subtopic, but ditto Frytown on the 55UD Skuttle. Great unit imo.
Mine is on supply side duct, right after my hot water coil, as recommended [warmer air absorbs more water].
A small solenoid controls water flow. Uses a 24v humidistat and transformer, but there are no "moving" parts in the unit itself. The "media" should be replaced yearly, but it's only $10 ish, iirc. [reminds me, I'm out of them, need to shop...]. With good water [low minerals], the media grid might last more than one season. Not at my house though, limestone deposits on the media. It wastes the small amount of water that goes through it, so it always has fresh water, and what runs off of the grid drains away. No place for anything to grow, really. Yearly cleaning is not much more than a wipe with a paper towel, and replace media grid. There is a water filter on the solenoid, but mine is still ok after years.
Pics at link below show how the media fits. http://www.skuttle.com/model_55ud.htmlhttp://www.skuttle.com/pdfs/55parts.pdfhttp://www.skuttle.com/pdfs/55_86_owner.pdf