It would be helpful if you posted the model numbers of the equipment in your quotes. You can also post the prices if you want comments but be aware prices vary by location. Is the basement 900 sq. feet? If yes then a 2 ton unit seems is very over sized.It does not take much to cool a basement if it mostly underground. My suggestion would not to spend money on a 2-stage unit and buy a dehumidifier. You could be the $300 big box store 70 pint type, but in your case you may want to spend the money on a better unit such as a Santa Fe brand. It is three times the prices but will last a long time and dry out your basement.

I'll try to address each specific claim individually. "He didn't believe the load calculation of 55,700." -- Many probably wouldn't although it is likely close to correct (don't know how accurate a load calculation was performed -- block load calc vs. room-by-room Manual J. Did you receive a print-out of the results with a software letterhead, or otherwise have any other details as to the accuracy of the load calc procedure?). That is why many oversized systems get installed daily, leaving the consumer with higher utility costs, a decreased level of comfort, and generally less reliable equipment (longevity). "Also, he states because Carrier invented the coolant used in most systems today, the all-aluminum coil is not necessary." Well, that is bogus. Google "formicary corrosion" -- it is an industry-wide problem that has plagued all manufacturers (Carrier included). That is why Carrier went to tin-plated copper coils as "standard" in the sense that they were just about all the supply houses provided anymore, due to the warranty claims on the non-coated coils. The tin-plating helped somewhat, but still left portions of the coil that were very much susceptible to corrosion; as a side benefit, they did include a 10-year warranty. "In fact, Carrier, he claims, does not make all-aluminum coils any longer." They sure do. Link below, which you may wish to forward to this dealer/salesman for their information. It is their responsibility to know the product line they are selling. "Because of my question he is willing to up the labor warranty from 5 to 10 years!" A salesman sensing you are somewhat informed and wanting to close the sale ASAP (i.e., before you catch on to any misinformation). Not at all surprising. Just my impression, not trying to pass judgment on a situation and/or person(s) whom I have not encountered. "And, Trane states that by going to the XV95 the seer rating drops under 15 and we lose the Energy company's rebate of $450." Possibly true that going to the 100k model gives slightly higher "ratings" in the AHRI database. However I am very against oversizing equipment for sole purposes of receiving rebates. You save a bit up front but pay the added costs (comfort and efficiency) down the road. Here is a link that might be useful: http://www.docs.hvacpartners.com/idc/groups/public/documents/techlit/cnpv-08pd.pdf This post was edited by ryanhughes on Fri, Nov 29, 13 at 22:30

The 17 SEER 2 speed is the value leader in this instance. The 20 SEER variable would provide the best comfort at the most economical operational cost view point, if your utilities are above 15 cent per KWH then this would be a no brainer. At a 15 cent operational cost it would take you 21 years to recoup the cost difference. At a 10 cent operational cost it would take you 31 years. This is in comparison to the units offered and doesn't reflect the additional savings you will get from the equipment you currently have. So comparing this to your current utility usage could put the pay back much faster than the 17 SEER system would provide. Sizing: yes sizing is important, but in terms of variable inverter systems they run via load matching so sizing is only important in terms of duct sizing / air flow distribution and so on. The variable speed / inverter AC will only supply the cooling need to match the load. So over sizing of these systems is really impossible, they don't start up in high stage. They start slow - ramp up and then match the load of the structure. While it's more costly to over size an inverter system, there are certain manufactures that sell only 5 ton condensers and 3 ton condensers. There are settings that can limit these machines to either 4 ton capacity or 2 ton capacity. I suspect this trend will only continue as HVAC industry looks to cut costs in the number of SKU's required. With that said sizing varies all day. Even a properly sized AC system can be oversized during specific time periods. A 2 speed AC is probably over sized somewhere around 50% of the time. A single speed AC is over sized 80-90% of the time. A variable inverter (full inverter) never over sized. A 5 step variable inverter rarely over sized maybe 5% during seasonal changes.

Given that you're renovating I would say it's a good time to do it. It can raise the costs dramatically depending on a myriad of factors. If what you have is flex duct / going back with flex duct the costs shouldn't be too extreme, but this can lead to boot replacement and vent replacement so that can increase costs further than just duct work alone. You should replace with at least R8 flex duct if you are going back with flex. This can pose challenges because circumference wise these ducts will be larger than what you have now most likely. By using a minimum R8 insulated flex duct, this will increase the efficiency of the system, marginally in most cases depending on climate. It also wouldn't be a bad idea to possibly add an additional new filtered return at a known hot spot / cool spot within the structure. (hot or cool --dead air space that might be getting trapped, by adding a filtered return to this spot it will aid in the comfort factor of the structure.)