Cleaned & checked gas furnace. Inspected heat exchanger & flue venting. cleaned burners & flame sensor. Checked voltage & amp draw of motors. Test capacitor. Replaced filter. Blower: 4.7A. Inducer: 1.33A . Adjust Gas pressure: 3.5 inches water column. Temperature rise: 52 . Supply: 120 . Return: 68 . Found basement pressure negative (-.01 inches water column) due to leaks in return-air ducting. This can downdraft of flu and fluegas spillage at water heater. No humidifier. Suggestions: Seal return leaks or install make-up air venting to balance pressures.