Homeowner Forum / Construction / November 2006

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

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.)

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