Today I arrived on site Initial Assessment: Addressed a call regarding an F5 error code on the boiler unit. System Check: Inspected the heating and DHW system. Operational Status: Found the boiler working normally; no error code was present during the visit. Upon arriving at the property, the client explained to me that the boiler unit displayed an error code a week ago. Since then the boiler continues to operate normally. I proceeded to verify the system visually. I removed the front cover of the unit. This is a WeilMclain high efficiency water boiler with x3 zones and indirect tank. I found oxidation inside the unit in some places.. I pointed out to the client what could be the cause of the previous failure. I showed him and recommended the client to consider doing the heater exchanger cleaning in these units. I told the customer that we have to make a special order to do the heater exchange cleaning. It is necessary to order the heater exchange kit and ignition assembly.