[gwd]

I retrofitted one on a house we bought a couple of years ago. I'm really happy with it, although it does add a constant background 'hum'. It wouldn't have been possible however if we weren't doing major renovation -- we tore out & replaced the downstairs ceiling and completely redid the second floor bathroom; that meant we could run the ducts down the corner of the bathroom and then through the joists in the first floor fairly easily. And the attic is a bit cluttered with the ducts.

You definitely notice a difference when you turn it off.