Ductwork Questions, Straight Answers
Flex duct vs hard pipe — when is flex actually fine?
Short, tight, fully stretched runs from trunk to boot — flex is fine and fast. Long snaking runs, sag between joists, or compressed bends are where flex gets its bad reputation. The material isn't the problem; the installation usually is.
Why is there water dripping off my duct in the crawlspace?
Condensation — the duct surface is below the dew point of humid crawl air. Fixes in order: seal duct leaks (leaking cold air chills the jacket), upgrade to R8 insulated duct, and address crawlspace moisture itself. Wrapping wet R4 in more plastic is not a fix.
What's the rule of thumb for duct sizing per ton?
The classic is 400 CFM per ton and roughly 0.1 in/100ft friction rate for residential — as a starting point, not a design. Real answer comes from Manual D against measured static. Most 'undersized equipment' complaints are actually duct systems choking a correctly sized unit.
Can I use flex duct for return air?
Code-wise usually yes; performance-wise it's where flex sins hurt most — returns run at higher friction and every sag costs you. If you must, go big on diameter, keep it short and tight. Hard pipe returns are the difference customers can hear.
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