Air conditioners are measured in terms of capacity and efficiency; the amount of heat removed is measured and described as "tonnage." A single ton of cooling capacity represents 12,000 BTUs (a single BTU is roughly the amount of heat a single match produces). The efficiency of a unit is given a SEER (Seasonal Energy Efficiency Ratio) rating, which describes the amount of electricity the air conditioner consumes in a typical season.
Single-stage units only operate in an "on" or "off" state. When on, the unit pulls the peak amount of power and removes the maximum amount of heat possible; the maximum amount of heat removing capacity, measured in tonnage, is available at all times during operation. Equivalent to running a vehicle either at full throttle or not at all, single-stage units consume more power overall than a dual stage unit and have no variance during their operation.
Dual-stage units have two capacity settings -- usually 67 percent and 100 percent, though other partial capacity ratings exist. The reduced capacity rating allows the unit to consume less energy to remove a lesser amount of heat from the air. Air conditioning requirements are typically lower at dawn and dusk, at which times it is cooler than in the middle of the day, allowing a dual-capacity unit to sufficiently cool the home without the added and unnecessary energy cost of running at full capacity.
Dual-stage units typically consume less energy than equivalent tonnage single-stage units; savings are greatest when the air conditioning requirements are light. Conditioning at dawn, dusk and during season changes with a dual-stage unit requires less energy than would a single-stage model.
Automatic Adjustment between Stages
A dual-stage air conditioner does not require manual adjustment between stages; its onboard computer determines when the additional heat-reducing capacity is required and automatically engages the full-capacity second stage.
Dual-stage air conditioners are more efficient at removing moisture from the air in most situations. Because the reduced-capacity rating allows the air conditioner to run more slowly and for a longer time to maintain cooling, the volume of moisture removed from the air in a home or office is greater than by a single stage unit's intermittent bursts of cold, dry air.
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