On Thu, 11 Jul 2013 15:13:27 -0500, dpb wrote:

On 7/11/2013 12:22 PM, Jim Wilkins wrote:
...

Just checked with a Kill-A-Watt. A muffin-sized centrifugal blower
draws 24W open, 18W with either the intake or exhaust blocked.

OK, let's recast what I was driving at...you're saying there's no fan
curve/as-shipped design point for which it's possible the OP has done
something to his ducting or louvers or the like that can result in
raising motor load above design so he shouldn't/needn't consider that as
a possible root cause/factor?
I'd say that sounds right. Short of physical restriction (drag) it
is difficult to overload a fan. Restricted air flow COULD possibly
cause a fan motor to everheat due to reduced cooling, but not due to
increased load.