Is the loose aluminum slinky style even allowed in your area? It isn't permitted in mine.

Why is a plumber installing duct work?

I have no idea if loose slinky style ducts are permitted...but it was installed by a licensed master plumber who installs a lot of heating systems. I assumed he would know what he was doing...

The only way to know is to test the CFM with a meter. You should get the near rated CFM for a straight horizontal run of duct.

I have a 115 CFM Panasonic fan in my bathroom that, like you, is a straight horizontal run out to the wall, about 4 feet, through 4" rigid duct. I installed it myself and put a louvered vent cover on the outside wall. I thought it was working well until I brought an energy audit guy in and he only measured 60 CFM. The fan seemed to be working fine so I went outside and took off the vent cover. The CFM jumped to 112.

Regardless of your ducting, make sure your vent cover outside is not hindering the air flow. My old vent cover was fine mesh with louvered metal slots. I replaced it with 1/4" mesh wire and nothing else. I have yet to find a permanent cover, but the fan now works really well and is even more quiet.

"but it was installed by a licensed master plumber who installs a lot of heating systems."

Plumbers install boilers, and the related piping. Not duct work.

Now's the time to get it right.

How long is the run for those ducts? Are they actually "slinky" like the old fashioned plastic dryer ducts that would collapse over time, or do you mean the flexible aluminum ones that "telescope", but otherwise are quite rigid? Those work fine for reasonable distances.

This post was edited by homebound on Wed, Jun 25, 14 at 21:31

Depending on the geometry of your duct run, sags may well become traps to collect condensation.

Solid metal ducts with appropriate pitch is what I used.

There is definitely a reduction in air flow flex to smooth wall. As well, the system will likely be quieter with Ridgid pipe. Is there an attic above this? If so it can be accessed and changed from there. If it is between floors and requires the removal of the Sheetrock, you have to decide if it is worth doing. It is not an enormous job...before the taping is done on the Sheetrock. After, a whole new ball game. Flex is an option for these fans, but there should not be sags in the line. You will be pumping saturated air into this piping and it may condense and create water pockets in the flex.

I just put two of the Panasonics in our new bathrooms last year. They are definitely the best fans. I ran 4" Ridgid pipe on mine graded up all the way. One was roof terminated and one wall.