An access device for installation to a condensation drainage system, such as a drain line of an HVAC unit, includes a main body with connecting ports at opposite ends for sealed, fluid flow inline connection to the drainage system. A top cover releasably secures to the main body to completely cover and seal an open top and interior cavity. A cleanout adapter has a flange that surrounds a bottom opening. The flange includes a seal around its outer periphery and is structured for sliding receipt within either of two transverse slots within the interior cavity to position the bottom opening of the adapter in sealed alignment with either of the connection ports, whereby a pressurized gas or liquid flow source, or a vacuum source can be connected to a top opening of the adapter for clearing a clog in the drain line either upstream or downstream of the access device.