Flex duct is a pretty cool product, and it is used in Florida a lot.
It comes pre-insulated, it is cheap, and it is flexible, which are pretty important for the way that we build houses here in Northeast Florida.
But have you ever lived in a house where one room was uncomfortably hotter or colder than other rooms? Sure, a bigger system could maybe fix that problem, but that is a pretty expensive solution.
It is more likely that you have plenty of cooling power to cool the air and a problem lies with your air delivery system (aka ductwork).
If you have been around new construction sites or been up in your attic, it is not uncommon to see flex ducts similar to the picture shown below.
Your HVAC contractor has a lot going against him when he gets to a job site. Often framing, plumbing and electrical are already done and he has to just “make it work.”
But let’s dive into a little building science to see the consequences of “making it work.”
ASHRAE (American Society of Refrigeration and Air Conditioning Engineers) has an eye-opening chart (at left).
Duct systems work because of pressure. Your AHU fan increases the pressure in your duct system, which forces conditioned air through ducts and into each room.
The pressure overcomes air friction in the ducts. The more friction or restriction in a duct the more pressure is required to deliver the same airflow.
“Pressure drop” is a measure of that friction in a duct. Flex duct should be installed “fully extended,” which means if you have a 25-foot long piece of flex duct and you installed it in a 20-foot space (without cutting off the extra five feet) you have “compressed” that duct lengthwise by five feet, or by 25 percent.
The chart shows what happens to our pressure drop when we compress our flex ducts in this way. For example, take an eight-inch flex duct that is 25 feet long (which is the length that is typically sold by the manufacturers) and install it between two points that are 22.5 feet apart.
We have compressed the duct length wise by 10 percent. By using the chart, we see that the pressure drop increases by three times.
Wait … what? The pressure drop triples? That’s right.
Makes a lot more sense as to why your master bedroom in the back of the house is a little warm with your AHU all the way in the garage.
This is minor compared to the photo (at right) where the HVAC contractor “made it work.”
The moral of the story: Pay attention to the installation of your flexible ducts and consider using more rigid materials such as metal or fiberglass duct board, which cannot be compressed.
Flex duct can be installed correctly to where you don’t have air flow friction issues, but it far easier to install it poorly and “make it work.”