Jt Some questions please. What is your location? What size home living area? What size furnace and eff are you replacing? What size AC are you replacing? I would like to know the exact mdl number evap coil for each quote. What thermostat is proposed for the Carrier quote? That is a big difference between the 96% eff Carrier furnace and the 80% eff Trane furnace-both 60 K input. That would be about 56 KBTUs output on the Carrier and about 48 KBTUs on the Trane. Not insignificant. I don't care for some shortcuts that homeowners make particularly on reusing linesets and using existing thermostats that shortchange a nice furnace's operational capability. Post back with the info requested. IMO

I suggest separate systems for the first and second floor. That is the best way to have even temperatures on each floor. It costs more up front, but you potentially have lower operating costs. You could zone the loft off the second floor system. However if you are going to be working 10 hours days with multiple pieces of equipment generating heat then spend the money on a mini split.

Patrick,I did option 1 in several commercial environments,a few residential homes and have pre-filters on my units. I use spun polyester and paper pleat merv 8 pre filters along with merv 11 finals. Those with health issues should consider using merv 13 or higher as the final. Big box store 99 cent fiberglass filters are slightly better than running without a filter. I did some testing several years ago using professional tools monitoring and evaluating air flow,refrigerant pressures,temperature drop/increase across coils and compressor amps/reaction/performance. In the field,pre filters proved to enhance performance,equipment life and operating.maintenance costs. I have written about and reccomended the arrangement a number of times on this forum. Almost without fail,the idea has been met with disagreement claiming it to detrimentally restrict air flow. When dissenters are asked about flow restriction as result of poor quality filtration,they claim annual coil cleaning solve's that. Anyone know how to spell self-serving bias? The increase in final filter life is easily better than five times that where no pre filter is used. It is beneficial to performance and cost savings to install as large a filter rack as room allow's. As for option 2,that is actually how many large homes are set up and to respond to the big scary dust mite story, common sense tell's us different. If and when anything set's up shop inside the return,it will be trapped by a high quility filter and wind up at the landfill next filter change. With that in mind it should be evident that if one chose option 2 then later went to option 1,duct cleaning will not be required. Beating the drum regarding one's own credentials is pretty common on this forum. I am retired and have no reason to participate in self promotion but for what it's worth,I didn't attend the 2 week trade school training but I do have more than a few college credits in mechanical and electrical fields. Let the nay saying begin.

WHAT IS A HIGH LIMIT SWITCH? https://www.newcentralairconditioner.com/ The high limit switch in your gas furnace is a small part with two very important jobs. This switch plays a role in the normal operation of your furnace. Turning the fan motor on and off is the high limit switch’s main job. This is the role it plays in every cycle of your furnace’s normal operation. But that’s not the only thing that the high limit switch does. It’s also safety device that helps prevent your furnace from overheating. The high limit switch consists of a long temperature-sensing probe attached to a metal mounting plate. From the mounting plate, the switch is wired to the blower fan motor and to the gas valve. The switch is generally located just past the heat exchanger. This is so that it can detect the temperature of the supply air. In other words, the air that’s about to be blown into your home.