PoP wrote:

I'm thinking of putting a canopy over our back door, and possibly
building a covered walkway between the house and our garage, the walls
of which run parallel and about 3 feet apart. However there's a
problem in so far that the central heating boiler chimney exits
alongside the back door, and this would end up with the chimney
venting into the canopy/walkway. Obviously I'm not going to do that!

There's also an extractor hood vent the far side of the back door, but
I'm more comfortable with extending that if I need to.

Here's a couple of pictures of our back door with the chimney clearly
seen to the left:

http://www.yorkshireplace.co.uk/Door.asp

I know it is possible to install a vertical extension to a CH chimney
because a place down the road has got one, but never having delved
into this sort of thing before I'm not familiar with what can/can't be
done. I'm assuming that if I were to extend the chimney then I have to
take care that the chimney can "breathe" in and out, plus whatever I
mount around the chimney (in terms of canopy/roof) has to be
acceptably fireproof.

The CH boiler is a Potterton PrimaF if that makes any difference.

Advice would be welcome. Does this require a gas fitter to do the job
to ensure it is done right, or is it in the realms of general DIY?

PoP

Actually the boiler instructions are likely to permit the flue to
discharge into such an area [1] except for one problem - you need 1200mm
from all the openings. You would seem to currently have about 500mm to
the door.

You will need to look at the installation instructions for the boiler.
It is likely that a flue with two 90 bends (one inside and one outside
the house) and a vertical flue terminal will comply.

You will need to obtain the flue parts ( 1 bend, 1 vertical terminal, 1
extension peice to replace the existing horizontal terminal).

First you need to really checkout what possible in the instructions,
they might require a minimum 600mm horizontally to the walls - both of
them - which could be the problem. 8-(

The vertical terminal will be designed to go through a wooden deck - the
flue/duct does not get hot because the flue is inside the duct.

The physically hardest part will be getting the old terminal out of its
hole - it looks like it's been cemented in.

However unless you are competant to work on gas appliance flues you best
get in a pro.

[1] "Car ports" are defined as having having a roof and open on 2 or
more sides.
--
Ed Sirett - Property maintainer and registered gas fitter.
The FAQ for uk.diy is at www.diyfaq.org.uk
Gas fitting FAQ http://www.makewrite.demon.co.uk/GasFitting.html
Sealed CH FAQ http://www.makewrite.demon.co.uk/SealedCH.html