Arrived to do a performance maintenance. Set up for combustion analysis temperature split and static pressure tests. Found that the system is operating with excessive static pressure. Verified operation of all mechanical and electronic components of the furnace. Checked blower for cleanliness and to see if there is any play in the bearings. Found that the fan is coated in dust. Checked systems heat exchanger to ensure proper venting of flue gas. Found that the heat exchanger is damaged, making the furnace unsafe to operate. Verified operation of the humidifier. Turned the gas off to the furnace due to it being unsafe to heat the home.