The most common type of central air conditioning is the split system, which features a large, boxy condenser outside the home and a fan-and-coil system inside, connected by pipes carrying refrigerant. The air is distributed through ductwork. However, not every home can accommodate the ductwork needed to install central air. Split ductless systems are an option for those homes because, as the name indicates, they don’t require ductwork.
The heat pump gained popularity in the 1950s in Japan and the United States. Heat pumps can extract heat from various sources, such as environmental air, exhaust air from a building, or from the ground. Initially, heat pump HVAC systems were only used in moderate climates, but with improvements in low temperature operation and reduced loads due to more efficient homes, they are increasing in popularity in cooler climates.
Heaters exist for various types of fuel, including solid fuels, liquids, and gases. Another type of heat source is electricity, normally heating ribbons composed of high resistance wire (see Nichrome). This principle is also used for baseboard heaters and portable heaters. Electrical heaters are often used as backup or supplemental heat for heat pump systems.