I tore out the old bathroom fan unit today and discovered a new problem: hard metal ductwork that is 3" in diameter. The connector is 4" in diameter. It ends just at the edge of the hole.

I cannot just put the new plastic connector around the old and tape it because it has a flap inside that won't open/close the vent when the fan is on in that configuration. The 3" duct blocks that flap.

I cannot easily just replace all of the ductwork since it appears I'd be up on the roof - see the photo of the pipe exiting the attic interior.

The metal is heavy duty rigid stuff, so cutting it seems to be a lost cause, too.

Is it time to call in a pro? Or is there a workaround within my reach? (Move the hole 5-6" towards the tub so I can install an adaptor, then patch the gap in the ceiling?) Right now, the tub-side edge of the hole is 12" from the outside of the tub area.

Photos are here .

If it were mine I'd cut the ducting approx 6" just beneath the roof sheathing using a hacksaw. Then connect a a 3" to 4" adapter to the stub using short sheet metal screws and aluminum tape. Then connect a run of 4" insulated flex ducting to your fan.

You definatly want the ducting to be insulated to prevent condensation from forming in the ducting before the steam can make it up and out. By looking at your current ducting it seams to be a bit rusted and water spotted indicating a condensation problem already. I'd also buy a small piece of insulation to wrap around the metal adapater at the roof sheathing and then cover it with 3 mil plastic to blend in with the ducting.

One really important installation tip is to make sure you don't have any droops in the ducting once installed. The straightest path allows for the least resistance and no place for any condensation to pool.

Sending me into the dreaded attic, are you? <grin> I was so hoping to avoid that.... but if it's what I have to do, I'll do it. Thanks Dora. I'm still going to cross my fingers that someone else has a non-attic adventure solution instead, though it may be a futile dream.

Dora touched on my solution, just add a 4" to 3" adaptor to your existing ductwork. You should be able to do that without going into the attic, assuming you can get some flex in that old metal pipe. You'll probably have to cut some of it off to fit in the adaptor.

Be advised that the new fan will not be as quiet as it could be if you use the old 3" duct. The 4" outlet reduces air friction and keeps the fan noise down compared to the 3".

Let us know what works out.

Bob.

Success!
I got the old duct out (not intentionally) and replaced it with a new one of the same diameter, connected to a flex elbow and reducer. The unit is in and working. It only took 2 trips to the hardware store, too! I didn't have to go to the attic, though it did feel like the attic was coming down on me at times. I hate blown-in insulation!

I still have to patch the hole in the drywall around the box in the ceiling. Getting the old unit out was tricky and I had to expand the hole on one end. Right now, the hole is stuffed with an old rag to keep the insulation in the attic.

Is caulk the best way to seal around the edges of the box unit? There is a small (1/8 ") gap on three sides. Leaving that open doesn't make sense.

Thank you all for the help. I have a working light and fan again, with the bonus of a heater, too.

Glad to see you got it done, however...


Are you sure your circuit can handel that? A heater draws alot of power. Usually a dedicated circuit is required. According to the code a perminate applaince can't exceed 50% of a circuit's rating where the circuit also serves lighting or receptacles.

It's on a 20 amp dedicated circuit, per the directions that came with the unit. Thanks for checking.