Puzzling ducting restrictions in home ventilation system


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Old 06-30-19, 12:37 AM
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Question Puzzling ducting restrictions in home ventilation system

Hi everyone,

Searched around the web and seems like this place might be able to help me out with a ducting question.. I hope so! I'm in Australia and local forums are pretty lacking.

I recently moved into an apartment where two out of three ducted in-line fans (2 bathrooms and a laundry) have failed since the place was built only five years ago. Looking to replace them but also wondering why they failed..

The current arrangement looks odd to me but perhaps it's fine:

All three systems have 20 feet of 6" flexible ducting from the room to an area in the ceiling where all the fans are installed. At the other side of the fans, there is 30 feet of rigit duct, but only 4". The current fans are sized for a 6" duct, but all have a 4" reducer on *both* sides of them (even the 6" side).

i.e. it goes: 6" flexible duct -> 6-4" adapter -> 4-6" adapter -> 6" in-line fan -> 6-4" adapter -> 4" rigid duct.

Here is a photo: https://imgur.com/oqC1xrW

So I'm wondering why the original installer decided to restrict the duct diameter to 4" *before* the fan, even though the incoming duct is 6" and the fan is 6". Is there is a logical intent here? Does this reduce stress on the fan or something? Does it prevent collection of humid air around the motor? I'm guessing here; nothing seems obvious to me.

Because otherwise it looks like they just didn't have enough length on the 6" duct and so stuck two 6-4" reducers back-to-back because they were nearby!

Should I get rid of the incoming restrictions when I replace the fans?

Any advice appreciated. Many thanks.

James
 
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Old 06-30-19, 04:04 AM
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I agree. It sounds like the used whatever they had on hand instead of running 6" all the way to the fan.

In most cases you do not want to restrict the infeed side. Any restriction is usually best done on the output side.
 
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Old 06-30-19, 04:32 AM
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two out of three ducted in-line fans (2 bathrooms and a laundry) have failed since the place was built only five years ago
That fan looks very similar to one I installed in my master bath, and agree they should be a constant size for optimal air flow!

However, even with the reduceres in place that should not be causing the fans to fail.
 
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Old 06-30-19, 05:26 AM
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I am unsure of what is standard practice in your area but here inline fans are not normally used to supply or boost air volume.
They are normally used as an attempt to fix a poor or inadequate duct system.
Any readily available fans i have come across are poorly constructed and seldom last more than few years.
You would expect a fan in a permanently installed duct system to last at least ten years.

You would need to locate a better quality fan which would have ball bearings rather than the sleeve bearings yours likely have.
 
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Old 06-30-19, 09:01 AM
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I recently moved into an apartment where two out of three ducted in-line fans (2 bathrooms and a laundry) have failed since the place was built only five years ago.
No problem, call the landlord, it's his responsibility.


I'm in Australia and local forums are pretty lacking.
Yet your new profile says you are in the U.S.
 
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Old 06-30-19, 03:47 PM
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Thanks everyone. (Except perhaps CasualJoe: I own the apartment and it is not possible to select a country other than US).

Sounds like I should just toss out the restrictors and put in a piece of 6" rigid duct!

Will certainly look out for a ball bearing fan for the replacement.

I guess I'll go with a 6" fan for the extra flow. I wonder if there's a higher chance of failure in future though, if the fan is driving air into a 4" duct?
 
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Old 07-07-19, 08:45 AM
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Thanks everyone. (Except perhaps CasualJoe: I own the apartment and it is not possible to select a country other than US).

That's fine, no thanks are required.


Other forum participants don't seem to have a problem selecting their country.
 
 

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