Clearly the new fan is just blowing the moisture into the other bathroom. The least complicated would be a separate duct.

Yes, you can only combine ducts if you are pulling the air (the fan is located at the wall/roof termination), not pushing the air (the fan is located at each bathroom ceiling).

I would run new separate ducts, and I prefer timers, I put them in every bathroom.

Definitely separate ducts.

Get a humidistat/timer that will turn on when it detects steam and run for X minutes. You need this feature for teenagers that always forget to turn the fan on until the bathroom resembles a steam room!

Agreed about the teenagers. And to add fuel to the fire, the old exhaust had a heat lamp feature that I'm sure was used instead of the exhaust.

I have a wonderful fan set up with two vents (tho both in the same large bath) vented to and joined in the attic. The motor that runs both is placed in the attic, so there is virtually no sound. Both vents have lights but run on the same timer. Look at Fantech. The fans are so powerful I have used them on summer evenings to draw cooler air into the house.

I think better fans might be the answer and who uses these bathrooms? Kids never turn on fans in my experience . As for the timers I always use them just to make sure

"Better fans" is not a real answer. No matter if the fan is 80cfm or 300cfm, high or low sones, timered or not, shiny or dull, the fan will be limited by the makeup air it can pull under the door when it is closed. A typical 30" door with a 1" gap at the bottom can allow from somewhere around 40cfm-60cfm.

Is there possibly a type of fan that will pull more of the air from under the door? The problem bathroom is 5x8, has no window, a typical 30" door, and is used by "kids" (20-24).

And now another problem has come up. The 20 year old fan we are replacing is a huge 17x19. Pretty sure they don't make those monsters anymore. That'll be a hell of a ceiling patch job.

We built a house in 2021 and ended up moving/adding some thing, so we had some holes in walls/ceilings. A good drywall/plaster guy or a very good painter won't have any issues patching a ceiling hole. You will want to repaint the whole ceiling. I've used Benjamin Moore Aura Bath & Spa paint for bathrooms and I think it is worth the extra cost.

In general, whatever passes through a hole is restricted by the hole size.

To minimize moisture causing problems:

- Have an adequately sized fan (yes even larger cfm than the underdoor gap allows)

- Have at least a 1" gap under the door

- Have HVAC supply serving the bathroom

- Locate it next to the shower

- Have the ductwork be as straight as possible, using smooth sheetmetal

- Start it immediately upon entering the bathroom

- Open the door during or after showers

- Let the fan run for 20-30 minutes after the shower

To answer an unanswered question, one can find blowers of a desired flow rate for a given pressure loss. They tend to be axial, and potentially noisy, and may want to run on 400 Hz, 3 phase power. Aerospace blowers used on aircraft and aircraft pods for equipment cooling are of this type.

Better is to make the door gap larger (as noted above) or put a louver assembly in the door, slot oriented for maintaining privacy.

All of this info is very mch appreciated! Thank you all so much!