Two-Stage AC

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Homeowners Guide to Building Performance
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HVAC Systems Field Guide
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Residential Ventilation Tech Brief
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Residential HVAC Tech Brief
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Two of the most important improvements to modern high-efficiency air conditioners are the two-stage compressor and multi-speed condenser fan.

During the hottest weather, you may need the full capacity of your air conditioner to keep your home comfortable. During this full-time operation, your air conditioner runs at its maximum efficiency. But during moderate weather your air conditioner will cycle on and off to keep from over-cooling your home. This stop-and-go operation is inherently inefficient, since a lot of energy is wasted during the start-up and shut-down part of each cycle.

Air-conditioning units with a two-stage compressor and a multi-speed outdoor condenser fan operate at lower capacity during moderate weather. This results in cycles that are longer and more efficient. It also allows your system to remove more humidity from the air. When the weather is truly hot, they step up to full-speed operation to assure your comfort.

Variable-capacity air conditioners are all more efficient than older units, but they work in different ways:

• Some two-stage systems have two small compressors side-by-side. The first compressor operates alone during moderate weather. The second compressor kicks in only when needed during hot weather.
• The two-piston compressors run in one-piston mode during moderate weather and two-piston mode during hot weather.
• The multi-speed compressors actually turn at two different speeds to match the cooling load.
• You can’t avoid the heat of summer, but with a variable-capacity air conditioner you can reduce the cost of cooling your home. You can learn more about air conditioners at the Energy Star® web site.

For more information on air conditioning see The Homeowners Handbook to Energy Efficiency or Residential Energy: Cost Savings and Comfort for Existing BuildingsChapter 6 for more information. For professional information about cooling systems and energy efficiency see Saturn Hydronic Systems Field Guide.