Performed heating maintenance on 3 year old Lennox unit. Cleaned return grate. Replaced filter with owner-supplied MERV 16 filter. Temperature rise is within limits. Inspected blower compartment and found blower wheel to be clean and balanced. Tested resistance of hot surface igniter. Cleaned flame sensor. Tested pressure switches. Tested safety switches. Flushed condensate drain with water. Inducer amp draw and total system amp draw are within limits. Prior to departure I ensured the furnace was operating normally.