There seems to be a dizzying amount of options to choose from when configuring a new A/C system. What about zones? Heat pumps? Stages? Among these choices is the option for cooling equipment with two speeds or stages, so the cooling equipment can scale its capacity to meet peak and part-load conditions. The goal here is efficiency and a more comfortable and cost-effective environment.
Let’s start with the basics of both the single stage air conditioning system and the variable speed variety. Most homes with central air conditioning use a single stage compressor. The defacto standard for decades, they continue to represent a large percentage of total units. Single stage compressors are pretty simple in principle, and they turn on anytime the temperature setting on your thermostat is exceeded. If the thermostat is set to 78 degrees, the air conditioner will run anytime indoor temperatures hit 79. When the AC cools the temp back to 78 degrees, the unit turns off. This cycle of on/off repeats itself as long as the unit is left in operation.
Now, with variable speed compressors, they operate continuously at less than 100% of their cooling capacity to achieve and maintain a comfortable temperature. A unit may run at 50%, or even as low as 25% but for an extended period of time. This differs from single stage AC units where the cycle is a continuous series of on/off throughout the day. In Florida’s hot summer, a variable speed compressor based A/C system might run the entire day to keep the temperature in the desired range.
So how does a two stage compressor fit into the big picture? Unlike the wide range of percentages that a variable unit operates at, a two stage A/C system offers just two levels: high and low. It will run at full capacity to achieve the temperature you set your thermostat to, and at the lower level when it’s economical to do so. Two stage compressors don’t turn on and off nearly as much as single stage units, thus they’re more efficient and “short cycle” less. In fact, two-stage units can reduce the “cooling” portion of your electricity costs as much as 30%.
What else? Indoor air quality with two stage or variable speed units can help keep the air moving through the filters removing more dust and contaminants from the air. Since the units cycle less often, they do a better job at dehumidify your indoor air and help prevent mold/mildew growth in bathrooms, kitchens, and closets. These healthier conditions are appreciated by allergy sufferers. Finally, the two stage and variable speed units are quieter as a result of less cycling and less disruptive. Even sit near a vent and “feel” it blowing on you repeatedly as the single stage unit turns on and off over and over again? It’s not providing a comfortable experience so it’s not really performing on the same level as a two stage or variable speed unit.
Finally, the frequent on/off cycling of single stage compressors adds strain on the system, potentially shaving time of the unit’s service life in what could amount to years of early wear and tear..