Central Heating Flue Pipe

[Noggin-the-Nog]

Can my new central heating (condensing) boiler output through a gable wall without the need to add an external pipe (in the same way as a hob extractor fan does)?

[frugalfred]

Yes, but needs to be protected by a cage if accessible (reachable) from the ground by person walking past.

[heathfield]

Yes. Pick up a Wickes leaflet. See page 4 here...

http://www.wickes.co.uk/content/ebiz/wickes/resources/images/gil/79.pdf

[The Builder]

Usually, yes. Heathfield's link explains it well.
It rather depends on what's the other side of the wall. If it's next door, then maybe a "high-level" flue position is needed. The other usual constraints apply, such as proximity to window and door openings etc.