noisy bath fan

[nizak]

Anybody here know much about ducting creating excessive fan noise?

I have 4 newly installed QT series bath fans. All are 80 cfm, same model,same mounting means,and same diameter flex duct.3 of the units are quiet, one is making almost like a rumbling sound. I put a new motor assembly in it and it's still the same. Is it possible that a partially restricted duct tube could cause the excessive noise? I am not sure of the path the duct takes through the attic but appears that the first 90 deg bend might be quite sharp. Have not mustered up the ambition to wade through 18" of blown in to explore further. Appears to be a decent amount of air coming out that exhaust vent on the side of the house. ????

 

[cowboyjwc]

One of the inspectors here says that the tight bend could be the problem. The bend could be causing turbulence in the duct.

 

[Smart $]

Can't help! I've yet to figure out how manufacturers manage to design 'em to make gosh-darn-awful noise, invariably, a month after it's installed. :blink:





Just kidding.



Can you remove the impeller without too much trouble and run the motor without it? If the rumbling sound stops, or subsides substantially, you have an unbalanced impeller.

 

[Smart $]

To test that turbulence theory, block off the intake while running.

 

[Dennis Alwon]

Sounds like something may be loose. Take the old assembly and try it in the fan housing that is quiet.

There may be a restriction and that would certainly make the fan run louder. QT are quiet test so they should be pretty darn quiet

 

[GoldDigger]

A quick differential test (not conclusive) is that air flow noise will not vary in frequency with air volume although it may decrease in loudness as you restrict the flow.
Vibration noise should change in frequency with motor speed but should be independent of decreased air flow.

 

[nizak]

Will install noisy motor/impeller assembly in one of the other housings and see what happens.

 

[rlundsrud]

I would suspect how the housing is mounted, you may be creating a harmonic vibration because of what the frame of the housing is mounted to. The cavity might also be creating a tuned harmonic (pretty improbable), you could try a small bat of insulation behind it if you can still gain access to the space.

 

[kwired]

I had a HVAC buddy of mine once fill me in on some air flow information. His problems of making a heating/cooling system run quiet are often targeting similar issues but at different places in the system. With a heating cooling system the fan is located away from inlets/outlets so they generally have more trouble with noise created in the duct and not by the blower itself, where with a self contained in/at the room exhaust fan you get direct noise from the fan more so then noise from air passing through duct.

First thing that is important is having sufficient sized duct to cut down on resistance to air flow. 'The easier that air is to move the less noise the fan will create (outside of just poor design, bad bearings, or vibration) Take same fan with no attached duct and it will run pretty quiet, attach even as little as 5 feet of duct and you may notice it starts making more noise. I asked my buddy one time what size duct is needed for a 110CFM fan and he told me at least 6 inch round duct - which of course one has to ask why do they only put a 4 inch duct adapter on most 110CFM fans. I can't answer that but can tell you I have started running 6 inch most of the time when they want to spend money on the ultra quiet type units and it does help quiet those units down compared to running 4 inch duct.

Elbows, I don't recall specific numbers but every 90 degree turn adds similar resistance to air flow as something like 10 feet of duct. The more elbows you have the harder it is for the fan to push same volume of air through and it will make more noise when attempting to do so.

Flex duct - is great for cutting down noise in a heating/cooling system - but it does so because it acts like a muffler and does cut down on air flow. Put flex duct on a typical bath exhaust fan and you increase resistance to air flow over what it would be with smooth interior duct of same dimensions, again making fan have to work harder to move same volume of air and will increase noise levels. Increasing duct size when flex is necessary is a way to compromise.

If you want to guarantee super quiet - use an inline fan located away from the inlet of the room to be exhausted - even if duct is undersized you don't have the blower right in the opening to the room and typically can't hear the noise of the blower, you could possibly have whistling sounds if the duct is too small for the volume of air you try to pull through it though.

 

[nizak]

Found the culprit today with the noisy fan. I went up in the attic and found that the 4" flex duct had gotten wedged into one of the truss crotches. It was a sharp 90 deg bend and restricted by at least 50%. As soon as I pulled it out and formed it into a gradual radius it quit rumbling and was as quiet as the other ones in the house.My guess is that it got snagged with the insulators hose and pulled tightly into the V of the truss.More wasted time in the wonderful world of residential construction.