The single-stage option has the simplest form of technology. It's lowest in price, but it's also the lowest in value. Almost every part of these single-stage systems can be repaired with universal parts. Meaning, you don't necessarily have to go through the distributor to get the replacement part. Single-stage motors, compressors, control boards, pressure switches, and gas valves are everywhere and readily available. Very likely on your technicians van right now. Two-stage systems have better technology. They run more efficiently and control the temperature in your house without fluctuating as much. The main feature of a two-stage system is that they all typically run at around 70% capacity in the first stage and 100% capacity in the second stage. These systems will run the majority of the time in the first stage, which is where you really start seeing the money savings. Two-stage systems are great for two-story homes that have two thermostats or zoning. These systems can be set up to run in first stage when only one floor is calling for air. The second stage will only come on when both zones are calling for air. When it comes to salespeople pitching new systems, a lot of them will say, But our system is an 18-seer, not a 16 or a 17-seer like the other brand. I tell my customers not to get too caught up in seer ratings and focus on the technology. Any two-stage system is going to outperform a single-stage system. And when you're comparing two-stage systems, the minuscule savings that you're going to receive by going with an 18-seer two-stage over a 17-seer two-stage is trivial. No matter what brand system you buy, two-stage motors and compressors needing repair will have to be ordered from the warehouse near your town that distributes them. There aren't a lot of universal parts available for two-stage systems. Now, capacitors, contactors, and some other parts are universal. But with higher-in equipment, you see safety components like special pressure switches to protect the furnace or air conditioner from damaging itself. And these parts have to come from their factory. With train, carrier, linux, all the way down to Goodman and York, I've never had a hard time getting these replacement parts. And I can't really say that I've seen any huge price difference in the cost of these parts from brand to brand. At the most, we've had to wait five to ten business days for a part to come directly from the manufacturer. There are always exceptions to this, but honestly, it would be the same for any brand. Remember how two-stage systems have a 70% and 100% capacity? Variable speed systems can adjust their capacity levels from about 25% to 100% in 1% intervals at a time. These systems can keep your home within a half a degree of the temperature that you want it. These are the quietest systems, too, because they typically run at a lower speed, they require less energy, and they create less noise with less vibration. There are a variety of steer ratings when it comes to variable speed systems. Once again, I would encourage you to focus on the technology and not the steer ratings, claiming higher numbers. Systems you install today will always be way more efficient than those of 20 years ago. All of these variable speed systems have Wi-Fi capabilities and are communicating systems. When it comes time to repairing these variable speed systems, only their proprietary parts are going to work. With such intricate technology comes priciness and a higher learning curve for who can actually make the repair for you. Train, Linux, Carrier, and all the other brands with variable speed lines will usually only make these parts available to their most respected dealers of those brands. The skill that it takes to handle in-vertor type systems is next level.