no, you aren't overthinking it. when i was doing my fan i did some reading an apparently 3" duct can olny handle so much air, so when you adapat a larger fan you often get backpressure because its pushing air in faster than it can go out.

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as for the changing it the vent it can be super easy or impossible depending on your house and how the vent pipe is routed because 4" vent isn't going to fit in a 2x4 framed wall cavity so if it's a 2nd floor bathroon with attic access thats easy, but if its a 1st floor bathroom that vents through the roof that's a different story

Make sure the exterior vent is not clogged with leaves or a bird nest.

HVAC engineer here.

We use 4" duct to move up to 35 cfm or so. If your fan is strong (most restroom fans are not) it could do more but will be loud.

My sizing criteria puts 3" duct at around 15 cfm for normal design conditions.

Your fan probably claims to be able to move something like 65-100 cfm. 4" duct works, but not well for long runs.

This doesn't help solve your problem, but it might illustrate why 3" isn't working.

Where is it located, exterior wall access where you could punch through? What part of the world are you in? Fart fans are not made for static pressure so when you shove a bunch of air into a small 3in duct it can't push it. It would be best to have a decent fan and 4in duct but if that's not possible downsize the fan. What fan did you install? What size is the bathroom?

If you want to use the current ducting, you need a fan that moves less air. Code-wise, you are not allowed to reduce the duct size coming out of the fixture.

If the bathroom is on an exterior wall and the joists are going perpendicular to it, you could vent out the exterior wall.

Thanks for your response, makes me feel better that some back pressure is expected.

I’ve done my bathrooms upstairs which was fairly easy but this time unfortunately it’s the downstairs bathroom. It does vent up through the roof so I unless I open up walls I guess this will have to do.

My fan does claim to move 50-100, it’s a Panasonic one with options for 50-80-100.

If a 4” duct is good for moving up to 35 then should I just set it to 50cfm? Why would it have the option to go all the way to 100? Marketing to make it sound good?

If someone wanted to upgrade the size of the duct in my situation with no attic access, is the only option to open up walls?

Good idea I’ll go take a look tomorrow, lots of trees around and my gutters are always full of them, thanks!

Oh really? I’ll take a look online for some that fit my current ducting. So the adapter I put on it is not meant for this type of application?

Would you know the concern for reducing the size of the duct?

The odd thing is that some of these exhaust fans come with the adapter.

Bathroom is located in a corner on the main level of a two level house. It has close access to 2 exterior walls that could potentially be vented through. That would be a good option and I would just not use the existing ducting.

I’m in the Pacific Northwest, in Vancouver.

It’s a Panasonic one with options for 50-80-100 cfm. Bathroom isn’t very big, about 50 sq. Ft.

Not sure why the fan would come with the 3" to 4" adapter unless the fan output 3" and you were attaching to a 4" duct. Or it is a fan with adjustable CFM settings, one of which was low enough for a 3" duct.

The concern is that you will overwork the fan motor and shorten it's life. You also get the problem you described.

When I installed exhaust ducting in my parents attic I used 6" so the full 100 cfm wouldn't be restricted.

4" is good to use in some instances where firewalls are present. They don't need special protection because they're so small.

If you don't have an easy path to outside from that bathroom, you may need to open up a wall and investigate running an oval or rectangle duct in it to get full airflow.

Otherwise yeah, run the fan at 50 cfm so it's not trying too hard for what the ducting can allow. You could lower the lifespan of the motor, or you're just wasting energy.

The fan output is 4”, I wish I had the opposite problem but it does seem to be a 4” to 3” reducer. Here’s the official Panasonic one;

https://na.panasonic.com/ca/home-building-solutions/ventilation-indoor-air-quality/ventilation-accessories/4-oval-3-round-duct-adapter-whispervaluetm-fan-and-fanlight-combinations

It does have adjustable settings so I’ve set it to 50cfm but has options for 80 and 100. Honestly even on the 50 setting the noise level seems almost the same as the other settings.

So, your fan isn't coming with a reducer like I was thinking. It comes with an adapter to go from a 4" oval to 3" round duct. The area of the both sides is probably very similar but without knowing the width or height of the oval, I can't be certain. This fan is designed to work with this adapter.

Your air blowback could be due to duct blockage or otherwise reduced throughput. Typically, there are maximum duct lengths and number of elbow (bends) allowed before throughput is reduced. Without putting eyes on your ducting, you've really no way to know. I don't know if HVAC companies will scope your duct or not, but I'm fairly certain they can clean them.