I am not an Heat and AC man. I just went thru the process of choosing a new system to replace my 16 year old broken unit. I learned that they have 2 stage compressors that use a lot less electricity. The condensers are much bigger and have lots more heat transfer coils than the older ones. Along with that my old unit used 7-1/2 lbs of Freon and this new one uses 14 lbs. The furnace ( gas ) has a 2 speed heater in it. The blower has a variable speed motor. The best way for most anyone to understand what this does for you is to relate it to driving in the city with stop lights. The light turns green and you slowly accelerate to the speed limit and when you see you need to stop you take your foot off the gas and slow down to a stop. The blower motor and compressor / furnace mimic this same thing. When the unit turns on you basically do not even know it. There is no hit with the start like the units that do not have these features. The lights do not flicker when it starts. Another benefit of the variable speed operation is that when the fan motor is running slower the air is moving slower over the cooling coils. This allows more moisture to be taken out of the air. Another benefit of having the 2 stage system is you can put in a larger unit. My house was between a 4 ton and 5 ton unit with the calculations. I chose the 5 ton. When it is not needed the compressor runs at 67% of it's capability ( 3.35 tons ) and if needed it can kick it up to the full output and be a 5 ton unit. The furnace is the same way. This is done automatically. On top of all of this being quieter it also saves you money on your utility bills. There are also units that have 2 separate compressors, communicate with data to the thermostat as to how it is doing, program your thermostat from the internet, etc. I chose not to get that for my use. It is quite a bit more expensive. Bottom line. Do lots of research before you buy your next unit. A few hours of research might result in many years of pleasure from your choice. Compare the brands and the features of each of the brands and models that each brand has to offer. Get several bids. My bids for what I needed / wanted varied from $10,000 to $15,000. Check out the dealers for feedback and the Better Business Bureau. In my case the $10,000 bidder was the best in the BBB, had the most professional quote ( listed every thing line by line with part numbers, not a description ) and had the best feed back between the 3 of the bidders. The brand is the same brand I am replacing. From what I have seen so far of their work on my unit it is first class. A word of caution. The SEER ratings say "up to" XX.x. The smallest matched unit they have is capable of that number. As you step up in size that number drops. My unit's literature said up to 17.2. My 5 ton unit says up to 15.2. It is only that if all the pieces are matched. Hope this might help someone with their choice when they have to replace as I am doing as I type this. They started my install yesterday and will finish it this morning. Edit / Add: They finished my unit today. I found out when he explained the thermostat that I can set the AC temp, Set the Heating Temp and then choose Auto for the mode of operation and it will switch to whichever is need for the current temperature. Also found out that the unit I bought has a Rebate. Mercy ! Cool and quiet and a Rebate .