Thanks all! I am learning a lot. The reason I am thinking it is coil that is leaking is because the AC guy who performed initial diagnostic last Spring used some kind of a device which looked to me like "electronic leak detector", according to the posted description. At this time, I don't have a reason to believe he did a bad job last year. He poked his instrument in different places. One of the places that (according to him) showed the "leak" was the coil (which was, by the way, located inside the furnace cabinet in the basement, not outside.) The reason I am not completely sure about the diagnostic is the following. He said that according to his instrument's reading the leak was pretty minor. That's probably why he recommended simply recharging puron. However, the puron he put back into the system apparently leaked out over the course of last summer. So maybe the leak at the coil was not so minor in the first place? Or it could be because there was more than one place where the system has a leak? Anyway, I am going to call a different company this time. Based on the info I have so far, the coil might be only part of the problem. That's why I try to keep an open mind and consider alternatives. Yes, I read that 16 SEER is a minimum rating to qualify for tax credit. My concern is that I already replaced the gas furnace not so long ago. Can I pair my furnace (Carrier 58CVA110-20) with another modern Carrier AC unit of 16 SEER or higher so that they work seamlessly together? If yes, what model would match my Carrier furnace? If a new (comprehensive) test confirms that the leaking coil is the only problem, how much I should be prepared to spend on fixing it? The company that quoted me the price of ~$1600 last Fall told me that they would have to first evacuate the puron from the system (which is expensive), then replace the coil, and then put another load of puron into the system. That's why it is so expensive, they told me. Does it sound right? I've seen quotes for complete AC system installs at around $6K. Considering what I have to spend on diagnostics and repairs anyway, plus accounting for $1500 tax credit, I figured I will end up only couple of $thousands short of being able to replace the whole AC system... Or am I missing something and the difference between repairs and replacement is actually a lot more?

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

Solution to this problem. When you update your system from single stage to 2 stage, you must have C wire from outside unit. My old single stage system came with 3 wires from outside unit to indoor (blower) and Indoor unit to thermostat had 5 wires. I solve this problem with new wiring the whole house. I install 4 wires from outside unit to indoor (blower) (red(Rh), blue(C), yellow(Y1), purple (Y2) (mine had a purple)); Indoor unit to thermostat install 7 wires, which following below. red (Rh) blue (C) yellow Y1 Cooling stage-1 Gray Y2 (Purple Y2) Cooling stage-2 Green (Fan) white (W1) Heating stage-1 brown (w2) Heating stage-2 If you don't want to rewire, I recommend to get CTK04 thermostat, which will be fine with your old wiring.

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.