An HVAC system utilizes one or more modular heat pump units installed within an outer wall or corner of a building, and one or more active registers. A central hub, which can be integrated into a modular HVAC unit, receives input from thermostats and sensors in the building and adjusts operation of the modular HVAC units and active registers to distribute conditioned air throughout the building. The system provides convenience and comfort of a traditional split HVAC system with significantly reduced installation and maintenance costs. The modular HVAC unit includes an interchangeable cartridge that contains the active components of a heat pump. Cartridges of different capacities can be installed as required, and are easily swapped out for maintenance, servicing, and replacement. A user can control the system using traditional-style thermostats and with a smart phone, tablet or computer. A cloud server provides backup services and remote access to the system.