Sorry, a little more info - exisiting house (and furnace) is only 5 years old. Good furnace, Carrier 58 STA/STX. The other driver here is that i have AC on present furnace and dont want to have to add another AC unit on 2nd furnace (for cost reasons). I will likely just have additional small high effieciency installed to avoid messing w/ existing vent (or any probs / backdrafting that could be associated with adding to existing vent). Will likely put 2 stories above grade on the big mid effecient one (with AC) and run basement on separate zone from smaller high efficient one (without AC). Just wanted to make sure options were evaluated. At present cost of nat gas the payout differential on mid vs high eff is ugly. Perhaps nat gas wont be so cheap forever but i think it will be for a few years.