Performed furnace maintenance on 4 year old Lennox unit. Cleaned return grate. Flushed condensate drain with water. Replaced filter with new owner-supplied MERV 16 filter. Inspected blower compartment and found blower wheel to be clean and balanced. Inspected electrical connections. Cleaned flame rod. Tested resistance of hot surface igniter. Tested all safety switches. Temperature rise is within limits. Installed Plasma Pure and 24v transformer in blower compartment. Upon departure system is operational.