What Is a Two-Stage HVAC System?

By Geoffrey Sperring

June 28, 2024

If your air conditioner causes cold drafts, it may be time to upgrade to a two-stage HVAC system. Most older AC units are single-stage systems that are either on or off. They run at full capacity until the thermostat senses the correct temperature and then switch off until the temperature falls.

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In practice, this means the temperature in your home varies by at least 2 to 3 degrees around the thermostat set point, and you could feel a cold draft. A two-stage air conditioner overcomes these problems because it can operate at more than one cooling setting or, in the case of a two-stage heat pump, at more than one heat setting.

How Does a Two-Stage HVAC System Work?

A two-stage AC unit has a compressor that can run at two different speeds. It has a high stage when the compressor runs at full capacity and an intermediate stage where the compressor runs at between 60% and 70% of capacity. When you first turn the air conditioner on, the unit runs at high speed until the indoor temperature gets to within a degree or two of the thermostat setting. At that point, the controller switches the AC to its intermediate speed and reduces the cooling power. The lower power setting prevents overcooling and helps keep a steady temperature in the home.

How Does a Two-Stage AC Differ From a Single-Stage System?

A single-stage HVAC system always runs at 100% capacity. This is fine in extremely hot or wintry weather when you need maximum heating or cooling. Typically, AC installers will always size the AC to cope with these weather extremes. The problem arises during periods of less extreme weather, as the single-stage unit is now too powerful and tends to overcool the house. This is why you feel a cold draft from the AC: Its cooling capacity is too large for the current heat load. A two-stage AC overcomes this difficulty because its cooling capacity is better matched to cool your home effectively at the low or intermediate setting. Additionally, it's more energy-efficient.

What Are the Benefits and Drawbacks of Having a Two-stage vs. Single-Stage HVAC System in Your Home?

While two-stage HVAC systems offer many benefits, they cost more than standard single-stage systems.

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Benefits of Two-Stage Air Conditioners

Higher energy efficiency: A two-stage HVAC is a more efficient system and saves on energy bills.
Improved comfort: The two-stage AC can easily keep the desired temperature, and the home is more comfortable because the cooled air isn't as cold on the intermediate setting.
Better humidity control: The longer running time improves humidity control in the home, particularly in very humid regions.
Longer equipment life: Two-stage HVAC units last longer, as they don’t work as hard or cycle as frequently as single-stage units.

Drawbacks Compared to Single-Stage Air Conditioners

The main drawback is the higher cost of a two-stage air conditioner or heat pump compared to the price of a single-stage unit. This difference in cost depends on the type of system, whether it's an air conditioner or heat pump, the AC capacity and its seasonal energy efficiency rating (SEER).

Typically, you can expect to pay between 30% and 50% more for a two-stage unit over an equivalent capacity single-stage HVAC unit.

According to HomeGuide, the cost of standard efficiency (single-stage) air conditioners with a SEER rating of 13-14 is between $1,000 and $4,200 (CAD 1,360 and CAD 5,700), while high-efficiency two-stage air conditioners with a SEER rating of 15-18 cost between $1,500 and $6,100 (CAD 2,040 and CAD 8,274), according to pricing data from HomeGuide.

All CAD conversions are based on the exchange rate on the date of publication.

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