Understanding Air Flow | Ask This Old House

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.

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.

Where to find it?
Richard and Ross use dampers and drain valves to demonstrate how balancing airflow throughout a home is achieved.

About Ask This Old House TV:
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

[ctfplumbing]

Richard, your plumbing work is the best! I’m your biggest fan!

[phototristan]

I certainly have experienced this. When I was in college I rented a room in a family's house and my room was the farthest away from the air conditioner unit. It would be 85 degrees in my room and nice and cool in the other rooms. They still made me split the electricity bills equally though so I ended up moving out.

[bigsparky]

Very interesting, wish i had that type in my house. I have window AC, old steam radiators

[tonystrollo]

Great new solutions for the future, the Iris is the best, then the cable adjust, is also a good product in the event you need to change air flow.

[Alex.AL_]

Surprised nothing was mentioned about automated zone systems. I have a thermostat in each room that is connected to damper that opens or closes electronically based on the temperature in each room.

[facklere]

This was really helpful for me. The upstairs and downstairs temps of my apartment were always drastically different - I measured the temp upstairs at 8 F warmer than downstairs last week. I had just resigned myself to the fact that we had a crappy AC unit because it's a rental. By closing off the vents downstairs somewhat (where the air handler is) I've gotten the upstairs and downstairs temps to match and it's so much more comfortable. Thanks!

[yahanwiwi]

Companies also make register boosters. AC Infinity has decently priced units starting well under $100.

[xslabcabxhearsex]

Been doing heating and air conditioning/ sheet metal mechanic for 33 years.the biggest problem is poorly designed duct systems.not sized right from the beginning.few companies are putting in proper sheet metal duct work that is sized correctly.instead it’s just a bunch of flex and box crap that doesn’t work well.the art of sheet metal is hard to find these days.

[bobm]

The pipes for the example are different. One has the outlets point down and the other is out the side. Of course it'll come right out the first one if it's pointing down. It would with the valve too.

[Jacarroll]

My bed room used to be very uncomfortable. We ended up using cardboard and blocking the register all together. It was so bad that even in the dead middle of Michigan winters Id have to crack a window because it was too warm. Me and my dad would fight over the thermostat, he would turn it up and I would turn it down.

[Lughnerson]

Open stairwells allow the stack effect, which causes temperature differences from lower to upper floors. But I have not seen this addressed. The stack effect means warmer air rises, so with open stairwells the downstairs warm air rises to the upper floor, whether it is heating or cooling season. I have seen, but not used, ceiling curtain tracks which hold long curtains that reach from ceiling to floor. Using those curtains would lessen the stack effect, I believe. Or build enclosed stairwells. Multi-level homes would have less temperature divergence, more comfort, and energy savings.

[hansjansen]

If the ductwork was laid out properly to start with airflow would balance. This would involve some prior calculations by the installer. There is even software available to help thru the process. But it takes time which most HVAC install companies don't want to spend.

[ThreeRunHomer]

That’s one of the reasons ductless split systems are becoming so popular, at least in smaller homes.

[tonyegg.]

I'm a Scottish plumber, I love watching Richard our systems are totally different from yours but He is always clued up and informative

[DemoniqueLewis]

My parents and I were talking about air flow today. We were trying to find the dampener for the Maine floor of a colonial. Looks like the primary duct is in the ceiling of the basement without any dampeners and ceiling registers. So the water example really helped explain why it is so hot on the Maine floor.

[morrismonet]

I had hot water baseboards put in when we put the house up over 35 years ago. Last year I finally replaced the original boiler. The whole house is a constant even temperature. Duct work sucks and is bad for your health.

[loganverdery]

It's always good to be back with The Times silver

[Craigthepope]

Not pictured: Flex ducts which are common in the South. Flex ducts also must be installed correctly no slack and no sharp turns,

[bonita]

Wish I knew more about this before I got screwed by Costco approved sub contractor. They installed my new air conditioning and ducts.

[markbyrd]

Does anyone else find that too often vent covers never seem to attach to the ceiling because the duct work was totally lacking appropriate fasteners? I would really like to hear how you all mitigate this particularly if you are unable to change the duct work.