I am limping along with a 100K Bryant 90+ furnace and a 4 ton 10 SEER AC unit. The house is ~ 4300 sq ft. with three zones of similar size. The furnace has a bad secondary HX and the compressor was making bad sounds at the end of the cooling season. Located in Michigan. Since the house was new in '94, the outside walls have been re-sided with 1" foam (taped) added, windows properly sealed, additional ceiling insulation added, etc. to minimize air infiltration and improve insulation. I have just received three quotes with multiple options in each and am overwhelmed with the choices of Bryant, Amana, and Lennox and a huge range of manufacturer rebates, tax credits and Utility rebates. I have been on a steep learning curve in the past few days and stumbled across this site which appears to contain excellent advice.

One of the contractors has performed a load calc (do not know result yet) but the other two have just responded with 100K and 110K for 90% - 98% AFUE furnaces.

On the A/C side, the options range from a 13 SEER to an 18 SEER two stage 4 ton unit with significant increases in price. We typically only use the A/C for 4 months a year and are on the boat for two to three of those with the thermostat set at 80 deg. F. On the heating side, only the main floor is heated to 70 deg. F consistently. The other two zones are maintained at ~60 deg. F with mostly damper bypass and the occasional damper opening from observing the system. These two zones are used at 70 deg. F about 25% of the time.

Our zones are configured with a total of five dampers on the duct runs. There are no dampers on the returns. The dampers and control board are Honeywell Trol-a-Temp. We have three Honeywell Chronotherm III thermostats

Our "Best" Lennox quote is for a SLP98 110K and an XC16-48 with icomfort Wi-Fi thermostat; the "Next Best" is an EL296 110K and an XC16-48 with a CS5000 thermostat.

Another "Best" quote is for a Bryant 987MA Evolution and a 187B Evolution AC with the option of a Wi-Fi thermostat and and new zone dampers at a huge upgrade cost.

Another "Better" quote is for an Amana AMVM 96% and a ASXC18 AC again with the option for a Wi-Fi thermostat and new zone dampers.

Does anyone have any input to help me make a choice? I am particularly interested in:

1) Should I get an load calculation for each quote? I am a bit concerned about going with a 110K when the existing 100K 90% unit has worked well and I have made some improvements to the building envelope.

2) Will a two stage A/C give me significant improvements in my circumstances?

3) Will the best Lennox system with a Harmony III control work with the existing zone dampers and maintain all of the features/capabilities of the furnace?

4) Do I need to install new dampers, etc with the Bryant and Amana systems?

5) What thermostats/sensors would be the best choices for each proposed system?

Obviously up front cost is a factor in my ultimate decision but we plan to be here for the life of the system (15-20 years). I want to end up with a high quality, efficient and reliable system that takes into account total ownership costs. I do not want to end up with the "ultimate" system that can never pay back its increased cost over its life.

I probably have not asked all the right questions but hope to learn from your expertise so I can ask some more of the right ones.

Thanks for any advice you can give me.