Answer: yes, the heater must be tubed at the roof outlet.
Please note: the installation of a chimney flue is highly regulated. The regulations stipulate that the casing of a duct must be continuous, from the heater to the roof outlet. It is forbidden to tub only part of the duct. Adapted products allow you to cope with all the configurations of your home. If your duct is bent, you can for example use a flexible tube. Every year, accidents due to poorly executed installations kill dozens of people in France - fires, but also poisonings not to mention fatal accidents. Also remember that in the event of a claim, your insurance will only compensate you if your installation has been carried out in accordance with the rules and in accordance with the legislation. You too, send us your DIY question.