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Our new-to-us home was built in 1850, so there's been a lot of oddball things done to it's guts over time. One of these is the dryer vent duct work that looks to have been installed sometime in the 1970's with an in-duct booster fan that died who-knows-how-long-ago and was alarmingly full of lint when we opened things up recently. The fan was very clearly meant for other kinds of ducting, not dryers. The duct that runs through the house to the outside is 7" in diameter. But of course dryer vents are only 4", so there's a huge adapter (a reducer installed backwards) that extends from the wall to allow the 4" vent to connect to the wall and the 7" duct.

Going forward, here's the question: can we install a 4" dryer duct fan into the 7" duct? Logically, it seems like it might not function well to use a fan designed for a smaller diameter in a larger duct. It's not a long run of 7" duct; only about 4 feet to the outside of the house, but the weird configuration of the laundry room means more angled duct work from the dryer itself to the wall port before reaching the fan. I don't want to install a product not designed for the purpose (ie using a regular HVAC booster fan instead of one built for a dryer, like was done before) but am having trouble finding anything built for dryers that is anything but 4"!

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    Why would you replace the fan if it doesn't seem necessary? I'd leave it out and consider re-ducting at 4". The 7" duct has more than twice the cross-sectional area, and therefore the air speed is much lower. This could result in more lint buildup. Maybe revise to describe the entire duct path (or post a link to a photo). – isherwood Jan 15 at 18:53
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    You mean more than 3x the cross-sectional area! – mike65535 Jan 15 at 18:57
  • The current fan is dead. Probably many many years dead. Really hoping not to have to demo my walls just to redo ducting! – Adrienne Jan 15 at 19:15
  • I'd remove the fan and see how the dryer performs. – isherwood Jan 15 at 19:34

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