duct work for bathroom exhaust fans

Hello, we just got some quotes to replace two existing exhaust fans and convert the canned lighting in the shower to a combo light/exhaust fan.

One company isn't licensed to run the duct work (for something that simple, I was surprised it required a licensed HVAC person), and the others could do both electrical and "hvac" work.

If we end up going with the company that isn't licensed to to the duct work, I plan on doing it myself. We have a walk-in attic with easy access to everything.

So, my questions about this are:

1. It seems most modern exhaust fans use a 4" connection. The current ones are 3". Can I buy a "reducer" of some type to connect the 4" duct work run to the existing 3" vent (the vents attached to the outside of the house)?

2. To save cost, I plan on buying a "Y-connector" to tie two of the existing vents together (that I would connect the existing exhaust fans to). Correct me if I'm wrong, but I don't see a problem with this since the air can only blow one way (as opposed to using a T-connection, which could allow some of the air to blow into the other room).

3. Where can I buy flexible duct work at? I checked Lowes but they only had really short runs (6' max). I need 30' and 11'.

Thanks,

Andy

 

You would not reduce, as that would restrict airflow... unless the mfg instructions indicate otherwise. Combining 2 vents should only be done if the mfg instructions say it's okay. You have to check their specs and instructions. And most flex vents are not allowed for hvac ducting, it must be solid pipe.

 

Thanks XSleeper. So, even if the existing duct work is the cheap, flexible piping, I can't install flexible piping?

 

Well, thats a gray area. But you might consider upgrading it.

 

If the flexible piping is to be run horizontally in an unheated attic space make sure that the flex pipe is held level with no "up and downs" which could cause the pipe to be restricted or totally plugged if the moisture laden air condenses. I have used 4" PVC drain or even schedule 35 or 40 and flex pipe at both ends for bathroom vent lines. Run the lines separately not connected together using a Y or T. Either fan running separately could back feed into the other room when 1 fan is running and the single 4" pipe may not carry all the exhaust air from 2 vent fans. Depending on the type of flexible vent you need I have seen them up to 20 Feet long and you can buy couplers to connect more lengths together. I would that you keep the vent pipes as short as possible since most of the less expensive vent fans "vent capacity" will be diminished with the use of long pipe runs.

 

In my area it is common to run insulated flexible duct for bath fans. Only on fan per vent as one could back feed into another. Lowe's carries what you need.

https://www.lowes.com/pd/IMPERIAL-4-...-Duct/50401790

 

Thanks for the info Steamboy and Ironhand. This house was built in 2001 and the builder put in really cheap flexible piping (white plastic basically). I'm debating whether or not to put in insulated piping or just the aluminum piping. Never had any issues in the attic.

Guess I won't do the Y pipe after all based on what everyone has said.

Thanks for the info!

Andy

 

No matter what type of vent you use, if it goes through an unconditioned space, it should be insulated.

I have seen that vinyl white duct disintegrate in an attic after about 15 years. Just food for thought.

 

OK, guess I'll go with the insulated duct work then. Thanks for the info.

Andy