2 stage cooling has been available from most major brands of residential air conditioning manufactures for a little over 18 years and over 30 in commercial applications. The original designs utilized 2 compressors to achieve the 2 stages. One compressor was half the size of the other. An example might be if your unit was a 5 ton, one of the compressors might be 30,000 BTU’s each or something or one small one and one larger one. That smaller compressor would engage on start up. On mild days it would run until it shut down. If it was too much of a load for the small compressor it would engage the other compressor.
As any Las Vegas AC Tech that has worked on these units will tell you, the big compressors die way before they should. The fact is, the smaller compressor doesn’t get used much. It is simply too hot in Vegas. When the thermostat shuts off the AC then turns it back on, It takes every BTU available to catch back up. Such is the price for living in an over 90 degree climate. There are many nights that will use the first stage, smaller compressor, and not need to engage the bigger one. Most of those nights will be in early spring and the beginning of fall. Anyone that has lived in Southern Nevada for more than a year will tell you there are many nights in the summer that do not cool below 90.
So what is a 2 stage cooling good for. In Vegas, not much. The big advantage of a 2 stage is it’s ability to run longer and there by remove more humidity. In Florida these units are great. There are many days where the temperature does not break 95. And 95 degrees isn’t that bad if the humidity is low. But the humidity is always high in Florida.
2 Stage in Vegas did pave the way for Variable Refrigerant Flow inverter AC units. These units are incredibly efficient and only about 15% more expensive to buy than a 2 stage unit. VRF systems like the Prestige Series from Rheem provide the ultimate comfort for a home. They do this in two way. First, many of these systems will turn on in early May and not shut off. They will instead dial themselves down to a lower setting. They will take a temperature reading every second and adjust the capacity accordingly. But wait, wouldn’t a unit that never shuts off cost more to run? Sure but that would be if it couldn’t run at a lower watt setting. Most of the Inverter systems can run at 10 to 30% of capacity.
Humidity is always part of the equation when you want to talk about cooling. To me, 77 degrees is very comfortable when the humidity is low. On days where the humidity is high, 70 degrees is not that great. But that changes when we talk about Inverter systems and Variable Refrigerant Flow. VFR will allow you to set your humidity in your home to your exact comfort. The value of this setting is rather hard to describe. It is something you would need to experience. Those that have find it difficult to live with out.
Unlike the 2 stage, the VFR works very well in Vegas. I have seen these system run as low as 40% of capacity when it is 110 degrees outside. It goes without saying that will save you money on your energy cost. Even if you could get that kind of energy savings out of a 2 stage system, they still do not control the comfort as well.
If you are considering a 2 stage system then you already know you will pay more than a single stage unit. In most cases two stage will run you about $1500 to $2000 more than single stage 14 SEER. For as little as $1500 more you can buy a far more capable home comfort system and save even more on energy cost. Not to mention the value of better humidity and comfort control.