Performed a preferred customer furnace cleaning. Cleaned the burners and the flame sensor. Checked the furnace heat exchanger. Wiped down the inside of the furnace. Checked the electrical. Cycled the furnace and performed a furnace combustion test. The furnace is running at this time. Noticed that the furnace is vented with 4 inch single wall tin. Type b venting is recommended. Office to call the customer to see if they want it fixed.