A hot water coil is a type of heat exchanger that is used to transfer heat from one medium to another. It is commonly used in heating, ventilation, and air conditioning (HVAC) systems to heat air or other fluids. Hot water coils are typically made from copper or steel tubing and are filled with a liquid or gas, such as water, oil, or refrigerant. The hot water coil works by passing the heated liquid or gas through the tubing, which then transfers the heat to the air or other fluid that is passing over the coil. The heat transfer occurs as the heated liquid or gas passes through the tubing, and the heat is transferred to the air or other fluid. Hot water coils are an efficient and cost-effective way to heat air or other fluids, and they are often used in industrial, commercial, and residential applications.