HVAC installers and I have a little game of hide-and-seek that we play. Of course, there are the obvious problems I find in duct systems, like the ductopus, ducts installed right up against the roof deck, and duct leakage galore. But there's one duct problem that I have to hunt for. When I go into an attic for a big, spread out house that's served by one system, I look for long runs of flex duct. Flex duct, you see, comes in bags that contain sections 25 feet long or less. Houses can be 100 feet wide sometimes, so this can lead to the object of my search.