Closed-loop systems are designed to transfer heat through circulating water for air conditioning, heating, or process applications.
In many HVAC applications, the closed-loop water is circulated through an air handler coil, which either cools or heats the incoming air. In some cases, the air handler is designed to first expose the incoming air to a chilled loop, which lowers the humidity of the incoming air, and then exposes the cooled/dry air to a hot loop to moderate the temperature of the air before it is introduced into the building.
In other HVAC applications, the closed loop water acts as the condenser for remote water-source heat pump installations. By design, closed loop systems should not lose water.