looking for advice for fitting a hob extractor

[plasticpigeon]

Hello PPpeople. I would be grateful for advice. I am going to fit a hob extractor up a chimney, as our cooker will go where the old range was (range was long gone before I bought the place). My idea is to duct it with 6" hose to the outside. I have found an air brick that matches the others in my house that I would like to use as the front for the vent. My question is does anyone know of a mounting plate that has a spigot for the 6" pipe and just a flat plate that can be fitted to the inside of the brick wall up the chimney (and yes I think I can just about manage to reach up the chimney far enough as the flues don't gather for a few feet. Many thanks.

 

[agbagb]

You should be able to find a plastic Grille and cut off what you don't need or "180 x 95 to 150mm round adaptor". Failing that just mortar in a flexible aluminium duct behind the air brick.

 

[Penners]

PP - Sorry if I'm being thick but I'm not sure that I understand exactly what you're planning to do. Are going to take the 6" flexible ducting from the extractor unit a short distance up the chimney, and then bend it at 90 degrees and vent it via an airbrick in the back wall of the chimney stack?

If so, I think my concerns would be as follows:

* An air brick has quite a thick terracotta matrix. You could find that this matrix reduces the area of the venting sufficiently to create a back-pressure that will reduce the efficiency of your extraction.

* Do make sure that, when you install the extractor at the bottom of the chimney, you leave some sort of provision for air circulation inside the chimney stack. A sealed-off stack full of "dead" air can become the source of problems.

* I think I would also insulate the flexible ducting, to ensure that the extracted air (with its suspended grease particles) remains warm until it is vented. If the air is allowed to become chilled within the ducting, you could eventually get a nasty build-up of grease on its interior walls.

 

[plasticpigeon]

PP - Sorry if I'm being thick but I'm not sure that I understand exactly what you're planning to do. Are going to take the 6" flexible ducting from the extractor unit a short distance up the chimney, and then bend it at 90 degrees and vent it via an airbrick in the back wall of the chimney stack?

If so, I think my concerns would be as follows:

* An air brick has quite a thick terracotta matrix. You could find that this matrix reduces the area of the venting sufficiently to create a back-pressure that will reduce the efficiency of your extraction.

* Do make sure that, when you install the extractor at the bottom of the chimney, you leave some sort of provision for air circulation inside the chimney stack. A sealed-off stack full of "dead" air can become the source of problems.

* I think I would also insulate the flexible ducting, to ensure that the extracted air (with its suspended grease particles) remains warm until it is vented. If the air is allowed to become chilled within the ducting, you could eventually get a nasty build-up of grease on its interior walls.

Hi Penners, yes you've understood what I want to do perfectly and described it more clearly and concisely!!!! I'm trying to avoid having a flappy plastic, or other plastic type vent on the outside. I'm not sure how much back pressure the reduced area of the airbrick would cause, I suppose it could be calculated, but I'm also not sure what the flow vs back pressure specs for the extractor are either, or the discharge coefficient of an air brick is!!!! I'm assuming that it will still move a reasonable amount of air out. I have ventialted caps on the top of the chimney, but it will be sealed at the bottom if I stick the extractor in there. Could that really be a big problem?? could i just rake out a vertical mortar joint or 2??? The chimney forms part of the back wall of the house and is only a half brick thick.

 

[Penners]

PP - I think you could maximise the ventilation through your airbrick by using an appropriately sized square-to-round transition ducting, like these:-

http://www.bestbuyheatingandairconditioning.com/Merchant2/merchant.mvc?Screen=CTGY&Category_Code=duct-square_to_round_transitions

If you place your round ducting flat against the back of a square or rectangular air brick, you are inevitably wasting a fair amount of the airbrick's open area. A transition duct like the above* could considerably ameliorate that situation.

As regards ventilating your chimney, it's good that you have a ventilated cap on top, but throughflow of air would be better still. Your idea of cutting out a couple of vertical mortar joints at the bottom would be a step in the right direction but could, of course, introduce other problems - rain ingress, rodent and pest access, and so on.

Could you not install a modest vent in the register plate that will be sealing off the chimney at the bottom?

* By the way, these are American, but show you what I mean.

 

[Feltwell]

We too have a cooker where the old range used to be (ripped out by previous owners only a few years before we bought the place :evil: ).

We don't have an extractor at all, just a grill over the bottom of the original flue. The top of the flue is capped off with a "pepperpot", but despite that there is still a bit of an updraft and it seems to manage to vent the cooker remarkably well.

Probably not recommended for a variety of reasons, letting damp air go up an old chimney flue, but it's been like that for over 10 years with no problems so far.

 

[malcolm]

What did you do in the end PP? I'm leaning towards a similar solution as I don't want to install a nasty plastic vent in the well. I have a terracotta vent but the holes are too small.

Has anyone else found a nice way to finish a cooker vent through a brick wall? I'm now looking at cast iron air bricks with bigger holes.

 

[a twig]

How about doing your normal plastic rectangular vent, but then covering it with something more traditional?

Here's some inspiration:

 

[a twig]

This company do some nice stuff, you could use something from here:

http://www.britannia-shop.co.uk/collections/cast-iron-air-bricks/9in-x-3in-(225mm-x-75mm)

A bit of finagling and you could cludge one of these to it on the inside:

http://www.qvsdirect.com/pipe-to-airbrick-adaptor

Hook your extractor duct work up and Robert is your father's brother...


OR


Stick with round and buy everything you need from here - very spendy but very pretty:

http://www.castironairbricks.co.uk/product-category/period-vent-grilles/

 

[malcolm]

I've found that eBay can supply "cast iron air brick" and "cast iron vent cover" (search for either) for about £12, though I'm not sure either their imperial or metric brick sizes are near the brick size of my wall. Ought to be a case of making the hole and worrying about it later when it is warm enough to go outside and I can see what size bricks I ended up knocking out.

My bathroom extract goes through a big hole in the wall covered by a terracotta tile. The hole is (badly) lined in cement sloping towards the outside of the wall. That could be a good fail safe to stop water running from the vent into the wall.

 

[Flyfisher]

What did you do in the end PP? I'm leaning towards a similar solution as I don't want to install a nasty plastic vent in the well. I have a terracotta vent but the holes are too small.

Has anyone else found a nice way to finish a cooker vent through a brick wall? I'm now looking at cast iron air bricks with bigger holes.

Something like this? http://www.ebay.co.uk/itm/like/131499280282

or this? http://www.castironairbricks.co.uk/product/con-4-conservation-vent-grille/

 

[Penners]

I'm now looking at cast iron air bricks with bigger holes.

Be a bit careful about having ventilation holes that are too big, Malcolm. Remember that vermin can squeeze in through some remarkably small holes.

Mind you, those ones that FF has found on eBay look good.

 

[Feltwell]

Mind you, those ones that FF has found on eBay look good.

Certainly do, thanks FF - a keen price as well. I've got to get my scaffold tower out at some point and drill a duct for a shower fan through the bathroom wall - black ducting through the wall and one of the round grilles to cover the end could look good. No backdraft shutter but there is one on the fan I've been looking at, hmm..........

 

[Bogart]

Read this thread with interest as I too want to vent an extract into a chimney stack. Though in my case the stack is in the middle of the property hence no outside wall available. What I have got is two
separate fireplaces one on ground floor now blocked with an inbuilt oven using the space and one on first floor just blocked off. Both transit to the roof in separate stacks but only a brick thickness separating them. Both have vents on the roof where they exit the stacks. Now I was thinking that if I just vent into the stack, near the hob position, is not an ideal situation as no guarantee the smells, steam etc will ever make it to roof level, in fact I am certain they would not. Am considering dropping a liner, probably insulated, down the stack connecting to the hob extract leaving some space around it so that the stack itself still has some ventilation. What do you think?
Regards

 

[paulc]

Be a bit careful about having ventilation holes that are too big, Malcolm. Remember that vermin can squeeze in through some remarkably small holes.

Had to install a small air vent last year when having a wood stove fitted. I sandwiched some very fine stainless steel mesh between the external fitting and the grill. For an extractor vent, the mesh I have would probably be way too fine (0.2mm holes), so perhaps a recycled flour sieve would work.

For rodent proof screen, I think they suggest something with holes no bigger than 6mm - But at this size, bugs such as wasps could possibly still get through and nest inside the ducting.

 

[88v8]

Quarter inch mesh is OK for keeping wasps n bees out. And in any case, they wouldn't like the airflow.

Provided that the stacks have a good draw, the only problem I can think of with venting into them is the potential build up of grease, but unless you're a burger fanatic, that shouldn't be an issue.

Ivor

 

[parkview094]

Now I was thinking that if I just vent into the stack, near the hob position, is not an ideal situation as no guarantee the smells, steam etc will ever make it to roof level, in fact I am certain they would not. Am considering dropping a liner, probably insulated, down the stack connecting to the hob extract leaving some space around it so that the stack itself still has some ventilation. What do you think?

This seems to divide opinion. We're currently in the process of installing a new kitchen and our builder refused to install an extractor venting up the old (already lined) chimney. His view was that it wouldn't work well and that building control probably wouldn't allow it. (Building control are insisting on mechanical external ventilation). A quick google reveals lots of people concerned with fat build-up in the liner increasing the risk of a chimney fire. Interestingly the Telegraph advisers disagree and think this is a perfectly acceptable setup: http://www.telegraph.co.uk/lifestyl...-can-I-vent-cooking-fumes-via-my-chimney.html Regardless, it's worth checking the height of your chimney doesn't exceed the maximum duct length for your extractor. Neff, by example quote 5m for their domestic extractors http://www.neff.co.uk/features-ventilation.html.

Our compromise is going to be to vent partially up the chimney and then run horizontally to an outside wall. I think the best vent cover option for us is the one mentioned above at: http://www.castironairbricks.co.uk/product/con-4-conservation-vent-grille/, but not sure I can bring myself to spending £125 on a vent cover. Shame there's not an *ahem* plastic equivalent..

 

[a twig]

Don't most decent modern extractors have grease/fat filters?

So as long as you keep your filter in good nick and replace when needed, there shouldn't much in the way of fat/grease to worry about.

Alternatively fit a re-circ hood - change the filters regularly - and keep a small window open when you are cooking

 

[Feltwell]

We inherited a "vented up the chimney" setup - and I have to say it works fine.

The chimney is not lined, it's just a sheet of plasterboard on a wooden frame set into the chimney breast above the cooker, with a vent in it. I'm not pretending for one moment that it's the best setup, it isn't how I would have built it, but we've been here for 10 years now and it's not caused any problems bar one.

Soot and debris falling down the old flue would come through the vent onto the hob top. So I passed a sheet of tough flexible plastic rolled up through the vent hole, with some spacers screwed to the bottom of it, so it sits spaced above the vent by a couple of inches - air flows around it but debris lands on top. I had a good look up the chimney when I did this - it wasn't greasy.

The chimney pot on the flue has a "pepperpot" vented cap on it. Despite this, unless something has been burnt we never have to open the kitchen window for cooking fumes, although admittedly it is a tall room.

I'd happily put it in this way but I would line the chimney personally. As it is, it falls under the category of "if it ain't bust don't fix it"!







I don't know if the re-circulate filters have improved but my parents had a hood set to re-circulate with a filter in it years ago and it was cr@p, frankly. Not worth switching on. Didn't reduce the cooking fumes in the room and of course doesn't get rid of steam from boiling pans.

 

[parkview094]

ooh- could I inquire as to the height of your chimney opening please? We need to get our chimney opening raised to facilitate the extractor above the hood, but I'm struggling to find a reference as to how high is 'normal'. I see an extractor should be 760mm above a gas hob, which gives us an opening height of of around 1800mm, but I'm wondering if this would look a little 'low'...