While doing some research on fresh air intake for wood boilers, I found this fascinating illustration
of a do-it-yourself fresh air return. I haven't tried building one myself, but I probably will, for my new Tarm wood boiler. And conveniently enough there's a company that sells miniature heat exchanger "coils"
and pipe lengths that will allow you to build this Rube Goldberg-looking contraption quite easily.
I haven't yet investigated whether this device would have any trouble with building codes. Some building codes do require a fresh air intake with a wood stove, so it could actually be considered a plus. I'll report more fully on the topic in the March issue of Green Builder