> furnace, run down to 10%

I hope that you accounted for the stack draft when you said this.
Synchronous fan motors are available with single phase and many (up to 12
- 5 is more common) speeds, in your power range, implemented as taps in
the motor coils. You can control them using a corresponding number of
thyristors or relays. If you use on of these then make sure that you start
on high and then step down (and use a turn sensor to detect start and
stall).

The other option is to use a universal or brushless 50W motor for the fan
(both are available - see automotive supply). This will ease your speed
control problems a lot (and increase cost).

hope this helps,

Peter

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