Furnace operating upon arrival. Performed Furnace tune up per Westin Air metrics.Replaced 20x25x1 filter by customer. Performed combustion analysis found no traces of carbon monoxide in return air or supply air at this time. Removed high limit to inspect heat exchanger with camera. NO signs of holes or cracks at the best of my ability. Set THERMOSTAT to customer request 70°. FURNACE IS OPERATING NORMAL AND SAFE AT THIS TIME. Recommend installing a whole house humidifier to maintain humidity level in winter.