A hot water coil for an air handler is a type of heat exchanger that is used to transfer heat from a hot water source to the air that is being circulated through the air handler. This type of heat exchanger is typically found in commercial and industrial applications where the air needs to be heated to a specific temperature. The hot water coil is made up of a series of metal tubes that are arranged in a serpentine pattern. The hot water is circulated through the tubes, and as it passes through, it transfers its heat to the air that is passing over the tubes. The air is then heated to the desired temperature and is then circulated throughout the building. Hot water coils are an efficient way to heat air, as they require less energy than other types of heating systems. They are also relatively easy to maintain and can last for many years with proper care.