sizing ducts: circular to rectangular
Taylor
| Posted in Energy, Heating & Insulation on
I’d like to try replacing circular flex duct installed by HVAC with solid duct where I can. Where ducts go through 2×6 joist bays in attic, I’d like to use the rectangular duct designed for walls. But I assume I can’t just take the CFM for the circular duct and match that with the area of the duct, since there will be friction on the corners of the rectangular duct. Any advice on how to compute CFM for rectangular duct as it relates to circular? Any pointers on how to do these calculations, period? So far I’ve only been able to find this.
As I’ve said, the pros have already been in, I’m just trying to fix, er, improve their work.
Replies
Because someone took money to perform some work DOES NOT make them professionals. Flexible ductwork can be a great time and cost saving item, and can be used in a manner that does not compromise the operation of the system. However, it is seldom used/installed properly.
See the referenced story:
Thanks for the info. FWIW I chose this crew because they have the best reputation for retrofitting AC into old houses, and they seem to have done a reasonable job.But I was surprised when they put in flex duct, even in the attic where the major duct runs are. There are no duct hangers anywhere, it just sits on the joists. In some places it's pulled through some 2x4 stud bays, I'm not sure it's supposed to be pulled that way, but what can you do....It was my idea to run ducts in the joist bays in the attic, thinking to dump insulation on the ducts later. In retrospect, the T&G plank floor is all chewed up, the ducts are flattened out, there's no room for insulation, probably ought to just insulate the whole attic.....etc. In any case, I'd like to try to retrofit rigid duct where I can....