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Variable Speed vs. Two Stage vs. Single Stage Systems

If you’re looking to purchase an HVAC system you’ve likely come across the terms variable speed, two-stage, and single-stage – but, what do they mean?

These terms are really referring to the capacity at which your system is able to operate as it conditions your home, and that’s all determined by the number of stages on the system. Single-stage systems have a compressor or furnace that can only operate at one setting. Meanwhile, two-stage systems have compressors or furnaces with two settings, and variable speed systems have compressors with many settings.

What is a single-stage system?

Single-stage, or single-speed, systems are the most basic systems. They have two settings: on or off, meaning they operate at full capacity or not at all. These systems work at full speed to heat or cool your home and the system shuts off completely once the indoor temperature has satisfied. Because of the frequent switch from on to off, and because they can only operate at 100% max speed, single-stage systems are usually the most expensive to operate over time.

What is a two-stage system?

Two-stage, or two-speed, systems take single-stage systems up a notch and control the temperature in your home a bit more accurately. While single-stage systems must be at either 0 percent or 100 percent capacity, two-stage systems add more variety by offering a high and low setting. In most cases, the low stage operates around 65 percent capacity, while the high stage operates at 100 percent capacity. Compared to single-stage systems, the two-stage system offers a middle ground for temperature control. This means that on milder days, you will be using less energy to condition your home. While these systems may seem to run for longer periods, they are using less energy to do so and lead to a more comfortable home.

What is a variable speed system?

Variable speed, or variable stage, systems are the most efficient and advanced systems on the market. When you set a temperature on your thermostat, a variable speed system takes into account the indoor and outdoor temperatures, the indoor and outdoor humidity levels, and the run time needed to reach your desired temperature, in order to determine the appropriate output. Variable speed systems can operate anywhere from 25 percent capacity to 100 percent capacity to meet your temperature needs. Variable speed systems reach and maintain your home's desired temperature within half a degree, and because they can operate at lower speeds, they consume less power, which makes them the least expensive to operate over time.

What to consider when choosing a system

Now that you know a bit more about each type of system, the next step is choosing which one is right for your home. Consider these factors so you can make the best purchase possible for your home.


Cost typically plays a significant role in most purchases, and your HVAC system isn’t any different. While single-stage systems are the lowest in cost upfront, they’ll cost more to operate in the long run. On the flip side, variable speed systems are the most expensive option upfront, but cost less to operate since they run on a lower operating speed and draw less power most of their operating runtime. Two-stage systems are the middle ground for both upfront cost and cost overtime.

Energy efficiency

Energy efficiency is an important measure to consider, as the more energy efficient your system, the less it’ll cost you in utility bills. Single-stage systems are the least energy efficient because they must operate at 100 percent capacity when they’re on. Since two-stage systems operate at a high and low setting, they use less energy than single-stage systems, but more energy than variable speed systems. Variable speed systems are the most energy efficient.

Humidity control

As we discussed in our IAQ post, too much or too little humidity in your home can have negative consequences, so it’s crucial your HVAC system can keep your indoor humidity at an optimal level. Two-stage systems are more efficient in removing moisture in the air than single-stage systems, which reduces humidity in the home. But, like most factors on this list, variable speed systems are the best at regulating humidity and are ideal for people living in high humidity environments.

Choosing the right HVAC system for you

Now that you have more information about the differences among single-stage, two-stage, and variable speed systems, you can choose the right HVAC system for your home. As you have learned, climate, energy efficiency, and budget can all play a role in helping you determine which system best fits your unique needs. Let us know if you need help choosing which system best suits your needs!

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