Depending on the model, Weil-McLain gas boilers can be cleaned with trisodium phosphate or specifically designed cleaning products such as Sentinel X400. The TSP cleaning method can only be used on systems that do not contain aluminum.
To clean the heat exchanger in Ultra-80 and Ultra-105 models, shut down the boiler and let it cool down to room temperature. Remove the jacket front door; close the gas valve; disconnect the gas line, wiring and connectors; and remove the air silencer. Pull the assembly out, remove any debris on the heating surface with a vacuum cleaner, and wash the surface with clean, warm water. Reinstall and reconnect the assembly.