Cleaned & checked condensing gas furnace. Inspected heat exchanger & flue venting. Complaint: exhaust smell in basement where PVC vent piping goes outside. I sealed with caulk pvc exits. Clean and check humidifier, replace pad, checked water flow and drain. I set up system for three stages instead of modulating for proper airflow through humidifier. Set humidifier to dewpoint Control. Checked temperature rise. 41. Rate: 60. Blower: 1008. Supply L131, R106. Suggestions: need to adjust perpetual maintenance plan to add humidifier.