The factory placement of the 25mm cooling fan right up against the heatsink results in only 2 of the 11 slots actually getting proper airflow. As a result my Ultimaker 2 would jam and stop printing after about 30 minutes of printing due to the hot end isolator and PTFE coupler getting far too hot.This duct is held in place by the back two thumb screws and the fan can be screwed to the duct with either the two included screws or 4 5/16" 4-40 screws. You may need to trim away some black plastic from the hot end carrier to let the heater and thermometer wires clear without pinching.I highly recommend putting thermal paste between the hot end isolator and the lower hot end carrier. Also between the lower hot end carrier and the heatsink itself.