why are flex or round duct not permitted sometimes? Post date Home › Forums › Materials, Trades › Mechanical, HVAC › why are flex or round duct not permitted sometimes? This topic has 0 replies, 1 voice, and was last updated 5 years ago by atuter. Viewing 1 post (of 1 total) Author Posts August 31, 2018 at 4:23 pm #244 atuterKeymaster what is the reason that flexible or round duct are not permitted sometimes ? Author Posts Viewing 1 post (of 1 total) You must be logged in to reply to this topic. Log In Username: Password: Keep me signed in Log In → Earthwork