“It’s all smoke and mirrors (well…, no mirrors)” Or “Have you seen my air?”
I just preformed an air leakage test on some of my ductwork today. The results were less than pleasing. In fact, I was quite disappointed.
Let’s first break down a home air conditioning system into 2 basic components.
- Equipment – The equipment. The furnace the air conditioning condensing unit (outdoor unit) and the evaporative coil (the indoor unit).
- Ductwork – All of the ductwork that connects the system together from the return filter you see in the wall or ceiling to the registers in the floors or ceilings where all your air is distributed throughout the home.
No matter how good your air conditioning equipment is, it’s only as good as the ductwork that serves it. You can purchase the latest, most energy efficient equipment on the market today but if the ductwork is bad you will still be spending more on energy than is necessary and you won’t be as comfortable as you could be.
Several problem can be related to duct work, such as;
Pinched flex duct
Too much flex duct
Let’s talk about those air leaks and ways to find them.
A very good way to check your duct work for air leaks is with a pressure test. All of the registers and the return is sealed off. A special fan and meters are connected to the ductwork. The fan builds up pressure in the duct and the meters measures how much air is still flowing through the duct work even though it’s been sealed at all of the openings. This will tell you how much conditioned air you are losing before it gets to the supply registers. The meter breaks it down into 2 very useful parts. One, is the total amount of air that is leaking out of the ductwork measured in CFM or cubic feet per minute. The other is the physical size of the air leak in square inches. So if you were to measure all of the air leaks and put them to gather how big that opening would be.
The sample test on my house gave me the two readings. I was leaking about 350CFM. Since it’s a 2 ton unit, the total amount of air that is circulated through the system is about 800 CFM or 400CFM per ton.
350CFM / 800 CFM total = .437
So I am leaking about 44% of the air out through duct leaks. So for every $100 I spend on air conditioning or heating I am throwing away $44. Unbelievable. The other measurement was the size of the area that is leaking. The meter says its 39 square inches. That’s about the size of I dinner plate. It’s not all in one spot but it sure enough air leaking, it needs to be fixed.
These numbers can be even exacerbated by the location of the duct air leaks. For example, if the air is leaking from return duct in an attic, it’s “sucking” air in from the attic rather than the conditioned space. Instead of trying to cool air down from, say 80 degrees inside the house you are trying to cool it down from attic air that maybe 140-160 degrees.
So, I have the data, now what. Here is the second really cool part. We take a theatrical smoke machine and feed smoke into the fan that is pressurizing the duct. The only place the smoke can escape is out of the leaks. Now we can see where those ducts are leaking we can seal them and improve our efficiency.
Duct leak testing is a fairly uncommon practice in residential work but will become a requirement in the not too distant future. The department of energy now requires duct systems in all new construct to be pressure tested.