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The heater core is a small, radiator-like unit that provides a large surface area for heat dissipation into the passenger compartment. It made of a series of tubes surrounded by fins. As air passes through the tubes and fins, heat is transferred into the air.
A clogged heater core is a common problem that can reduce heating system efficiency. The inside of the core can become filled with rust from engine coolant system, especially if it has been a while since you last changed the coolant. Therefore, periodically, a car's heater core has to be flushed in order to rid it of the debris and dirt that builds up within it over time.
Symptoms of a clogged heater core: