I know that I have talked about this before, but below is an excellent photo of how dryer flex ducting should not be used. Okay, let us be practical. People like convenience and using a short little section of this accordion style flex duct right behind the dryer is par for the course even at new homes. They are known as "flex transition ducts" and they are allowed by codes but with some stipulations: They cannot be concealed in construction (crawl spaces, attics, going through walls and floors) and the maximum length must not exceed 8 ft. Even for these short runs the semi-rigid ducting is better, just do not crush it. The ducting in the concealed space must be, again per code, smooth interior ducting with joints in the direction of the airflow and with zero screws to catch lint. How long the smooth ducting can be depends on the number of elbows and the manufacturer's instructions. The duct above, situated in the crawl space, is more problematic than most similar crawl space ducting. In this case, the duct is taking so many twists and turns that, no doubt about it, it is a potential fire hazard and it can be hard on the appliance above if the vent is, essentially, restricted. The other thing with this flex duct is that it tends to tear or come apart. Therefore, before long, you end up with the dryer spewing into the crawl space. Since we are setting the record straight here, not only should ducting under the house be smooth metal, it should, also, have R4 insulation around it. That reduces the amount of condensation that forms in the ducting when warm air is running through the duct in the unconditioned crawl space. Thanks for stopping by, Steven L. Smith
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