A furnace hot water coil is a device used to heat air in a furnace. It is typically made of copper tubing and is heated by hot water that is circulated through the coil. The hot water is heated by a boiler or other heating source, and then circulated through the coil. The heat from the hot water is then transferred to the air passing through the coil, thus heating the air. The hot water coil is an efficient way to heat air, as it requires less energy than other heating methods. Additionally, it is relatively quiet and requires minimal maintenance. The furnace hot water coil is an essential component of any heating system, and is used in both residential and commercial applications.