Energy efficiency: get your ducts in a row

Getting ready for the summer? Want to be more comfortable and save some money? Then pay attention. A good percentage (up to 30% by some estimates) of energy for heating and cooling homes is lost through leaky ducts.

The California Energy Commission rightly figures that connecting a high efficiency furnace to leaky ducts is like ordering a diet drink along with an ice-cream sundae. It helps, but not much. So the commission in 2006 began requiring that contractors who install furnaces or air conditioners in homes check the home’s ducts first.

But what if you don’t want or need a new furnace or air conditioner and want to do what you can right now to save energy, money, and the Home Planet? Got a coat hanger and a garbage bag?

Many people in the building performance field know about a certain, easy-to-perform air flow test that involves common household items. The Canada Mortgage and Housing Corporation (CMHC) began publicizing the Garbage Bag Air Flow test in the early ’90s and tested it in the field. One of the first uses of the garbage bag test was for someone in Canada who was getting no heat on the second floor in his new house. Several contractors tried to solve the problem with no success. The man’s furnace was oversized, as is true in most new housing, so there should have been no shortage of heat. The garbage bag test showed that only 2 of his 18 supply registers had flows greater than 10 cubic feet per (CFM); as my mother used to say, this is slower than molasses in January moving uphill in a snowstorm. The Canadian homeowner called a contractor to fix the problem and now his whole house is warm in the winter and cool in the summer and his utility bills are not outrageous.

Performing the Test

To perform the test, you'll need:

• a garbage bag
• a wire coat hanger
• adhesive tape
• a watch

In its literature, CMHC specifies using a Glad 66 cm x 91 cm garbage bag. When your bag is laid out flat, it should approximate that bag's dimensions: 26 inches by 36 inches (about 2 ft x 3 ft). Easy enough to measure, or even approximate.

Bend the coat hanger into a circle, square, or rectangle big enough to fit completely around the heating and cooling supply registers in your home. Fit the opening of the garbage bag around the coat hanger and tape it in place (you don’t have to make it tight, just so it stays on the hanger). Now you have the necessary technology. The biggest challenge is avoiding poking a hole in the garbage bag with the coat hanger. The blogger welcomes any tips regarding this point.

Now, test your duct system. Turn on your air conditioner or heater. Put the completely deflated garbage bag rig that you’ve created over a supply register. use the watch to time how long it takes for the bag to fully inflate. Or, put the fully inflated garbage bag rig over a return register and time how long it takes for the bag to deflate.

The visual nature of the test makes it easy to interpret. A 2-second inflation (75 CFM) is a healthy, robust flow. An inflation of less than 10 CFM (roughly 15 seconds) is almost comically slow, and you better call a contractor to diagnose your problem and fix it.

For the more precise among you, CMHC came up with a table that more accurately determines the flow rate at each register:

The relative performance of several supply registers is obvious even without a watch. You can get a good idea of the performance of all of the ducts in your house, usually within ten minutes. And you can keep one plastic garbage bag out of a landfill and give it green employment.

(And by the way, never use duct tape to seal your ducts. It tends to dry up and fall off over time. Use mastic or foil-based tape. But that’s another blog.)

Jim Gunshinan is Managing Editor of Home Energy Magazine. He holds an M.S. in Bioengineering from Pennsylvania State University, State College, Pennsylvania, and a Master of Divinity (MDiv) degree from University of Notre Dame.