Heat Pump
Older furnaces may run to 70-ish% efficient; increasing to 80 and 90% efficient with condenser (Amann et al., 2012). The most efficient furnaces operate at 98% efficient. However, heat pumps operate well above 100% (furnace) efficiency but do require an air duct system for air distribution.
An air-source heat pump operates similar to a refrigerator in that it removes hot air from within the appliance. A heat pump can remove heat from the air, thereby cooling it and replacing a central air conditioning system. In addition, it can remove coolness from the air, thereby warming it and replacing a furnace. These are rated using a Heating System Performance Factor (HSPF). An HSPF rating of 10 delivers 3 kWh of heat for 1 kWh, providing an efficiency of 290% (Harley, 2012). Ground-source heat pumps are also available, but less common.
Heat pumps traditionally operate well down to 40-45° F, but few models operate well below 30°. Therefore, many are not recommended for cold temperatures, unless paired with a regular furnace. With advanced technologies, it is possible to select one to use with colder winters.
Air Conditioning
Increases in solar heat arise from solar radiation (sun through windows, skylight, roof), internal home gains (through cooking, showers, people, appliances) and air leakage (ducts and house leakages) (Harley, 2012). It is best to reduce the need for air conditioning first, addressing the three sources of heat gain: sealing ducts and leaks, improving attic and wall insulation, using a ceiling fan, installing a light-colored and solar-reflective roof, upgrading windows, and/or shading externally (best) through awnings, shutters, trees, trellises; and/or internally, through cellular or lighter shades.
In addition to cooling, air conditioners also dehumidify air for comfort and air quality. Using exhaust fans for cooking and baths/showers helps to remove humidity caused by internal gains.
If an air conditioner is 20 years old or older, a new device will be cost-effective. It is best to buy as efficient as possible. Central air conditioners are rated using Seasonal Energy Efficiency Rating (SEER), which measures British thermal units (Btu) per hour of cooling output per watt input (Harley, 2012). Room air conditioners are rated with Energy Efficiency Ratio (EER), which contrasts slightly lower than SEER (Amann et al., 2012). Higher (S)EER values are more efficient, but recommended standards differ between northern and southern climates.
Other systems include (Amann et al., 2012)(Harley, 2012):
- A (ductless) minisplit heat pump has a high purchase price to cool 1-2 rooms, but may cost 1/3 to ½ as much in operation.
- Evaporative coolers are useful in hot, dry areas such as the southwest U.S. They can lower temperatures by 30° by sending outside air through moist pads generating evaporation of cooler air, via fans.
- Economizers are sophisticated air conditioners that select air from outside when outside is cooler and dryer than indoors. Of course, this can be done manually through opening and closing windows at opportune times.
- A whole house exhaust fan can cool a house in minutes, when it is cooler outside by blowing air into an unconditioned attic or outside.