert Richard Trethewey and his son, home technology expert Ross Trethewey, to talk about airflow. Richard explains that most folks with forced hot air or central air conditioning dislike their air distribution, and the Tretheweys explain the options.

Most Folks Aren’t Happy With Their Heating and Cooling System
Most American homes have hot air heating or ducted cooling systems. And, seven out of ten of the folks in those homes are unhappy with how they operate. Much of the reasoning behind their unhappiness is air distribution and how some rooms receive too much air while others receive too little.

Air Travels the Path of Least Resistance
Physics is constantly fighting against ideal airflow. As the main duct routes through the home, smaller ducts branch off. If these smaller ducts are left wide open, the majority of the air will pour out through the ducts closest to the furnace or air conditioner, while the ducts at the end of the line will receive very little airflow. This is because air travels through the path of least resistance.

Dampers are the Answer
It’s possible to control airflow by installing differently-sized ducts or adjusting louvers on air registers, but most homes can benefit from dampers. Dampers can control the amount of air that flows through the branch, helping distribute air more effectively.

Different Types of Dampers
Like most things, there are lots of different types of dampers. Some do a better job than others, so it’s a good idea to become familiar with each type.

Metal butterfly-style dampers [] install inside a standard duct. They’re relatively easy to adjust, but they rely on a thumbscrew to keep them in place. Vibrations and airflow can loosen these thumbscrews and affect the flow.

Iris-style dampers [] have adjustable and lockable lenses that allow for properly targeted airflow. They install between pieces of duct, and their lenses open similarly to a camera shutter, providing quiet and reliable airflow.

Automatic volume dampers [] self-regulate airflow. These devices have settable airflow volumes that the user can choose, and the spring inside the damper automatically adjusts to ensure the proper amount of air is passing through the ductwork.

Cable-operated dampers [] have long, stainless steel cables that travel from the damper’s locations to the register. Inside the register sits the damper adjustment, and tweaking the adjustment opens and closes the damper. They’re an excellent choice for ductwork hidden in ceilings or walls.

Where to find it?
Richard and Ross use dampers and drain valves to demonstrate how balancing airflow throughout a home is achieved.
Cable operated dampers [] can be found at Home Depot [].
CAR3 Constant Airflow Regulator [] is manufactured by Aldes [].
Duct Iris Damper [] is manufactured by CFM Inc. []

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From the makers of This Old House, America’s first and most trusted home improvement show, Ask This Old House answers the steady stream of home improvement questions asked by viewers across the United States. Covering topics from landscaping to electrical to HVAC and plumbing to painting and more. Ask This Old House features the experts from This Old House, including general contractor Tom Silva, plumbing and heating expert Richard Trethewey, landscape contractor Jenn Nawada, master carpenter Norm Abram, and host Kevin O’Connor. ASK This Old House helps you protect and preserve your greatest investment—your home.

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Understanding Air Flow | Ask This Old House