There is a need for a compact and high capacity HVAC system for a vehicle cab. This need is met by a roof enclosed HVAC system. The system includes a roof upper panel, a roof lower panel, a fresh air duct communicated with a fresh air inlet exposed to an exterior of the cab, a recirculation air duct communicated with a recirculation air inlet exposed to an interior of the cab, and a conditioned air duct communicated with an air outlet exposed to the interior of the cab. An HVAC unit is mounted between the roof upper and lower panels. The HVAC unit includes a fan unit, a heater unit and an evaporator unit. The heater and evaporator units are concentrically arranged around and surrounding the fan unit. The fan unit draws air from the fresh air duct and the recirculation air duct and blows air through the heater unit and an evaporator unit and into conditioned air ducts