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I am upgrading an undersized bathroom fan that is 50 CFM and has (possibly) a 3" to 4" exhaust pipe. The new fan, which is 150 CFM requires a 6" exhaust pipe. Do I need to rip out the existing vent pipe and replace everything with 6" (including the roof vent) or can I get by putting a vent reducer (from 6" to 4")?

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    Is the motivation for this change more airflow, or a different motor/fan for quietness? – Criggie Oct 1 '18 at 22:42
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    More Airflow for the room. – Dan Pollack Oct 2 '18 at 18:52
  • Then shoving more air through the same pipe will be louder. Upsize everything. – Criggie Oct 2 '18 at 23:23
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You can install a reducer, but you'll cut your flow volume by an equal proportion:

28.3in2 - 7.1in2 = 21.2in2 (an area reduction of 75% when going from 6" to 3" duct)

This will negate a significant amount of your fan upgrade, will make it work harder, and may shorten the motor's life due to reduced cooling.

You might ask yourself whether you really need 150 cfm (and the accompanying noise).

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    This answer could be improved by explaining what are the 28.3" and 7.1" are, respectively. Also if the existing vent is 4", the reduction is "only" 55%. – stannius Oct 1 '18 at 18:58
  • I mean, it's clear in that it's unambigious if you take the time to reason it out. It's not clear in that you have to think for a couple seconds to determine what measurement means what, especially if you're not familiar with the area formula. If you 'showed your work' on the cross section reduction it would be a more complete and easier to parse answer for visitors. – Adonalsium Oct 1 '18 at 19:26
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    Believe it or not, I bet the 150CFM fan is quieter than the 50CFM it's replacing. I ripped out a shoddy 50 in my house (4 sone) and replaced it with a 90CFM that's so quiet (0.8 sone) it's easy to forget it's on. – Machavity Oct 1 '18 at 20:24
  • Yeah, I redid my vents too and the 50CFM ones were all loud and the 130 CFM is the quietest (1.5 sones). – tk421 Oct 2 '18 at 16:51
  • Looks like I'll need a larger vent for the roof. The bathroom is pretty large and the current fan is about 1/3 the size that is needed. The builders cheaped out on the fans. – Dan Pollack Oct 2 '18 at 19:00
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Let's back up here for just a second and note something

You almost certainly don't need a 150CFM fan.

So your original fan was a stock 50CFM (probably NuTone or one of its predecessors) that's a 4-sone fan, meaning it sounds like it's been cleared for takeoff when you flip that switch. So you went out and bought this bad boy and though "I'll solve this right now!". I bet it cost you a lot as well (that's one hefty motor there).

Anything 100CFM or more will have a 6 inch vent, but you you need to look at the size of the room. About 1 CFM per square foot should do the trick. Since it's rare to have a bathroom over 100sq. ft., you're building overkill. Downsize to a 70-90 CFM fan. You'll probably save money over the old fan and it will have a 4" vent.

Make sure your exhaust line is insulated as well! A lot of older fans had bare metal ducting.

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    A lot them are run in crappy 4" plastic flexible. That's why they don't work. – Mazura Oct 1 '18 at 22:42
  • And while we're giving advice... Do yourself a favor and buy a Panasonic. Enjoy the silence. – Tom Hundt Oct 2 '18 at 3:38
  • I actually went up into the attic and found out the lazy builder just slapped in a 2" pipe in there. Decided to re-pipe with a 6" diameter pipe and accompanying roof cap. 150CFM isnt overkill for the size bathroom we have (master bath). Based on the calculations, it actually came out to requiring a 150CFM fan. I live in FL, so the air can get pretty damp in the summer time, and that side of the house doesn't get adequate airflow from the HVAC fan (in my opinion). – Dan Pollack Oct 10 '18 at 21:02

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