quiet in duct booster fan suggestions

[TP]

We have a booster fan built-in to a 8 inch round duct. This was needed for the long run, 50+ foot. It has a rheostat for speed adjustment. Seems to move the air just fine at 60% of full power. Problem is the noise. When the air is moving the noise travels to all the rooms. We did replace it, thinking the first one was defected. Does anyone know of a quiet booster fan?

Thanks, TP

[Joseph Meehan]

I might suggest isolation the vibration. A short flexible section of duct before and after as well as some sort of bend in the flexible duct after may well eliminate most of the problem.

[Joseph Meehan]

I wanted to add that locating the duct fan well away from the supply and close to the destination would reduce the noise to other areas other than the one being served by that specific duct.

I will also add that if there is any other solution, like a better or larger duct or improved duct distribution design and the elimination of the fan, that may be the best method.

[effi]

[Ken]

I'll add my own experiance with this sort of thing.

There are two sources of noise; fan motor noise, and the noise of the air being moved through the duct. We put in the 8 inch version of this fan to warm up a bedroom at the recommendation of the local HVAC company:

The first thing I discovered is that it moves _WAY_ too much air. (It's nearly half the CFM of the blower fan in the furnace that supplies the entire house.) In addition to moving way too much air, it was noisier than heck. So next I went to the electrical supply place and got the recommended motor speed control for this fan and installed it. I was then able to dial down the fan speed to reduce the CFM. At that point, I discovered two more things; that at the minimum setting that the motor would still run, it was still way too much air movement, and also I learned that motor noise _increases_ as you decrease the fan speed. So the air movement noise was lower, but the fan motor noise was higher, and together it was still too much noise, and it still moved too much air. So next I started backing down the damper in the duct.

We now have the damper nearly shut off completely, the fan runs at the lowest speed that I can stand the fan motor noise, and it still moves too much air, and it is still too noisey, to the point where we shut the fan off during the daytime because of the noise and only use it at night when my son is sleeping in the bedroom and the room needs to be warm.

My conclusion from all of this is what everyone else already knows, that it is far better to solve the original problem for why the room in question is too cold. The in-line fan solution is only a band-aid, and not a very good band-aid. I've tried doing a bit of research on the matter, but there seems to be little information from many of the manufacturers. The last I looked a couple years ago, there were numerous in-line duct booster fans on the market, but most many sellers of these devices don't even list how many CFM the fan moves, or any sort of sound rating.

Sorry I'm not much help other than to say that in my experience, you're kind of stuck.

Ken

[Warren Weber]

A muffin fan is the way to go. We have a food dryer that had a 4 bladed fan and was very noisy. I replaced with a muffin fan of same physical size. Has same airflow and so quiet can't tell if it is running without feeling air flow. W W

[TURTLE]

This is Turtle.

The only cure for noise was given to you by Joesph Meehan in this thread. Now i use a 10 foot run of flex Duct and put the fan in the middle of the flex run.

TURTLE