Natural Science Forum / Physics / Acoustics / November 2005

I've read some old posts about furnace noise, so maybe someone here can
help me out.

I have a Trane XE80 gas-fired furnace.  It has two fans in it, the
blower that moves air through the house and an induced draft fan to
pull the exhaust out of the house.  My furnace is installed in my
attic, just over the door to the master bedroom.  The induced draft fan
operates at 3000 rpm.  Not sure about the other blower.

The furnace starts up by turning on the induced draft fan, waiting a
few seconds, then igniting the gas and finally turning on the
whole-house blower.  When it shuts off, the gas is cut off, then the
induced draft fan, and then about a minute later, the house blower.

I've noticed a change in the sound sometimes when both blowers are
running, where the furnace will seem very quiet and then I can hear a
low-pitched vibration sound that comes and goes.  When I go up in the
attic, I can put my hand on the furnace and feel a vibration coming and
going.

My guess is that this is caused by one of three things.  It's either
the bearings starting to go in one of the blower motors, or some kind
of resonance in either the furnace enclosure or in the flue pipe.  The
flue pipe is about 15-20 feet long, and probably 6 inches in diameter.

I only hear this when both blowers are running, which makes me believe
it's not either of the motors starting to fail (plus the house blower
is less than 1 year old).  Also, both blowers are relatively quiet when
running individually, so they are balanced pretty well.  What bothers
me is that the sound changes.  I would think if a standing wave is set
up, it should not change, because that would mean something physical is
changing in the furnace enclosure or in the flue pipe.  I can see the
gas flames (4 of them) and they don't change when the vibration is
present or absent.

Anyway, I'm an electrical engineer who works with radio communications,
so I'm familiar with electromagnetic wave resonance, and I'm just
trying to understand what's going on with the furnace.  It drives me
crazy since it's right over my bedroom, sitting on my ceiling, which
acts like an acoustic guitar!

I guess what I'm after here is if anyone is familiar with this and can
set me at ease a bit, if this is a common issue with furnaces, and also
if anyone can explain what is happening.  The time-varying nature in
particular is curious.  Thanks for any comments!