Single-stage heating and cooling systems operate at a fixed level of operation for the entire time that the mechanical unit runs. These older, conventional systems produce a set rate of heating or cooling per hour. Because units can only run at an off or on level with no middle ground, they cycle frequently, ostensibly putting a higher load on a building’s electrical system.
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Conventional single-stage furnaces, when properly sized to fit the square footage of a home, are designed to heat a building during the absolute coldest weather in the area where the building is located. This means that units always run at their maximum capacity, with every burner firing, when they turn on, no matter what the outside temperature.
Cold Air Blasts
Single-stage air conditioning compressors run in a similar manner, delivering the coldest air possible, despite conditions outdoors. In addition, these systems are less efficient at removing humidity from buildings in areas that experience high levels of water in the atmosphere in addition to heat.
Higher Electricity Costs
Because single-stage units cycle on and off so frequently, many heating/ventilation/air conditioning (HVAC) professionals content that these systems produce higher electrical bills. Others, however, indicate that savings are negligible because the units run at lower capacity for longer periods of time. Single-stage furnaces are considered optimal for buildings in colder climates, while single-stage air conditioning compressors are best for very hot, humid locations.
The majority of HVAC contractors note that single-stage systems produce less overall comfort inside a building. Temperature fluctuations of up to two degrees for heating and cooling are common. Single-stage air conditioning compressors remove only about half the amount of humidity from the environment as their two-stage counterparts.
Some newer single-stage units are available with variable speed blowers. These switches do not affect the rate at which heating or cooling is delivered throughout a building, but control airflow to different areas of a building, providing a slight increase in efficiency.
Today’s single-stage furnaces and air conditioners are more efficient than their 15- or 20-year-old counterparts. Look for furnaces with Annual Fuel Utilization Efficiency (AFUE) values of 90 or higher and air conditioners with Seasonal Energy Efficiency Ratios (SEER) of at least 13.
Properly matching the size of furnaces and air conditioning units to the size of the building in which they are installed is important. A complete heat/gain loss analysis will help homeowners determine if a single-stage system is sufficient for their needs. Contractors should also provide manufacturer specifications and system ratings to properly compare units being considered.