I have been a heating engineer for nearly forty years and have never had any trouble with any kind of flue until condensing boilers came on the scene.
We have just installed 51 Glowworm 38 CXi combi's in a block of flats, all with extended horizontal flues, that the manufacturers instructions say, must rise to outside by 44mm per metre, which they do.
When it rains, water runs into the outer flue, air intake, and down the flue back to the boiler and drips out of the case. It also leaks sometimes out of the flue joints as Glowworm flues do not have a seal on the outer pipe, and they are made of steel, so they rust !!!!
We have contacted Glowworm technical who admit that this " can be a problem " but they don't have a solution !!!
Surely it's not rocket science to design a terminal with the air intake facing downwards, with an extended turret so that rain water drips off of the bottom to outside. They could even make a retro-fit unit to click on the end ( like the anti-plume kit does on the inner flue )
So anyone who has this problem, as far as Glowworm are concerned, although I know other boilers have this problem, you're on your own !!!