HVAC by the Numbers

13 or more: Seasonal Energy Efficiency Ratio (SEER) rating. Thirteen is the minimum rating for air conditioners manufactured after Jan. 26, 2006. SEER is the ratio of the cooling output in BTU divided by the power consumption in watts per hour.

8 to 10: Heating Season Performance Factor (HSPF) rating. Eight to 10 are high efficiency ratings for air-source electric heat pumps. The ratio is determined by dividing seasonal heating output in BTU by the power consumption in watts.

83 percent and 90 percent and higher: Annual Fuel Utilization Efficiency (AFUE) rating. The efficiency percentage at which Energy Star-qualified oil and gas furnaces start. It is a measure of the amount of fuel converted to space heat in proportion to the amount of fuel entering the furnace. However, a high percentage, such as for an all-electric furnace for example, must be balanced out with the cost of the energy.

1/2010: In accordance with the Montreal Protocol, the date after which chemical manufacturers may only produce R-22 refrigerant to service existing air conditioning equipment but not new equipment. Only pre-existing supplies, such as recycled R-22, could be used to produce new air conditioners and heat pumps.

1/2020: Date after which chemical manufacturers will no longer be able to produce R-22 to service existing air conditioners and heat pumps. The servicing of R-22-based systems will rely on recycled refrigerants.

R410A: A refrigerant substitute for R-22. It is a blend of hydrofluorocarbons (HFCs), substances that do not contribute to depletion of the ozone layer, but, like R-22, still contribute to global warming.

30 percent to 50 percent: The amount of relative humidity needed to make a home comfortable. Depending on a home’s location, it might require spot ventilation or humidification or a whole-house humidification or dehumidification system.

70 percent: Relative humidity at which mold and mildew start to grow.

20 percent: Percentage of heating and cooling system efficiency you could be losing by having leaky ducts.

1 in 4: Number of furnaces in U.S. homes that are more than 20 years old.