Heat exchangers are central components in heating, air conditioning, and ventilation systems. They transfer heat between two fluid streams, typically water, air, or refrigerant, without mixing them. Common designs include plate heat exchangers, shell-and-tube exchangers, and fin coils. In ventilation engineering, cross-flow exchangers or rotary heat recovery units are used to transfer heat from exhaust air to supply air, reducing energy consumption. The efficiency of a heat exchanger depends on the surface area, the flow arrangement (parallel, counter, or cross flow), and material properties. Regular cleaning and maintenance are essential for efficient operation and long service life.