What is typically found in a closed loop heating system?

In a closed loop heating system, fluid coolant is essential as it circulates within the system to transfer heat efficiently. This fluid can be water or a mixture of water and antifreeze, designed to absorb heat from the source and deliver it to the spaces that need heating. The closed loop prevents the coolant from escaping into the environment, allowing for efficient heat exchange and maintaining system integrity.

In contrast to the other choices, open air circulation would allow for heat loss and is not characteristic of a closed loop system. Refrigerant recovery is relevant in systems that use refrigerants, such as cooling systems or heat pumps, but it is not a defining feature of closed loop heating systems. Direct evaporation implies a method where refrigerant changes from liquid to gas directly in the evaporator, which primarily pertains to refrigeration and air conditioning systems rather than closed loop heating systems. Hence, fluid coolant being crucial for heat transfer and system function solidifies its role in a closed loop heating system.