Performed furnace maintenance on 4 year old Lennox unit. Cleaned return grate. Inspected filter and found it to be in good condition. Flushed condensate drain with water. Temperature rise is within limits. Inspected flame sensor. Tested resistance of hot surface igniter. Measured external static pressure and found it to be high. Inspected ductwork and found it to be an inefficient design that utilizes a remote plenum which is increasing the static pressure. Inspected blower compartment and found the blower wheel to be clean and balanced. Electrical amperage draws are all within limits. Upon departure system is operational.