Came out to perform preventative maintenance for gas furnace. Inspected all the electrical components. Confirmed proper high and low voltages. Replaced the 14x25x1. Cleaned the burners, flame sensor and burner compartment. Performed a combustion analysis on the furnace. System is operating at 81.2% efficiency with a CO level of 64ppm. Found system return ductwork undersized as well as filter is undersized for a 100,000 btu furnace. Steve went over system improvement options and customer is interested in updating to an air cleaner and upsizing return duct work. Steve will follow up on Monday after he draws up the specs. Found microbial growth on the wall in the utility room that needs to be inspected/ tested by a remediation company. System is operating but not at peak performance. These upgrades will maximize system performance.