wrote in message ...
Don Young wrote:
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Having inspected what I could, one thing that doesn't seem right is the
fan assembly (modern direct-driven drum type). The impeller blades are
inclined outward in the direction of rotation i.e. as if to claw back
air being forced out by centrifical force. Can this be correct?
I could be wrong but it seems to me that blades should be straight, or
if bent to a venturi curve and inclined as in this case, then the high
(outboard) edge should be the trailing one.

You are right to be concerned about the motor failures and improper air
flow could be a contributing factor. The only thing I can tell you is
that fan wheels are designed with both leading and trailing vanes. I
think AC blowers normally have leading vanes, where the vanes appear to
scoop air inward although the airflow is outward. I do not remember the
characteristics of each, but they definitely exist.

That's a new one on me, this biting vane thing, over my head too.

The furnace is a Dettson and it's blower is also used to move air through
an inline downstream wood furnace. The AC motor is temperature regulated
by either furnace for high-flow ON or OFF, plus one of three continous
steady speeds optionally selectable by one of three lugs to which an extra
wire is connected (in series with an on/off switch). Installed in 2003,
the first burnout in 2006 was replaced on warranty and the next higher
motor was recommended and used as replacement. Last year the furnace ran
all winter long on LOW flow (between sensor-commanded HIGH runs). The last
burnout came this spring when I tried to start it up on low flow following
filter cleaning. It just threw a fireworks fit and croaked with burnt
windings.

I'm only suspecting incorrect design/installation with no real grounds one
way or another thus far. The fact is that both motors have been sluggish
getting started on low flow and I think this feature should always begin
with high-flow before backing itself downto low, but that's just an
unqualified opinion. Meanwhile I intend from now on to
get hi-flow turning before switching the low-flow to on as an interim
precautionary workaround.

A check of the fan current under running conditions with all covers
installed might give you a clue as to whether the motor is overloaded. I
know it sounds wrong but some blowers will overload the motor if operated
in the open without back pressure from the ductwork. The pitch of the
vanes causes some spillage of the air with back pressure and this reduces
the load on the motor. Of course these motors are commonly "air over" and
depend on normal air flow to keep them cooled. Also check that your
running voltage is neither too high or too low.

I'm not too familiar with meters but will become so if needed to do this.
Whatever the cause I have to nail it down and fix it or get rid of the
furnace. The last motor cost me $150 and I'm not at all prepared to have
to keep replacing it.

Thanks very much for the time you took to explain :-)



When you replace the motor, make sure that it has thermal protection. Some
motors have it built-in or you can add a magnetic starter or relay with the
correct thermal protection for your motor. That will prevent it from
burning out due to overload.

Have you tried contacting the factory for some advice on this matter? I
mean by telephone; not their web site.