Single Stage vs. Two Stage vs. Variable Speed For Heat Pump/Air Conditioner


If you’re shopping for a new HVAC system or looking to upgrade your existing unit, chances are you’ve already encountered a lot of different options. Luckily, this variety includes single-stage, two-stage and variable speed, each one designed differently to fit every kind of home. To determine which best suits your personal setting, considering things like price, efficiency, noise and humidity control and overall air quality, will ultimately help you to decide. Although they all serve the same function, this breakdown will help you see just how different they really are and in which environment each one excels so you can be confident in your choice when the time comes to buy.


3 AC Efficiency SEER Level

In terms of air conditioning, individual units are rated by the Seasonal Energy Efficiency Ratio, commonly referred to as SEER. Essentially, SEER measures how much cooling a system puts out for each unit of energy it consumes. The higher the SEER rating, the more efficient the unit runs.

Like its name suggests, a single-stage system only has one setting other than the off position. Though it’s still common in many homes today, it’s by far the least efficient of all three units. Because of its limitations, even when the system doesn’t need to operate at full capacity, the only option is to run at 100%. This causes the system to use much more energy than is necessary, which has its repercussions.

A two-stage system is a step up from the single-stage unit, both literally and figuratively. Not only does it have a second operating position, but that position allows for increased efficiency. A typical SEER rating for a two-stage unit is between 16 and 18. Unlike the single-stage unit that must run at full capacity, the second position allows it to run at 67% capacity on milder days. Of course, if the temperature demands it, it can run at full capacity, but in many cases, that’s not necessary. Because of the decrease in demand, it ultimately uses less energy.

If you’re looking for the most efficient system for your home, variable speed is the technology for you and also holds the highest SEER rating at 25 or higher. Unlike the single and two-stage units that are limited to one or two positions respectively, variable speed can operate at a number of different levels that can be tailored to your home’s demand. In addition to operating at full capacity, it can go as low as 20% and everywhere in between. The result is a great reduction in energy consumption, especially compared to single-stage and two-stage system.

Humidity Control

humidity control for ac and heat pump

In order to prevent the growth of mold and mildew, humidity control is essential in your home. In fact, in some parts of the country, it’s even more important than temperature control alone. It’s no wonder then, that choosing the right HVAC system is so important.

A rule of thumb when controlling the humidity in your home is that the longer your AC unit runs and creates a steady air flow, the less humidity there will be. This, of course, begs the questions of which unit is best at this job.

A single-stage unit, though powerful when it does run, is prone to starting and stopping—it runs at full capacity and shuts itself off when the desired temperature is met. A two-stage unit will run longer, switching to a second position, in turn resulting in further dehumidification, but it’s variable speed that will perform this task even better. Designed to run the longest and at a lower capacity, variable speed systems allow for constant air flow which decreases humidity the most effectively, while also using less energy to ultimately create the most comfortable environment.


As is the case with most HVAC systems, the better it performs, the more costly it will be to purchase.

A single-stage system, though the lesser of the three units, will be the most affordable to purchase outright. Due to their inability to run as efficiently as the other units, however, they will also cost more to operate long-term. Because of its improvements over the single-stage unit, a two-stage system will also be more expensive, on average around $600 to $1200 more. Of course, because of its efficiency, it will cost less to operate than a single-stage unit.

A variable speed system boasts the largest price tag, on average, costing up to $1500 to $3000 more than a fixed stage system. Though it is the most expensive choice, it’s also the most efficient and can create a comfortable indoor environment with quality air and humidity control that the other two systems can’t quite match. This efficiency can help long-term with operating costs.

Air Quality

ac air quality matters

Much like with humidity control, the longer a system runs, the better the air quality will be in your home. This is because with more air passing through the filter, it has a better chance of catching and reducing dust, debris, and potential allergens.

When running a single-stage unit, the “start and stop” of air flow means less air passing through the filter. Like in most cases, a two-stage system is a step up, though it fails to meet the capabilities of a variable speed system. With the longest running time, all while at a lower capacity, the flow of air from a variable speed unit means that the quality will be the most satisfactory of the three. This continual air movement also means that your home will have fewer to no hot or cold spots, with even and comfortable temperatures and clean, breathable air.

Noise Level

Noise level isn’t always something that homeowners consider when purchasing an HVAC system, but it is definitely an important factor.

If you’re upgrading your single-stage unit, you know just how loud they can be. Because of the limitation of the single position, the only choices of operation are on and off. Once the indoor temperatures are met, it shuts off, only to start up again at full capacity when the temperature needs adjusting. This constant “start and stop” can be quite loud when the air kicks in, especially if the unit is located close to a bedroom or living room. Two-stage systems limit this noise to some degree, though they’re still relatively loud compared to a variable speed fan.

A variable speed system, unlike single and two-stage units, can run along in the background at a low level without much noise. Because it can adjust it’s output level, it doesn’t have to start and stop the same way a single-stage unit would, freeing homeowners from hearing the constant “kick in” whenever air flow is required. Even though it runs for a longer period than other units, the lower capacity means it stays much quieter.

Which One Is Right For You?

In most cases, a single-stage system can’t quite operate on the level of two-stage and variable speed systems when it comes to efficiency and energy savings. Though they may cost less, they also provide less. If you’re not ready to make an investment in a variable speed system, a two-stage unit can provide improvement over a single-stage system in humidity control, air quality, noise level and efficiency. A variable speed system, of course, can further improve these factors, though it may not match everyone’s budget. If you’re building your dream home or are in that home already, however, the increase in efficiency and reduction in long-term operating costs may mean it’s the perfect HVAC system for your house. In any case, most two-stage and variable speed system will be the most appropriate choice in heating and cooling and can offer flexibility over fixed stage units any day.

10 comments… add one
  • I was wondering if you have any information on the Carrier Hybrid system
    Heat pump/gas heat ? If so, where would I find it ?

    • Sorry, but we plan to update Hybrid system this year.

  • I have a heat pump XL20i with variable speed and thou it’s very efficient it’s very costly to repair, going on it’s 4th summer season coming up the larger variable compressor went out, the part was under warranty (1800 dollars) the labor is estimated to be over 2000 dollars to install. Have decided to back up on technology a bit and go to two stage instead.

  • I am considering replacing my entire system, compressor, evaporator coil and heater/fan. Reason is that the coil has a leak. I have a Trane unit that is 12years old and want to replace with Trane and live in zone 3. When you discuss variable/two stage/single stage systems are you talking about the compressor fan or the air handler fan or both?
    Thank you

    • Hi

      It is not about fan, just the compressor type.

      For the fans, most fans no matter indoor or outdoor are multi-stages.

  • william davis ;

    Thanks for your valuable insight.

    After reading your viewpoint, I went for the two stage , XR-17 AC and saved a lot of bucks .

    Thanks !!

    • Mr. Davis,
      When you say you saved a lot of bucks do you mean on the purchase of a two stage vs single or variable? Since it’s a recent purchase I’m thinking your operating savings has yet to be determined. Thanks.

  • What is the difference between a two stage vs a variable system? I am in process of purchasing a new heat pump system. Currently have a single stage heat pump system.

  • Does Trane have different models for a 5T 16S unit, if so which is best ? Are they all aluminum?
    Thank you very much

  • Wondering if the price difference between 2 stage and variable stage is worth it. And which is the better brand to buy? Ruud or Bryant?


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