HVAC systems require the use of filters to ensure that the air remains clean and free of harmful particles. There are several types of filters available, including mechanical filters, electrostatic filters, and HEPA filters. Mechanical filters use a physical barrier to capture particles, while electrostatic filters attract and hold particles through an electrostatic charge. HEPA filters are the most efficient and can capture particles as small as 0.3 microns.

The cleanliness level of an HVAC system refers to the amount of clean air that is delivered to a space. It is measured in air changes per hour (ACH), which is the number of times the air in a space is completely replaced per hour. The recommended ACH for commercial spaces is around 4-6, while the ACH for hospitals and laboratories can be as high as 12-15.

When designing an HVAC system, it is important to consider factors such as the size of the space, the type of occupancy, and the expected level of air pollution. Proper ductwork design and placement of air registers are also important considerations. The system should be designed to provide even distribution of air throughout the space and prevent the accumulation of stagnant air.