reply to misiek
Re: External air supply for an 80% AFUE furnace? HRv's are mainly used to exchange air. Their major benefit is that they can save a bunch of energy on what you're exchanging.
The 80% furnaces are required to have a combustion air opening, some places require 2 as previously mentioned. That being said, the older homes are usually considered to be quite leaky meaning a combustion air wasn't originally required. Newer, better insulated homes are tightly sealed which creates humidity problems, hence the need for HRV's. Chimney's on older homes help to circulate air in the home keeping that humidity down.
I have a furnace in my home that I swear must be original. It's a big brown 5 foot Tall Coleman that's still going strong. I've contemplated switching it out but it continues to run! When i do i think i'm going to find a gently used 80% efficiency furnace for 2 reasons, one they're WAY cheaper. I'm looking at about a 5 year old Heil SP80 for $50 bucks. A new condensing furnace will save me more, but it'll cost me at least $4000 to get installed which will take years to pay off. My worst gas bill in the winter is $250 bucks. I have 2 furnaces, 1 newer 80% and the older one, (maybe 60%). Say they're split evenly, at $125 each, upgrading the 60% to an 80% will save me $25 a month for say 4 months a year where as going with the 95% will save me. $50 a month for 4 months. The 80% will pay for itself in the first year, the 95% will probably never pay as there's bound to be repairs down the line.