On all Navien Condensing 98% "A" Series models, there are two pre-heating modes in which the unit can operate: Internal Circulation Mode and External Circulation Mode. The desired mode is selected by manually adjusting a three-way valve inside the unit and adjusting the DIP switches on the computer board. Unlike other tankless water heaters, Navien's Condensing 98% doesn't require an additional pump or an electric control device, significantly saving on supplementary installation costs