I am replacing a bathroom exhaust fan. The roof vent has a 4 in. duct. The new fan has a 6 in. outlet. I plan on using two reducers, first to get to 5 in. then a run of duct and another reducer to get to 4 in. The length of the duct run is about 12 feet. Is this reasonable?
Also, the old fan had a 180 degree turn of the duct immediately out of the fan. Would this cause airflow issues?

Thanks!

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The new fan has a 6 in. outlet.

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That seems awfully big for a bath exhaust fan. What is the make and model number of this fan.

Any turn reduces the air flow. Why would old fan have 180 degree turn? Why wouldn't fan exhaust be turned to face the direction of flow?

Thanks for your reply. The fan is a Panasonic FV-30VQ3 rated at 290 cfm. I can't explain why the old fan wasn't turned around. It was originally installed that way and might help explain why there's a moisture problem in the bathroom. I plan on coming out of the 6 in outlet straight for 2 or 3 feet before reducing to 5 in. then two more feet before reducing to 4 in.

I would eliminate all turn possible and keep size as large as possible, as long as possible. Where you reduce probably won't make any difference as far as air flow is concerned. You will not get the 290 CFM because of the reduction to 4" but probably won't make that much difference. The further away the reduction is the quieter the fan will run. The 180 degree turn would create a significant reduction in air flow. At 290 CFM you should be changing the air in the room about every two minutes, provide fresh air can be drawn into the room. Can't pull air out of a room unless make up air can be drawn in.