That price quote sounds out of line... something doesn't add up. I'm in the SouthEast, where cost of living is generally lower than most other parts of the country. For lower end equipment (13 SEER A/C with 80% furnance), I didn't get any quotes below $6,500. Most quotes for equipment that qualifies for the tax credit were generally closer to the $8,500 number. In the end, we went with a recomended private licenced contractor that got us a system to qualify for the tax credits at a cost just a little south of $8,000 and it seemed like a bargin. Equipment cost sounds about right, but the labor cost seems low. The worry there is that it's an installer that might be taking short-cuts to come in as a low bidder. What I learned from my experience in getting the HVAC system updated is that WHO is installing the equipment is more important than WHAT is getting installed. Basically, I've been told that most manufactures are getting major components (like the compressor) from the same source. But it's also possible to take short-cuts to get a system installed. A short-cut system WILL run. But the prognosis for long life isn't as good. Basically, you can get a longer life with cheap equipment installed correctly than you can from expensive equipment installed poorly. So my advice to people who are looking to replace their HVAC system is to get a quote from three or more places. No one can properly quote an HVAC install without a sight visit. Use that time of the sight visit to in effect interview the company/installer. You SHOULD get quotes that are all about the same even if the equipement isn't exactly the same. Ignore quotes that are "out-of-line", and then of the quotes that are left, ignore cost and go on your instincts on which contractor is likely to do the best job.

Thank you both for the replies! @mike_home: Perhaps because I usually set the AC temperature to be 77-78F, at times I do feel a bit uncomfortable on the 2nd floor when the outside is only moderately hot. The 2nd floor doesn't go below 80F. Therefore I can't be sure whether humidity contributes to the stickiness on the 2nd floor. Maybe a little bit. But I'm hoping the variable-speed blower can help and I don't have to pay more for the 2-stage AC. @tigerdunes: The contractor's proposal says the systems qualify for federal tax rebate of 10% of the total cost up to $500. I'll be sure to get a thermostat with the cool to dehumdify function. The contractor said 95% furnace would need PVC flue piping. I'm not sure what that meant, but I guess I should just get a quote for XV95 with the PVC flue piping done.

You may have better comfort and dehumidification with the 2 stage Performance 17. How will your return air be filtered? You have adequate returns on main and second floor? Your current furnace is 80 K 80% efficiency? Generally, you will get better dehumidification with a furnace that has a variable speed blower. While your furnace selection is two stage, high efficient with high eff blower, it is not var speed. Just so you know. IMO

I was able to get info on the existing furnace from the data plate. It's a Carrier Model 58ss140-BC, Series 110, 167,000 BTU. The data plate said the blower is "157", whatever that means. The motor spec was 1/2 hp. Mfg date 1982. That's all I found on the furnace. I found next to nothing on the a/c. The data plates outside are gone. I could see where it was but there was just rivets. A single, long rectangular unit outside. Like I said, vintage 1974, from the builder. According to my mother, the quoting contractors guessed that the system was 4 or 5 tons. I don't know anything about who said what. Turns out the Lennox/Costco guy was the only one to quote an upgraded unit. He also quoted: Option #3B. $9,185. Platinum Package. A/C XC17-048 and 2 stage gas furnace SL280UH135 with variable speed and I-Comfort Communicating Controller. I'm having a hard time getting her to look at the Am Std quote because they didn't quote her an upgraded unit option and their basic units were only about $1,000 cheaper than the upgraded Lennox units. ($9,145 vs $7,945). About the house, I tried to describe it above. Built 1974. 3200 sq ft. and two stories. Attached garage. Full sun exposure--the property is not shaded. Brick exterior with not a whole heck of a lot of insulation. (I remember my parents being mad that the builder gipped them on the insulation.) High quality house though, expensive neighborhood. Stick framed over a full basement. Attic with pitched shingle roof. Colonial. Original single pane wood windows with storms windows, maybe a tad drafty. After being widowed with 4 kids in 1978, she never had the money to remodel anything and everything is pretty much 1974. Anything else you want to know about the house, please ask. This is all I could think of.