An installation can be done in a day to keep water shut down time as short as possible.  The process includes the removal of existing old boiler parts, then the installation of the pre-designed tankless water heaters and components.  Once installed, the multiple units allow one unit to be off-line at a time while the remaining units carry the load during the maintenance.  Please see video below for a full installation that started in the morning, with the rising of the sun, and completed by afternoon the same day!