Performed preventative maintenance on 3 furnaces. Changed out filters for customer. Inspected the blower assemblies, motors, other electrical components, burners, heat exchangers, inducer motors, and overall condition of equipment. Cleaned flame sensors and brushed dust off the burners. Checked thermostat operation, wiring, amps, temperatures and gas pressures. All furnaces are heating properly within manufacture specifications at this time. Lennox unit had no visible cracks in the heat exchanger, but is showing signs of wear.