Only noncorrosive materials should be used to avoid the pitting that corrosion creates. Likewise, the sprayed foam commonly used on drain pans and other interior surfaces in the past is no longer acceptable because it is so difficult to clean. All areas of the HVAC equipment and duct system should be totally accessible for inspection, cleaning and maintenance. It's an established fact that equipment that's easy to service gets serviced first and most often. Areas accessed frequently, such as filter and cooling-coil sections, should be equipped with access doors.
Easy access is especially important for fan coils and unit ventilators.
These types of systems typically consist of many units, making inspection a time-consuming process. Although fan-powered VAV terminals do not have cooling coils or drain pans, they must still be designed for easy filter replacement and accessibility for inspection and other normal maintenance within the tight confines of a ceiling plenum. Remember that access for inspection and possible cleaning is required in addition to the standard service clearance requirements.