However if a vertical flue is badly fitted then yes you could have a leak on your hands.
Boiler flue through conservatory roof.
Especially if it s not sealed properly.
The boiler or its flue as close to the conservatory as you re proposing to put the conservatory to the boiler flue then you or your conservatory builder would be in contravention of section 8.
No boiler builder will allow to put a elbow out and then a vertical channel through the roof of the conservatory as the terminal would be too close to the wall of the house.
Note teh gsiur make this a criminal offence not just a matter of a tiff with.
How far this is will depend on the size of your boiler but is typically between 30 60cm above below or to the side of an opening.
If everyone who installed a vertical boiler flue through a roof had to fix leaks right left and centre then no one would install them at all.
If the flue terminal is more than 2 3m away from the taller main house then the flue does not need to rise above the main roof.
However the boiler flue manufacturer s instructions are final so if there s nothing specific in the vertical flue kit installation manual then call the manufacturer for advice.
However to comply with the approved document j of the building regulations the flue still needs to rise a total height of at least 4 5m unless the appliance manufacturer specifies less.
Think of it this way.