There are two main types of ventilation in HVAC systems: natural and mechanical. Natural ventilation relies on the natural flow of air through open windows and doors to provide fresh air to the space. Mechanical ventilation uses fans, blowers, and other equipment to move air in and out of the space. This type of ventilation is more efficient and can be used to control the temperature, humidity, and air quality of the space. Mechanical ventilation can be further divided into exhaust, supply, and balanced ventilation. Exhaust ventilation removes stale air from the space, supply ventilation brings in fresh air, and balanced ventilation combines both exhaust and supply ventilation to maintain a balanced air flow.