Hi, just had a complete system installed yesterday. I live in dallas where cooling means more than heating. cost 7k even for everything. here is a list of what was installed. Trane XR13 4 ton AC condenser. Trane XR80 5 ton 1 stage with 4 speed blower. New matching evaporator coil with the new R-410A refrigerant. New return and supply plenums, all sheet metal construction with exterior wrapped insulation. New manual air dampers at each supply duct. Added an extra return air duct, grill and filter in double existing volume of return. New drip pan with overflow float switch (cut off system if drainage get blocked - keeps ceiling from getting water damage). New electrical disconnect and wiring (located outside by the condenser). New concrete pad. New programmable thermostat. Did all the work required to rid and clean system of old coolant, hauled away old system and did a relatively good job cleaning up. This new system is so much cooler and better airflow than the old one and should be more efficient. I really wanted the sheet metal plenums with EXTERIOR insulation and the manual dampers. This should really pay off in the long run for comfort and health. Next time I should just have to replace the AC condenser. Furnace later. All in all I think I got a good deal and I'm pretty sure the work is A+. I paid a little extra for the plenums, dampers and for Trane products.

"replace the a/c and air handler to a high efficiency unit?" I do no thing Spacepak or Unico are in the 'high efficiency' game. They are in the 'at least you can have AC game.' Relocating the air handler would be about as much labor as it took to install the existing system. You could make a rough room in the attic to contain the air handler with light construction and foam insulation panels. If you then remove all the ceiling insulation inside the 'room' it becomes at least partly conditioned space. Other than that or adding more insulation to the air handler and ducts (hopefully you do NOT metal ducts but ductboard) there is not much that can be done.

You are right, your condenser does use R22. It must be the last of the R22 condensers Carrier manufactured. You should not sink $2500 in repairing the condenser. My suggestion is to buy a Carrier condenser and coil. No advantage to introduce another manufacturer's equipment unless you can't find a competent Carrier dealer. You should change the line set also. You can't add a 2-speed condenser to a single speed furnace. If the furnace is working well then there is no need to replace it. The furnace is probably over sized for your climate.

"The two contractors below came up with a 2.5 size but heat pumps only come in whole sizes. Am I over sizing the unit if I go with the 3 ton heat pump?" The heat pumps you were quoted are 2-stage models. All 2-stage heat pumps come in whole ton sizes. You would have to get a single stage heat pump in order to get the 2.5 ton size. I am in central NJ so my climate is very similar to yours. Did you review the load calculations? Is the 1800 sq. feet all on the second floor below attic space? Given attic poor insulation (R19 or less?), the 2.5 ton may be slightly under sized. In my opinion I don't think a 2-stage 3 ton heat pump is over sized. We would have to look at the data sheet, but most likely the lower stage will be less than 2.5 tons. It is difficult to comment on the price because of the required duct work. It sounds there is a lot of labor involved in the new duct work and attic installation. Contractors don't like to break out line item cost. Getting one price is very common. You have gotten 8 bids, so you compare prices. You did the right thing by rejecting people who don't even go into the attic. The answer you other questions, I think 2-stage equipment is worth the extra money. It is going to do a better job of lowering the humidity in the summer. It should also be quieter when running on the low stage while you are sleeping. I think suspending the air handler from the rafters is the best way to minimize noise. I don't have experience with attic installed equipment, but it seems logical you would cut down the vibration from the ceiling. You want R8 insulated ducts in the attic. The main duct should be metal and all joints should have mastic. The flexible ducts should have no kinks. If it looks bad to your eye then it is probably bad. You are doing the right thing by investing in improved duct work. I like Carrier over Lennox. You also want a 2-stage thermostat. Find out the model number and make sure it is in the quote.