Ducting for an extractor hood - what's the point?

[James Carmichael]

Hello,

I live in an Edwardian house with an original 9 inch brick flue in the kitchen. This is as built above the original stove and has spent most of its 100+ years merrily transporting cooking fumes and smoke out of the kitchen, from the days of cast iron and coal onwards. It's in good nick.

Don't worry, this isn't a question about whether I can vent my nice new cooker hood up the chimney. I can. I've worked with a specialist cooker hood design company to design one that will have the lift necessary. That's not in doubt, neither is it in doubt that I *do* want to vent up the chimney. I'm not in any way interested in digging through the walls of my house, even if it might end up being a cheaper option or allow me to go 'off the shelf'.

However, the cooker hood people are 'recommending' that I line the chimney with flexible aluminium ducting. I suppose I can; it'll involve scaffolding, removing and re-bedding a castle pot and hiring in a man for the day to push at one end while I pull at the other, but it can be done.

But what's the point? Given the chimney has spent the last 100 years venting kitchen crap unlined, and the extraction design doesn't involve any inline fans half way up the chimney, why can't I just the vent the hood up it unlined?

Many thanks

James

 

[SFK]

Hot coal fires heat the bricks all the way up the chimney and reduce the amount of creosotes that stick to the bricks. (Note that log burners run cooler in the chimney and so they also need ducting).

For an extractor fan, the bricks will disrupt clean air flow, and because they are cold cause the water and fat to condense/stick to the bricks, cause damp and be a nightmare to clean.

 

[johnny2007]

2 words:
Elf & Safety
...
Or was it Health & Nonsense?

In any case, I personally don't understand why you're going through so much trouble when it would take no time and effort coreing a 5 inch hole in the wall.
That's my opinion, don't take it as a negative feedback.

 

[James Carmichael]

2 words:
Elf & Safety
...
Or was it Health & Nonsense?

In any case, I personally don't understand why you're going through so much trouble when it would take no time and effort coreing a 5 inch hole in the wall.
That's my opinion, don't take it as a negative feedback.

I don't want an ugly looking duct vent smiling at me when I'm in the garden, or filling my face with cooking fumes when I walk past it. That would be an outcome that is beyond stupid, given I already have a method for venting all of this stuff that was designed in to my house when it was built.

 

[James Carmichael]

Hot coal fires heat the bricks all the way up the chimney and reduce the amount of creosotes that stick to the bricks. (Note that log burners run cooler in the chimney and so they also need ducting).

For an extractor fan, the bricks will disrupt clean air flow, and because they are cold cause the water and fat to condense/stick to the bricks, cause damp and be a nightmare to clean.

Many thanks. Ahh, so it's a heating issue? So, why doesn't that stuff all condense in the ducting too? The duct - if I understand it - is flexi so will not be fully smooth walled? Or is the point that it does condense in the duct but no-one really cares/it's not an issue if it does?

I guess that's what I really can't get my head round. One way or the other, the hood has to extract all the crap up 7 metres of hole to the sky. I don't get why a slightly bigger brick hole is worse than a slightly smaller aluminium hole. But, maybe it is the case that crap stuck in the aluminium hole can't leach out into the fabric of the building and cause staining on the chimney breasts above etc?

 

[^woody^]

Nothing to do with heat.

The high moisture content and low temperature of cooking extract air makes it highly likely that it will react with the sulphates from flue gasses and attack the mortar and together with mortar crystallisation will degrade the flue and stack.

 

[James Carmichael]

Nothing to do with heat.

The high moisture content and low temperature of cooking extract air makes it highly likely that it will react with the sulphates from flue gasses and attack the mortar and together with mortar crystallisation will degrade the flue and stack.

That makes more sense to me - thank you.

 

[domdee]

James, I am interested to know the company you have used if possible as I will require the same set up myself in the not to distant future. Thanks

 

[James Carmichael]

Thanks everyone for the replies to this.

To the poster that asked who I had found to provide a suitable extractor: WestIn.

 

[catlad]

I don't want an ugly looking duct vent smiling at me when I'm in the garden, or filling my face with cooking fumes when I walk past it. That would be an outcome that is beyond stupid, given I already have a method for venting all of this stuff that was designed in to my house when it was built.

All you need now is an old fashion stove and your cooking as your house was built to do!

 

[mrrusty]

Personally, I think venting a cooker hood up a chimney or a ducted chimney is a silly idea. The temperature of the vapour extracted starts off not very high, and is definitely going to significantly condense in a long vertical flue. After a bit it will be dripping back down. Yuk. A proper chimney is hot enough to keep the flue dry.