I came across this thread while searching for a suitable booster fan for my new kitchen.
Roland's experience is probably similar to mine. The fan that comes with a combination microvave/hood is pretty lame and only works well if you have a short straight vertical run (so you can take advantage of heat rising).
My installation is in compliance with the manufacturer's instructions with regard to length, diameter, and number of bends, but works poorly. The microwave manufacturer is no help; I can forward a chain of to/from emails, which basically results in "we can't help, and don't touch our components".
In response to bc, if you have an exhaust duct, yoes, you'll have some grease; but unless you are frying everything you eat, or a commercial kitchen, it takes a long time to build up a lot of grease.
Is BC advocating only a recirculating system, so you can keep the heat, humidity, grease and smoke in the house? The filters for residential units have an extremely limited capacity to absorb stuff.
In noodling around on the net, I've seen some fans that were promising. The easiest installations are the end-of-line units that mount on the wall or roof; they're likely to be the quietest as well. There are also some round units that would go in-line; they look like they would fit only in an attic space, however.
I'm looking for something that would fit in rectangular duct, and be easily removed for cleaning. My problem is that my duct goes down, then runs horizontal between floor joists to a vent at the base of the exterior wall. Vertical space is limited because it's close to the ground.
Anyone have suggestions?