In an equally mild manner

. I was disagreeing with your inference that the butterfly valve would not be effective, but I do think that it may have been a more sensible design to have put the flue lower down in the chamber. They may have tested this however and found it not to be the case.
Anyway, their design is what it is and and without talking to someone who has tried one first hand any theoretical musing about its strengths and weaknesses are just that. On the same note though, it may also not have been very fair for it to have been so openly criticised without first having talked to someone with experience of using it.
Anyway enough has probably been said on this subject. I will be quiet now and await the responses to the questions that you have asked of them...