Kitchen extractor advice sought

[blackworx]

Having read multiple threads on this issue in various forums, I expect I will probably get mainly "argh don't do it" type replies to this, but I can't for the life of me figure out any other way to make this work.

Two storey Edwardian terraced villas (1 flat lower/1 upper). We are on the ground floor and fully renovating.

We do a LOT of cooking (OH is a former chef) so vented extraction for the kitchen is something we really, really want to have if possible. I am currently planning this however the kitchen has no external walls, and no access to external walls. The only way out is up the chimney, which is about a 5 metre vertical run.

As part of the structural work already done to maximise floor space we opened up the chimney breast and placed the lintels very high up, exposing the chimney throat in the process and leaving lots of vertical headroom to accommodate an extractor/hood.

I'm not deluding myself that a standard residential extractor hood is going to be able to create enough pressure or move enough air to vent properly this way, so I am thinking of installing either a small commercial extractor hood like this one, or, as an alternative, one of these inside a custom-made hood. The latter claims to suit ducting runs up to 30m - doubtless pure fantasy - however 5m of smooth flue liner with a couple of bends and a cowl should theoretically be well within spec.

On that note I'm also aware I'm going to have to line the chimney, as well as fit a condensate trap. The need for a condensate trap is making me lean towards the custom hood option; it would allow space to fit a concealed condensate evaporator which I'll need since there is no access to drainage.

Given all the arms and legs my plan is now growing, I'm on the verge of throwing in the towel, finding a decent recirculating extractor and resigning myself to replacing carbon filters every six months.

I'm hoping for input from anyone with knowledge or experience of doing the same or similar. As I mentioned before: from everything I've read so far it seems like repurposing chimneys for extraction purposes is generally frowned upon, and for good reason. Either tell me I'm mad and should just get a recirculator, or give me a glimmer of hope that it has been and/or can be done, and is worth doing. The practical aspects, additional cost, time and trouble aren't an issue for me; it's more a question of whether or not it will all be worthwhile or just wasted effort.

 

[foxhole]

Second unit looks best option, you can get grease filters to add to a purpose built unit . Maintaining it is essential , so regular cleaning of filters .Allowing fan to run for a few minutes after use will reduce any condensation .
Is running vent thru adjacent room to exterior via ceiling boxing not an option?

 

[blackworx]

Thanks, yeah no suitable vent through options that aren't way more complicated. To the left is stairwell for access to upstairs property, to the right is 1950's kitchen extension, and through the wall on left and right is neighbour two doors down. There is a small area of external wall in the opposite corner, however it's entirely taken up by the window and its lintel so no way out that way either.

 

[jj4091]

Before you do any work put a smoke bomb in the alcove and check that the chimney is not blocked and draws air. I presume you have some form of ventilation in the kitchen.

 

[blackworx]

Should've said already had it checked and cleaned so we know it's not blocked.

I don't think we need it to have a draw for a forced air system like an extractor, although now I think about it, the draw will certainly help when it's in use. Might be a heat loss/noise issue when it's not though... Hmm.

 

[endecotp]

Is this going to pass through your upstairs neighbour's flat? What will they think about the noise? Whose permission do you need?

 

[blackworx]

I'd be very surprised if they can hear air moving through a flue liner, three courses of brick and an inch of horsehair plaster. Also they have the whole chimney breast hidden behind a stud wall. Technically we don't need anyone's permission to line our own chimney, however we are on good terms with the neighbours and have spoken to them about our plans as a courtesy and to advise them we'll need access to the shared roof to insert the liner, and they are fine with it.

 

[mrrusty]

I think I would seek advice from a professional catering extraction company. The condensation hazard is not just water, but cooking fats and grease that will condense on the inside of the flue potentially presenting a fire risk, which the chimney effect will exacerbate should it ever ignite. In the commercial world cleaning of ductwork is regulated and an insurance requirement.

 

[blackworx]

Good point. I'd plan to inspect it every now and then, especially in the early years, to get a handle on just how quickly grease buildup is occurring and whether and when it's going to need cleaning.

That said, whilst we do a lot of cooking, I mean "a lot" in domestic terms. So a use rate of a few hours a week, and without a huge amount of grease being generated. As opposed to a chippy pumping frying fumes of food to feed dozens or hundreds of people up it 12 hours a day, 7 days a week.

Plus it's not as if grease build-up would be unique to our situation - all domestic kitchen extractor ducting will experience that to some degree - especially the "crinkly" flexible stuff that the majority are installed with.

 

[jj4091]

Can't see why you should have a problem then. Using a chimney is no different than boxing ducting in.

 

[mrrusty]

Plus it's not as if grease build-up would be unique to our situation - all domestic kitchen extractor ducting will experience that to some degree - especially the "crinkly" flexible stuff that the majority are installed with.

It's not the distance, or even the direction - it's the temperature gradient. A normal kitchen extractor will be extracting warm humid air full of cooking byproducts. The short duct runs mean that the gases and vapours are still warm gases and vapours when they get to the open air.

In a chimney situation you inevitably have a much bigger temperature gradient between the input and the output, and quite likely a slower airstream because you are pushing air upwards, so inevitably more often the top of the chimney will be cold enough for the gases and vapours to condense inside the flue before they escape.