[ddawg16]

As Bud mentioned, how much air can get in? Have you tried leaving the door partially open to see if it pulls the air out better?

Think about it....I've seen a lot of bathroom doors with a gap of less than 1/2". At 1/2" and a 24" wide door...that is at best 12" sq inches.

A 4" duct has an area of a little over 12 sq in. With the space under the door, the fan is being starved for air. Open up the door a bit and airflow will improve.

 

[ddawg16]

Have you gone up on the roof and checked the air coming out the exhaust? Are you sure you don't have a kink somewhere?

Mine uses a 4" duct...short run of metal flex and then smooth metal duct.

 

[ddawg16]

I have 2 lights in my bathroom. One is the vent vent fan/light combination. The other is the light over the sink.

The vanity light is slaved off the vent fan light. I'm also using a motion sensor for the vent fan. (the people on my house have not figured out how to turn off lights)

Hence, when you go in the bathroom....you turn on the light....that turns on the fan...but it's quiet enough you don't notice it.