It's a window unit: http://www.purifresh.com/erv.html It's the only such kind I could find when we were in the market a few years ago. (Was renting a place with mold issues.) Works well for a single room. It's wonderful to pull fresh air in during the winter.

We also have its accompanying HEPA unit: http://www.purifresh.com/cleanroom.html They can be linked (as http://www.purifresh.com/freshairsystem.html) or used separately. (Apparently this is out-of-stock and being redesigned.)