Most air conditioners and heat pumps sold in the U.S. & Canada, including most split-system air conditioners and ductless mini splits, 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 mini-split system, or it can be a large fan-coil unit connected to a conventional forced-air duct system.
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