Normally they beef if the flue pipe is "Backwards" or if the water vapor can condense and not run back down the flue proper. That plus clearances around the flue pipe. Could the word Backdoor or DRCKDR maybe be the word "backwards" hopefully spelled frontwards? Might be a form of pigeon English from a girl that got too much diversity training up.
You can put the flue sections together backwards so each section joint will leak if any condensate is running back down. Sort of difficult to explain but each flue section has a fluted tapered end and a smooth (belled) end. If you do it one way, the section joints will not leak, do it the other, they may.
That and they want the flue pipe to not have lots of elbows, horizontal runs or a type of cold trap built in. Depending on the run, a pitched sloped run is preferred over a straight up run with elbows and then horizontal sections.
Maybe the good olde boy is a former doctor turned heater guru and he still thinks he is writing prescriptions.
Calling about the problem is sure to get, ...... For English press 159...... for Malyaese Press 1.