A split system is designed with  separate outdoor unit for cooling and an indoor unit for heating. The indoor unit can be a gas furnace with an evaporator coil, an oil furnace with an evaporator coil or an air handler with built in evaporator coil and electric heating elements. The indoor unit usually supplies the airflow for the heating system and the cooling system.

The outdoor unit is called the condensing unit. It is connected to the evaporator coil by 2 copper lines. The small line supplies high pressure liquid refrigerant to the evaporator coil. The large line returns the low pressure refrigerant vapor back to the condensing unit. This line will be below ambient temperature and should be insulated.

Since the indoor unit is used for heating and cooling the air filter is used for both modes. Keeping the air filter clean is critical to the proper operation of the system.