I like flex duct. Really! In fact, I did an interview yesterday for the Energy Saving in the Home Radio Show and surprised the host, I think, when I said that to him. He's an HVAC contractor himself and is used to building science types being purists on the use of rigid ducts. (You can catch the show when it airs on Saturday, 10/20/12, or download it from iTunes afterward.) What I don't like is that flex duct gets horribly abused by a lot of installers. If you've been reading this blog for a while, you've seen lots of evidence already.
As I was looking through old photos the other day, I came across the one above. Do you see what the problem is here? Those deep sags will kill air flow. Flex duct has an inner liner made of plastic and a wire spiral. When it's not pulled tight, the inside surface has a very high air resistance. High air resistance means reduced air flow.
Another problem is that the sag means that the air doesn't flow in a straight line, as it should in this case. It goes up. It goes down. It gets tired and poops out quickly. The rooms don't get as much air as they should.
Yet a third problem here, albeit one that's a bit hard to see, is that the one support you can see in each of these ducts is a piece of wire. If you're hanging rigid ducts (hard pipe), wire is suitable. With flex, it's not. Wire has a greater tendency to constrict the size of the duct at the point of support and create an extra bump that the air has to get past. To support flex duct properly, use straps that are at least 1.5 inches wide. The photo at right shows such a strap.
Here are three things to look for so you can see if your flex ducts look OK:
- Flex duct should pulled tight to reduce inner air resistance.
- It should be supported with straps that are a minimum 1.5" wide.
- The straps need to be spaced no further than 5 feet apart, but closer is better. Local codes or manufacturers instructions may specify shorter intervals.
Flex duct can work well when it's sized and installed properly. It's a versatile product, which is both a blessing and a curse. I wish I could say that it's usually done right, but the truth is the opposite.
Now go check your flex and see if the opportunity for enhanced comfort and efficiency awaits.