Furnace fan design/installation question

Hi,
I'm trying to troubleshoot possible causes for three consecutive fan motor burnouts in my oil furnace. The dealer/installer is no longer contactable and I'm getting nowhere with the manufacturer's website.
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.
The rotation direction matches the arrow on the drum housing but if motor-mounting holes were drilled as part of the installation then incorrect assembly would have been possible.
I've been on baseboards for over 25 years and the last oil furnace I had was still running without problems since 15 years when I had bought that house. I don't remember how it's fan was configured though, it was an older generation belt-driven rig.
TIA
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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.
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.
It is possible that the motor HP is inadequate for the blower size if either has been changed.
Don Young
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Don Young wrote:

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.

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 :-)
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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.
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John Grabowski wrote:

I've a technical background but don't know much about electric motors, this one has a capacitor on the outside. It does not have the customary reset button (thermal breaker), it's an Emerson K55HXFZW-8023 "assembled in Mexico".

Yes, once. The secretary told me "we only make them" and switched me over to someone who ended up being an open line. What better place BTW to provide efficient and fast support than on a web site? I also sent an email to the parent outfit, nothing yet. I should be getting some other contact numbers tomorrow as I've just got in touch with the original installer who is disabled. Also tracked down the original retailer who sold him the whole lot, they've fired the only person who knew all about furnaces. The manufacturer has been bought out and the buyer also in turn. It's getting more complicated every day :-)
As it is I need to hit three objectives, get the warranty refund for the last replacement that I paid for pending, verify that the assembly/design is solid (I have serious doubts about that at two levels), and see how to head off any future problems because the warranty expires in a month or two. I should have more info in 24 hours.
Cheers
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