One hot water storage heater, six operating modes. That's how flexible the new DEM C wall-mounted storage heaters from AEG really are. Whether of an open (vented) or closed (unvented) design, in single circuit, dual circuit or boiler mode - in terms both of connection and performance the DEM C series is entirely made to suit your individual circumstances. One hot water storage heater, six operating modes. That's how flexible the new DEM C wall-mounted storage heaters from AEG really are. Whether of an open (vented) or closed (unvented) design, in single circuit, dual circuit or boiler mode - in terms both of connection and performance the DEM C series is entirely made to suit your individual circumstances.