Performed routine maintenance on Navien NPE-240A tankless water heater. Cleaned all inlet and fresh air filters. Located leak coming from condensation drain line. Line is undersized and piped with metallic fittings which corroded due to the high acidity of the condensation. Reworked drain line with cpvc. Client also was having temperature issues at all the showers. Internal check valve on the unit has failed and there was no isolating shutoff or external valve on the recirculating return line. Added both missing valves and hot water issue was resolved. Tested everything for leaks and proper function.