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Joined: May 2003
Posts: 1,158
I went on a service call today to replace a bathroom exhaust fan.What i found was amazing.

The worse hack work i have ever seen for a ventilation system.
These people have respiratory Issues.
They have the air tank with hose they walk around with.
Our POCO sponsors a power smart program which is contracted out.
10 years ago these people spent the money to use the existing bathroom vent fan opening .
What the so called contractor did was wired a timer and a humidistat into an in-line exhaust fan that they mounted in the Attic.

The duct work was horrible the fan was mounted on a truss brace with flexible ducting .

the intake side is 4 inch insulated plastic flex duct . the exhaust side of the fan was 6 in insulated plastic flex duct.

What they did was run the 6 inch stuff about 12 ft away from the fan unit and it was placed between the truss support and because it was flexible as soon as the moisture started to gather in the duct the weight of the water just made all the ductwork sagged between the truss supports this fan could not exhaust any of the moist air to the outside as the duct was full of water.

The inline fan was full of water and all the exsiting ductwork is contaminated with mould.

I feel the whole system needs replacing. This is a side by side duplex the bathroom is on the same wall the divides the 2 units and it is about an 15 ft or so run out to the gable end for the vent cover.

Now should this replacement systen still run horizontal or is it better to have it go staight up and though the roof.

I fell bad for these people as they piad good money from a sponsered program but this system could never of worked properly .

other question is to do this properly how many hours with new duct work and a fan. the wiring seems to be fine.

this link is for referance only

Thanks in Advance

Last edited by dougwells; 03/11/08 05:53 PM.
Horizontal Ad
Joined: Jun 2001
Posts: 161
use rigid metal duct all around and you can go either direction. Usually out a sidewall is easiest. The only limit is imposed by the fan manufacturer. hth

Mike Wescoatt
Joined: May 2006
Posts: 20
In the atic you will also want to use insulated duct. you do not want the exhaust air to get cold until it is outside otherwise you will have condensation in the new pipe. Check the manufacturer's directions if you are using an in line fan you may need to connect it with flex because of noise / vibration. There is nothing wrong with flex, I have used it with your problems. Flex has a bad rap because it and shoddy workmanship go so well together.

Joined: Nov 2005
Posts: 141

I made the mistake of using that flex duct stuff ONCE. I installed it in October of 1984 back when I was still in Winnipeg and I was back a month later because the HO complained (and rightly so) that the fan "gurgled" when it was turned on. We had just gone through a week of minus 30C weather and it had obviously collected condensation which froze up when the fan was off because I had taken the easy way out and run it out the side of the house and had a few feet of semi-horizontal run (about 3 feet of diagonal)

I wound up doing something completely different and used 4" white PVC going straight up through the roof with a really good vent cover at the top of the roof that closed up when the fan was off. I also wrapped the PVC with about 6 inches of fibreglass batt insulation to try and stop the condensation issue but it still dripped a bit of water when the fan came on the next time.

I wound up wrapping the pipe with a 25 watt strip of heat tape that was hooked to the bathroom light and putting the fibreglass insulation back on and it's worked fine ever since. That was about 25 years ago too.

I could have used aluminum or galvanized pipes here but I've seen way too many of those corrode away to a piece of nicely vented pipe with pinholes all over it because of the corrosion from the moisture.

Joined: Jul 2007
Posts: 1,337
I very rarely touch ducting. If done wrong, it can lead to mold and moisture problems. I tell people up front that they are resposible for the ducting and tell them why. I typically get a good response back. If the HO ducts it into their attic and rot it out, it is their problem, not mine.

Last edited by sparkyinak; 03/12/08 11:22 PM.

"Live Awesome!" - Kevin Carosa
Joined: Aug 2005
Posts: 265
We see the HVAC guys installing the insulated flexible ducting in new homes all the time, but they install it without any dips and sags that would trap condensate. Very rarely do we see rigid metal ducting in attics for exhaust fans.


"Will it be cheaper if I drill the holes for you?"
Joined: Nov 2005
Posts: 141
For the HVAC guys doing new construction, I know they do good stuff these days with the flex. I still don't like it because of the restricted and turbulent airflow through the flex which limits the output from the fan to a percentage less that the rated CFM's.

Regardless of the flex, when we do new or reno installs I've stuck with the old method I've used for years with white PVC and really good roof vents that don't make noise in a good wind outside. There is no corrosion and no condensation problems, and yeah, it costs a bit more, but a good explanation with the HO and I've never had an argument or complaint down the road.

Call me a radical for going overboard with installs and unconventional methods, but it works.

Joined: Mar 2005
Posts: 1,213
Flex has about twice the static pressure drop as round metal duct if pulled taught. Any slack adds extra drop, as do excessive bends which are all too convenient when using flex.

Over short distances, static pressure drop is largely irrelevant. The fan manufacturer should have a table stating what the equivilent head loss is, often listed in feet for a certain diameter of duct. Add 15' for every 90 degree bend in the run, and then double it for flex duct.

Joined: Dec 2000
Posts: 4,294
Flex duct also traps moisture, both in the low spots and in the convolutions.

Any mold that may grow, expect it to grow more so in flex than straight pipe duct.

(One jurisdiction out here that I know of won't even allow any flexible ducting from bathroom exhaust fans)

Joined: Aug 2004
Posts: 186
In all the recent new construction I have been on they used ridgid metal pipe. They make the connection via a short 12" piece of flex vent to the ridgid and then vent out the side of the home for first floor and eve or side for second floor.

Last edited by NJ Wireman; 03/22/08 01:18 PM.

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