BECT's Duct Test and Sealing Video

BECT's video at http://bect.us/contractors/duct-test-and-sealing-video/ demonstrates how to get your HVAC system to pass a code duct test. It'll show you how to seal the connections on any HVAC system. When you understand that, you can seal all the connections from the furnace to the plenum to the start collars to the ducts to the boots to the interior of the house. When you do that there is not much room for the system to leak air.

You'll need the right supplies first. California Code requires that all supplies in the duct system must be marked UL 181. At any big-box home supply store you can buy air duct mastic, tape (Separate types for flex duct and metal to metal connections),  big zip ties, a disposable paint brush and disposable gloves. You'll also need a zip tie tensioning tool.  

Tape alone won't get you as far as you need go to get ducts sealed properly. The best technique is to apply a thick coat of air duct mastic over every single connection. This will seal up any imperfections.

The day before installation, paint all the metal "wyes", start collars and plenums with mastic. This will create solid connections where air is less likely to leak. When you connect the ducts to the start collars and boots make sure to use a hand tensioner tool and get the connections tight. There is an inner and an outer lining to flex ducts, make sure that you zip tie and tape both layers. Then, on the outside layer, seal around the whole connection with air duct mastic.

Be comprehensive about the this process. It is much easier to get it done right the first time rather than staring at a seemingly sealed duct system and wonder where it is leaking. I've spent too much unpleasant time with with contractors standing there telling me that "It can't be leaking!" when the manometer tells us otherwise.

Your work will pay off. All the technical reports I've read about well-sealed duct systems say that they just work better, are more energy efficient, stay cleaner and need less maintenance than leaky duct systems. While new duct systems can get away with 6% leakage in California, crews who compete with others in their own company routinely get down to 3%. There are some elite HVAC companies who call themselves Home Performance Pros who make it a matter of pride to get their ducts so tight and well constructed that the test equipment we use can't even register the duct leakage, it just says, "low". "Low", that is something to shoot for when constructing a new duct system. Go for it.