Last year we had a new kitchen fitted, which included, for the first time, a cooker hood.
Cooker hood worked fine until a few days ago, when my wife noticed a problem. When it's not in, air comes in from outside, making the kitchen a little cold. (This didn't happen over the winter.) Clearly there is some sort of mechanism that normally keeps the cold air out, which has stopped working.
I might add that when it was fitted, the pipe that connects with the exterior of the house didn't have a grille put over it - hence there is a hole in the wall with a pipe about 6" in diameter. Just the sort of thing a bird could fly into - and indeed, a bird did fly into it last month.
What do we do?