Swapping old oil Aga for electric induction range - extraction question

[BodgeWary]

Hi, hoping to get some pointers on this conundrum..

We have an old 1970's oil fired Aga that's being removed, in its place will be a new electric induction Aga (lead time is 18 weeks so gives me some breathing space)..

Got a question on the extraction. I've got no professional trade background so apologies if this is a dumb question.

The current Aga has its fumes extracted as you'd expect via ducting, but it's on an internal wall and so the ducting heads up an internal path, through a concrete slab on the 'ceiling' of the Aga's inglenook, through the first floor (ducting is boxed in), through the loft & out of a vent on the roof.

Since there won't be any need for gas extraction after the electric Aga gets installed, would it be possible to repurpose the current fume ducting for the induction hob extraction? Everything I've read seems to say the maximum extraction length from a powered fan is around 5 metres, obviously getting the steam & smells out of the roof is a much longer stretch than 5 metres, so I'd be worried about condensation building up inside the ducting, running back down and dripping out onto the hob.

Would this be a legit concern? Can you get high powered extraction motors that can make the distance, or would a recirculating fan be the best option?

The slab of concrete is pretty thick (at least 2" we think) so will be a pain to work with regardless, but i'd be fine cutting into this to recess a rectangle recirculating fan (with lights) if needs be, but it would be a shame to waste the opportunity with the current ducting if it can be repurposed. Unfortunately the slab is relatively low (around 1.62m from the floor) so not sure we could install anything that isn't recessed..

Any ideas?

 

[Exedon]

As a business we have installed many cast iron (and enamel steel) cookers on all fuels.
First if the concrete register was cast in place its probably 100m at least thick with rebar .
Sounds like you have a prefabricated flue probably 5in but sometimes 4in were fitted.
In truth I would not use it for venting, there is no doubt that at some stage of its use the aga won't have run cleanly.
High probably of very black soot on walls of flue pipe.
That mixed with possible condensation not what you need dripping down!

 

[BodgeWary]

As a business we have installed many cast iron (and enamel steel) cookers on all fuels.
First if the concrete register was cast in place its probably 100m at least thick with rebar .
Sounds like you have a prefabricated flue probably 5in but sometimes 4in were fitted.
In truth I would not use it for venting, there is no doubt that at some stage of its use the aga won't have run cleanly.
High probably of very black soot on walls of flue pipe.
That mixed with possible condensation not what you need dripping down!

Good shout on the soot, had it sweeped regularly and the guy said there's basically nothing there, but would still have a thin layer on regardless... agree it's no good risking that dripping onto the hob so reusing the ducting is definitely out the question

Removed the fireboard and seems you're right, looks like it's cast in place, 100mm.

So looking up, just realised that it's basically a brick chimney housing the ducting - so the slab & nook is load bearing.

Which presumably means I can't cut out a rectangle to recess a recirculating fan regardless of it having rebar, since the whole thing could collapse?

Ideally don't want to remove chimney & rebuild the inglenook, so options are running out here.

Option 1) Plaster over hole, install lighting only & put extraction elsewhere - only exterior wall with space available is the opposite side of the kitchen, and that's going into an attached unheated lean-to.
Option 2) Remove ducting & refit steel ducting meant for the job.. i guess this would be a big job, scaffolding needed? And would need access to the chimney internals somehow as i think there's a couple of 45 bends before it reaches the 1st floor. I reckon its a 5-6 metre run from the slab to the top of the outlet, so its at the max range of extraction fans from what i read anyway & that's not considering the bends.

1st option probably best all things considered and just vent into the garden room instead of outside. Any legal considerations there in terms of building regs?

 

[Exedon]

That appears to be a section of flexible stainless liner coming down (kopex) Internally it will be filthy! Will need care to remove it oil based soot isn't nice!
Will leave it to others to comment on construction/building work.