I have an kitchen extractor hood that vents air through a 5-inch flexible ducting to a louvred cover in the outside wall.
I'd like to add a similar piece of ducting to vent the tumble dryer outside, plus I'd like to add another length to add an extractor fan to vent the bathroom
The layout of the flat makes it quite easy to bring all three lengths of ducting to the inside of the property next to the existing hole.
Normally I'd just cut a couple of extra holes in the wall and have one duct per hole. However, my lease-hold agreement prevents me making any change to the exterior of the building. Also, the wall is quite high so cutting some new holes would require the expense of scaffolding and/or a cherry picker.
So, is it possible to merge three sets of 5 inch ducting, without the danger of one duct back-venting to another (for example, the bathroom venting back through the kitchen extractor hood).
I know I can get back-draft shutters for a single length but what I really need is a junction box with three entrances and one exit, is this allowed?