Don Young wrote:
wrote in message ...
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 :-)