Originally Posted by Quattro

Question...why do builders still use this type of system? High efficiency furnaces have been around for what, 25 years? Combustion air comes in via PVC, and is exhausted via a matching vent pipe. No need to fill the basement with frigid air. It makes no sense to me.

I guess the only reason is to exchange air. Outside winter air is dry, so it helps to keep humidity low in new houses. Seems like a cheap way to avoid a proper heat exchanger (heat recovery type) that would be 80% efficient.