Thanks for the quick responses! The existing furnace is an oil-fired York, 125 KBTU, which is at least 30 years old. I have been told it's horribly inefficient, but I don't know the number. The AC condenser is 3 tons and 25 years old (also not efficient). The house should probably have 2 zones (especially to deal with better cooling the second floor in the heat of summer), but I have been told that's not possible (at least not for a reasonable price). Otherwise, there are no noticeable issues with the current system, other than those endemic to scorched air heat. Each room has at least one supply vent and one return. The basement is below grade and is used as living area. There are two supply vents and one return in the basement area. There is no insulation between floors, but the basement walls are insulated (R-7.5). Beefing up the attic insulation is on the near-term to do list. The house currently does not have a humidifier, and while no one is suffering nose bleeds, the air is dry. So, I thought a humidifier would be helpful. I would definitely like to tighten the house (and new windows may be on the radar in the near future), but given that it is an older home, I think there is only so much that can be done. The Carrier dealer said the furnace would be no more than 100 KBTU, with the final determination after the house is measured and a load calculation performed. Both companies will also do a whole house energy audit prior to commencing work. So I would assume (and hope) the Lennox dealer will modify his numbers accordingly after the load calculation and energy audit. Our natural gas marginal price/therm is $0.83 Our electricity marginal price/kWh is $0.22 for on peak, and $0.07 for off peak. With regard to the Carrier condenser, what makes the 24ANB6 superior to the 24ACC6? I know a lot of people on this forum prefer Carrier over Lennox. All else equal, I have no problem going with Carrier (I know the Infinity controls are fantastic), but there is a $1,500 price difference... Thanks again for your help.

lroz 1 and 2 are identical furnaces-three stg top of line Carrier/Bryant. 3 and 4 are two stg furnaces. 1 and 2 will require the Infinity/Evolution controllers which can be pricey. are those controllers included in price? no substitute here! with an existing 30 yr furnace, I do think you should strongly consider upgrading now and take advantage of all rebates/tax credits, etc. what about AC? IMO

With fifty replies in the original thread, by starting a new thread you will deny future folks that search and find your original thread the oportunity to follow to conclusion. Suggest you post this first post at the end of the original thread and if possible edit your post here to read, "Deleted." Just a thought.

I agree with mike_home. To be fair, if we're listing folks in the residential design and construction business who know little about HVAC, we need to include architects, and a lot of low-bid mechanical contractors to the list, too. If your builder is suggesting 4,000 SF as a rule of thumb for going to multiple HVAC units, that recommendation would be based on unit capacity considerations--not comfort or efficiency. HVAC systems need to be designed with regard to the specific home plan (windows, doors, insulation, crawls space design, attic design, etc.); the specific climate zone; the specific orientation with respect to solar heat gain; the availability of local service for the HVAC equipment type; the ability to run ductwork; the cost and availability of utilities; and the owner's preferences and budget. 2nd floors, areas with cathedral ceilings, and sunrooms gain and lose heat more quickly than their 1st floor counterparts with standard ceilings. Ensuring comfort of occupants in such spaces generally requires zoning which can be achieved by installing separate systems for each level or implementing a zoning strategy.