Flue in flue...

[John Rumm]

Was looking at possible positions where someone could site a new boiler... There is a nNice space on the wall in the kitchen which would be ideal (above where the old floorstander boiler is currently fitted). However there is a partially removed chimney breast on the outside of the wall that is in the place the flue would normally exit. (its corbelled out from the wall about 5' off the ground, and then continues up to a normal chimney stack).

So I said that they would probably need to have an extra extension in to boiler flue to run half a meter to the side in the kitchen first, before turning and exiting the wall in the normal way so as to miss the old chimney stack.

That got me wondering though, although it does not seem "right" I can't put my finger on any rule for not sticking the normal flue right through the old stack - its completely unused and only about a foot deep. Can anyone think of a reason?

[Michael Chare]

Try and download boiler installation manuals. They will tell you the flue requirements.

[Andrew Gabriel]

Internal flues (and it might count as that, not being external) now have to be accessible for checking (e.g. if enclosed, behind removable panels). How accessible would it be inside an old chimney stack?

That run of flue would probably cost more than the boiler, unless the boiler can use bog standard muPVC pipework as a flue.

You could do this with my Keston split flue, but there were rules about placement of the air inlet - it didn't have to be next to the exhaust, but it couldn't be on opposite of a building or anywhere where wind could create a pressure differential. Also, a long exhaust flue needed a separate condensate drain point fitted, not just running it all back into the boiler.

[John Rumm]

Not the kind of question they answer... (I know all the normal rules on positions and distances from windows, corners etc)

[John Rumm]

Ah, I think you are over reading the question - I was not suggesting routing the whole flue up and along the old stack, simply drilling a core right in one side of the stack and out the other - hence a standard length flue kit will go horizontally straight through it rather like it were a 24" thick wall.

Yup sure...

I was just thinking of normal concentric standard flues here. i.e. whether it would be worth avoiding the slight extra expense of an elbow section and a visible run of flue in the kitchen.

[Tim Watts]

I know it's a balanced flue, but is it any worse than showing other flue liners through old stacks?

Can the flue pipes be assembled, locked together and fed into the stack whilst being certain the joints have not parted in the process? Do boiler flues need to be supported every x centimetres?

[meow2222]

Bricks, stones etc tend to fall down old chimneys

NT

[John Rumm]

Everyone seems to be misreading this... ;-)

Cross sectional view:

#C| #C| #C| #C| /====F BB#C| BB#C| BB#/ BB# # # # K W O

B = boiler, F = boiler Flue, C = disused chimney, K = In kitchen, W = Wall, O = Outside

There would be no joins - the chimney is only 12" thick and the flue would go across it, not up it.

(this is only an academic discussion, since in reality a quick dog leg in the kitchen will let the new flue miss the chimney and that ought to cause less confusion - which is generally a good thing)

[Tim Watts]

:)

I cannot see *anything* logically wrong with that - especially as you could clearly get an unjointed flue section right through the chimney so no hidden joints.

The next logical option would be to bore a bigger hole right through then line that with a big metal sleeve then pass the flue through, sealing on the outside, bit like passing a gas pipe through a cavity wall.

Actually when you think about it, what's the difference between the above and passing a flue through a cavity wall?

Cheers

Tim

[John Rumm]

Well indeed - its already a dual skinned arrangement... its one of those cases where it somehow feels not right, but I can't think why.

(although I suppose it would be harder to core drill and get both holes in line)

[Andrew Gabriel]

My biggest worry would be the structural integrity of the chimney stack, particularly near the corbeling.

[Dave Liquorice]

I didn't ... B-)

Others have mentioned stones etc coming down the chimney, I'll add to that a possibly corrosive atmosphere so a metal flue might not last very well.

I'd go with the "gas pipe across wall cavity" liner approach. Totaly isolated from the old chimney path for the new boilers flue and using a decent bit of somthing for the liner physical protection as well.

[John Rumm]

Yup I think that probably is the greatest unknown - the hole would probably be above the corbelled section, but one does not know what is sitting in the bottom of it. I think for the sake of a slight increase in flue price its worth side stepping it with a short run to the side and an elbow.