Coils can be used for many different applications such as: cooling, heating, pre-heating, pre-cooling, and re-heating. The three types of fluids used are: water, refrigerant, and steam. Preheating and precooling coils are used to heat or cool outside air coming into the back of an air handling unit. When temperatures get below freezing the preheating coil would come on to keep water coils downstream from freezing. There are a few very important things to remember when balancing flow through a chilled water coil. Always check to make sure that the chilled and hot water is being piped to the correct coil in the unit. The heating coil rows are generally much less than the cooling coil. Also check for correct pressure tap, balance valve, and control valve locations. The most common thing to be installed wrong is piping the coil backwards. This is where the supply and return lines get crossed when piped into the coil. A great way to check this is to remember the saying "Warmest Water hits the Warmest Air". This means that the warm air or return air should be hitting the warm water or return water first.