A support surface system provides alternating pressure therapy and low air loss therapy simultaneously using a single pump. The system includes a plurality of cells. Each cell has a foam-filled lower chamber and an upper chamber. Fluid communication between the lower and upper chambers is controlled by a normally closed valve that opens if the pressure within the lower chamber exceeds a predetermined threshold pressure. Pressurized air is alternatingly supplied to the lower chambers of a first subset of cells and the lower chambers of a second subset of cells, thereby providing alternating pressure therapy. When the pressure in the lower chamber exceeds the threshold, the valve opens to allow air from the lower chamber to flow into the upper chamber. Air is expelled from the upper chamber through perforations therein, to remove moisture and humidity and possibly reducing the temperature of the micro-climate beneath a patient.
