Heating & AC Repair

Single Stage vs. Two Stage and Variable Speed

Let’s start with a little primer. There’s more to the function of an HVAC system than just heating and cooling.

It really has three functions: temperature control, humidity control, and air filtration. Obviously the heating and cooling part is the primary function, but the other two shouldn’t be overlooked. And that’s where two stage and variable speed really comes into play.

Properly sizing the HVAC system for the building is important, but with single stage systems it is critical to the system working as intended. 

  • Too Small — You’ve obviously got big issues if your system is undersized. Most times it will appear to work fine, but in extreme heat or cold it won’t be able to keep up. Your thermostat will be set to the right temperature but the HVAC system doesn’t have the capacity to maintain your set temperature. You will be forced to accept whatever temperature the system can deliver. 
  • Too Big — This one isn’t as obvious. You might think, I’ll go big. Get an oversized HVAC system and it’ll run less, save energy and last longer. It might run less– but that’s actually a problem. Oversized systems without variable speed capabilities will “short cycle”, especially in moderate temperatures. Your house will be the right temperature, but because the system isn’t running long enough, humidity isn’t adequately removed from the air (in summer), and there isn’t enough circulation to properly filter the air. 

When your house was built, a “load” should have been calculated to determine the proper size, based on various factors such as the windows, insulation, and orientation of the house to the sun. Maybe these factors have changed over time, or maybe the load wasn’t done properly (or at all) at the beginning.

Single Stage — Where it all started

It wasn’t too long ago that there was just plain old HVAC. No two stage, no variable speed. Wasn’t that a simpler time and place? Well maybe, but we won’t get into that. 

Single stage has always worked just fine, why did they develop anything new? As far as performing the basic functions of heating and cooling that’s true. The issue with single stage systems is they are either off or running at full blast. There is no in between. When temperatures are moderate, the system will “short cycle” (defined above). Because humidity is often moderate at moderate temperatures, this is acceptable and the single stage function was good enough for years and still works just fine.

Two Stage — An Innovation in Comfort and Efficiency

A two stage system is a big step forward, because it gives the system a “medium” capacity option between maximum on and off. A two stage system has two speeds. During moderate temperatures it runs on the lower speed, avoiding the short cycle condition and providing better overall comfort and more efficient operation.

There are several components necessary, depending on the type of system, to deliver two stage operation.

  • Two Stage Compressor — Heat pumps and air conditioners operate by compressing and expanding refrigerant in a closed system. In a two stage system the compressor bypass ports are opened, allowing it to run at reduced pressure. 
  • Two Stage Furnace — Instead of just being all the way on or all the way off, a two stage furnace can operate at two combustion settings.
  • Two Stage Fan — Coupled with a two stage compressor or furnace is a two stage fan. When the compressor or furnace is running at low speed the fan also operates at low speed to save energy and provide a more even air distribution.

Variable Speed — The latest technology

Variable speed systems represent the latest in engineering for your home. As the name suggests, instead of just having just two speeds, variable speed systems can ramp up and down at the optimal capacity necessary.

Variable speed air conditioners and heat pumps run almost continuously– And in summer the slower the air moves through your cooling system, the more humidity is removed. As a result, you will feel more comfortable at higher temperatures so you can set your thermostat accordingly. And since they don’t go through the energy-guzzling startup phase as often, variable speed AC units lower your utility costs at the same time.


Each of these HVAC technologies can be useful in the right conditions. A variable speed system is tops for comfort and energy savings over time, but its extra cost may not always make sense. A single stage system will still work fine when properly sized. And a two stage system is a nice middle ground that provides significant improvements to comfort and efficiency without as much extra up-front cost. For help figuring out which type of system is right for your home, contact the knowledgeable folks at Comfort Monster or another high-quality HVAC company near you.