Heating and Air (HVAC) Single and Variable Speed motors.

We’re talking about compressor types:
⦁ Basic: A single-stage compressor always operates at 100% capacity
⦁ Better: A two-stage compressor has a low capacity, which is about 70% in most models, and high capacity (100%)
⦁ Best: Variable-capacity (also variable-speed and modulating) compressors vary capacity from about 40% to 100% in increments of less than 1%.

Now, let’s talk about how the pros and cons of each compressor type.

Single stage compressors in ACs and heat pumps have only one speed – 100%. Even if your AC is asked to cool your home one degree, the compressor runs at full capacity.
More energy is used than is necessary, and the system might overcool, since it is still running at full speed when the thermostat setting is met and the AC turns off. All the cold air it has already made is pushed into your home.
If you’re familiar with cars, you know that each vehicle has an optimal speed to achieve the best gas mileage. It certainly isn’t 100% — driving as fast as the car will go!

1). Single-stage SEER is the lowest. The technology is getting better, so the current SEER range for single-stage ACs and heat pumps is 13 on the low end.

2). Two-stage compressors are more efficient because they run on low most of the time. You’ll get better gas mileage driving 70 than 100, right?
Two-stage compressor SEER is in the middle. In current models, it starts at 16 SEER and goes to 19 SEER.

3). Variable-capacity compressors are most efficient. Their lowest speed is about 40%. Even if your home needs more air conditioning, it can boost capacity just a few percentage points when needed. This is much more efficient than starting at 70 and speeding up to 100.
Variable-speed compressors can run at 100%, of course, but speeds above 75% are only needed on the hottest days. The current SEER range for most variable-capacity compressors is 20-24 SEER.

Air Quality
When running a single-stage unit, the “start and stop” of air flow means less air passing through the filter. A two-stage system is a step up, though it fails to meet the air-filtering capabilities of a variable speed system. With the longest running time at a lower speed, the flow of air consistently cycles through the system for the best filtration. This continual air movement also means that hot or cold spots will be reduced or eliminated. An added benefit to clean breathable air.

Noise Level
All ACs have insulation designed to reduce noise. However, some inevitably escapes.
Here are AC sound ranges for each type of unit:
Single stage: 72 to 76 decibels
Two stage: 67 to 76 decibels
Variable capacity: 55 to 76 decibels
All ACs are loud when running at high capacity. But the slower an AC can run, the quieter it can be.