A heating, ventilation and air conditioning (HVAC) coil is part of an air conditioning and heating system. There are actually two HVAC coils in a typical system. There is a condenser coil in the external air conditioning condenser and an evaporator coil in the indoor unit. The coils are what increase or decrease the temperature and pressure of the coolant gas or liquid before passing it to another section of the HVAC system.
To understand how the coils work, one first must understand how an HVAC system works. In summer, the HVAC system pulls hot air into the system, cools it, then distributes it throughout the building. In winter, the system pulls cold air in, extracts the heat from it and passes that heated air through the ductwork in the building. HVAC coils play an important role in this process.
The compressor in the exterior unit is a pump that pulls hot, low pressure refrigerant or coolant gas from inside a building and will convert it into hot, compressed gas. That hotter gas is forced over the exterior coil and gets cooled off as the hot air from inside the building is blown to the exterior. The loss of heat from the coolant, however, turns the gas into a liquid form that is still very hot and pressurized.
The liquid flows into the evaporator coil that is found in the inside HVAC unit. As the liquid enters the evaporator HVAC coil, it is able to expand and cool. It is cooler than the surrounding air, so it absorbs heat out of the ductwork that is part of the HVAC system. This leaves cooler air in the ducts to be distributed throughout the building. As the cool air is dispersed through the duct system, the cooler, low-pressure gas recycles back to the exterior compressor HVAC coil to start the cooling cycle again.
In winter, an HVAC coil works in the opposite way. The heat pump pulls air in from the outside and runs the air over the evaporator coil. The colder air is warmed by the evaporator coil and then pumped through the ductwork to heat the building. Even though the air outside a building might be freezing cold, the HVAC system is able to pull some heat from it. In fact, an HVAC coil can pull heat from air that is well below freezing.