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Central air unit air flow noise problem Parse 11-28-2006
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Posted by Parse on November 28, 2006, 12:56 am
I have a 5 ton TRAIN heating/cooling unit. The duct work in my 32 year
old house is steel. When the system comes on, the noise from the air
flow is so loud that we can't hear the TV unless we increase the volume.
Besides, its very annoying. (All the registers are open)

I had a AC service company check it out and he said there is not enough
air flow from the intake. So he added a bid 18X18 vent and a large flex
tube to go with it. It did not help at all.
I replaced the duct on one of the registers with flex duct and it is quiter.

My question is: does replacing the extension ducts that branch off of
the main duct with flex duct solve my problem or I am gonna do all that
work and discover that there is other issues?

Posted by tbasc@bellsouth.net on November 28, 2006, 8:35 am

Parse wrote:
> I have a 5 ton TRAIN heating/cooling unit. The duct work in my 32 year
> old house is steel. When the system comes on, the noise from the air
> flow is so loud that we can't hear the TV unless we increase the volume.
> Besides, its very annoying. (All the registers are open)
>
> I had a AC service company check it out and he said there is not enough
> air flow from the intake. So he added a bid 18X18 vent and a large flex
> tube to go with it. It did not help at all.
> I replaced the duct on one of the registers with flex duct and it is quiter.
>
> My question is: does replacing the extension ducts that branch off of
> the main duct with flex duct solve my problem or I am gonna do all that
> work and discover that there is other issues?

There are a couple of possibles that occur to me - a non HVAC person.
Are the ducts sized to accept the air flow?
There are ASHRAE manual calculations for duct sizing.
Are the metal ducts not insulated or mounted in a way that emphasizes
the noise?
TB


Posted by PPS on November 28, 2006, 1:47 pm
Looking back over the ACCA's Manual D, the chapter on "noise" states "Noise
is created when turbulence is generated in a duct run. This turbulence is
usually associated with inefficient fittings (elbows, tees, transitions and
takeoffs) and sternly throttled dampers." ..... "Regardless of the source,
generated noise will propagate downstream through the supply duct and
upstream of the return duct." .... "will enter a room via the supply ducts"
... "can be transmitted through the duct walls to the surrounding space."

Another place that subject to noise is the registers themselves. "Noise is
generated when air flows through a grill, register or diffuser The intensity
of this noise depends on the velocity of the flow" ... "Normally a supply
outlet will not generate objectionable noise if the face velocity is less
than 700 FPM. (Return air should be held to 500 FPM or less.)

Basically, if the duct is to small, that's a real problem. If it's fittings,
you could try listening to each turn or takeoff starting at the furnace and
try to isolate which ones are the real troublemakers. (If you do find
specific duct problems, you will probably need to have the HVAC company back
to make the changes.

My first assumption would be that duct work is sized to small. (Five ton is
a rather large unit). And if that is the problem, it's not heating nor
(especially) cooling properly as well. (Did you have the Trane installed
recently? If so, you may need to have additional ducts installed to lower
the velocity to acceptable levels; quieting the system and moving more air.)


>I have a 5 ton TRAIN heating/cooling unit. The duct work in my 32 year old
>house is steel. When the system comes on, the noise from the air flow is so
>loud that we can't hear the TV unless we increase the volume. Besides, its
>very annoying. (All the registers are open)
>
> I had a AC service company check it out and he said there is not enough
> air flow from the intake. So he added a bid 18X18 vent and a large flex
> tube to go with it. It did not help at all.
> I replaced the duct on one of the registers with flex duct and it is
> quiter.
>
> My question is: does replacing the extension ducts that branch off of the
> main duct with flex duct solve my problem or I am gonna do all that work
> and discover that there is other issues?



Posted by on November 28, 2006, 7:26 pm
One possibility you might check is the blower in the furnace. If the
original design used a blower driven by a v-belt but was later replaced
with one with an internal motor, noise can increase markedly.

Hul

> I have a 5 ton TRAIN heating/cooling unit. The duct work in my 32 year
> old house is steel. When the system comes on, the noise from the air
> flow is so loud that we can't hear the TV unless we increase the volume.
> Besides, its very annoying. (All the registers are open)

> I had a AC service company check it out and he said there is not enough
> air flow from the intake. So he added a bid 18X18 vent and a large flex
> tube to go with it. It did not help at all.
> I replaced the duct on one of the registers with flex duct and it is quiter.

> My question is: does replacing the extension ducts that branch off of
> the main duct with flex duct solve my problem or I am gonna do all that
> work and discover that there is other issues?

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