Most air conditioners and heat pumps sold in the U.S. — including most split-system air conditioners and ductless minisplits — are air-to-air heat pumps. During the winter, these appliances extract heat from the outdoor air and deliver warm air to a house through ducts or small fan-coil units. During the summer, these appliances deliver cool air to a house and dump unwanted heat into the outdoor air.
Another type of heat pump, an air-to-water heat pump, produces hot water (or chilled water). When used for air conditioning, an air-to-water heat pump is called a chiller. Almost all air conditioners cool an air stream by blowing air past a copper coil. In a conventional split system air conditioner — called a DX (or “direct expansion”) system — the fluid in the copper coil is a refrigerant. In a chiller-based system, however, the fluid in the copper coil is water (or in some cases, a solution of water and antifreeze).
When used for space heating, an air-to-water heat pump fills the role usually held by a boiler; it requires a hydronic (water-based) distribution system. The heat pump makes hot water; the hot water can be pumped through fin-tube baseboard units, in-floor PEX tubing, or a fan-coil unit. (A system with such a fan-coil unit is called a hydro-air system.) The fan-coil unit can be a small wall-mounted unit that looks like the indoor unit of a ductless minisplit system, or it can be a large fan-coil unit connected to a conventional forced-air duct system.
Hydronic systems have pluses and minuses
The main reason that American homes usually use ducts to distribute heat is that most Americans require air conditioning. In climates where air conditioning isn’t required — for example, in northern Europe — space heating systems are usually hydronic…