Circulation and external venting unit

An air circulation and ventilation unit comprising:

• a housing configured to fit at least partially over the ceiling of an equipment cabinet including fitting over at least two vents in the ceiling of the cabinet, so that at least a majority of the interior of the housing is in permanent communication with the interior of the cabinet so that air can always flow between the interior of the housing and the interior of the cabinet through the vents in the ceiling of the cabinet;
• the housing having a housing vent comprising a closure member moveable between an open position and a closed position, wherein in the closed position the closure member closes the housing vent so that little or no air is remitted to flow through the housing vent between the inside of the housing and the outside of the housing, and which enables air recirculation in which air is remitted to recirculate between the interior of the housing and the interior of the cabinet by flowing from the interior of the cabinet into the interior of the housing through one of the vents in the ceiling of the cabinet, then through the other of the vents in the ceiling of the cabinet from the interior of the housing into the interior of the cabinet, and wherein in the open position air is permitted to flow through the housing vent between the inside of the housing and the outside of the housing and in which said recirculation is substantially prevented by the closure member, the housing vent and housing vent closure member being positioned so that in both positions of the housing vent closure member air is able to flow between the interior of the housing and the interior of the cabinet;
• and
• a controller to control the position of the housing vent closure member.