Kitchen hob extract

[Bogart]

As the boss has decided we needed a new kitchen and I have agreed everything is straightforward, as much as it can be, except the extract from the hob.
The old hob just recirculated, I do not like these. So where to run the ducting. It just so happens there is an old chimney stack in the kitchen which surprisingly has two 100mm
soil pipes running up it. One is the vent for the downstairs wc fan and the other just ran to ground level in one of the old cupboards and terminated with a plastic vent, most odd.
So my question is would this second soil pipe be suitable to carry the extract from a hob considering it has to go around 3m horizontal then 2 floors plus the stack on top, around 7m
vertically. As far as I can see both these soil pipes are terminated in some sort of mushroom shaped cowl where the exit the stack.
Venting out through the walls is possible but more more work entailed than going skywards up the chimney.

 

[Feltwell]

My natural inclination is keep the run as short as possible.

We don't have an extractor - we have a range cooker set in the opening where the original range would have been (previous owner ripped it out :evil: ). The old flue directly above the hob provides ventilation and seems to do a good job of it - it's the unlined flue, it's not got greasy or damp inside.

Sounds like your hob is not in the old range space though. 3m horizontal + 7m vertical sounds like a lot for an extractor that most frequently would just go straight out through the wall - they're not very powerful. I also wonder if you'd get problems with water vapour condensing on the walls of the pipe and running back down. I'd go for a shorter run through the wall personally, as straight as is possible. The rectangular ducting is quite good for hiding on top of cabinets.

I agree about recirculating fans - they're hopeless, even with new filters.

 

[Flyfisher]

I can’t see a problem with running ducting up a chimney per se (assuming the chimney isn’t also used as a heating flue). I’d be more concerned with the length of the run and being careful to use a powerful enough fan to cope with the added ‘load’ caused by the extra bends and length.

I also wonder if there is the potential for the extracted gases to cool inside long runs and condense, possibly leading to a build up of grease?

Ha! I see that Feltwell beat me to basically the same points.

 

[paulc]

Soil pipe is nominally 100mm diameter. Most extractor hoods require either 125mm or 150mm dia ducting. Duct length is usually limited to 3m to 5m depending on the diameter and the number of bends involved (subtract 1m for each 90° bend). Using a powerful centrifugal fan unit might enable you to use the soil pipe, but it is going to be expensive and quite likely very noisy.

Keep the duct length as short as possible with the minimum of bends. Avoid using the flexi concertina hose as it will seriously affect the air flow - A short length to connect the fan unit to rigid ducting is OK.

 

[Bogart]

Hi

Thanks for the replys guys. Am in agreement regarding length of run and the motor power. I too envisaged the extract getting so far up the pipe then running out of steam, maybe literally, then all coming back down after the extract is switched off and the grease just hanging around. Me thinks will duct outside using horizontal ducting to fit between the joists.
Interestingly did find behind the brickwork covering the old chimney the remnants of a fireplace, assuming Victorian, sweet little thing tucked away.

 

[plasticpigeon]

I have done something similar I think. I have a gas "range" in the space where the old range would have been like Feltwell describes, and an extractor in the flue and a metal concertina pipe going from the extractor out the back wall of the chimney which has a flap valve on it. In my case it actiually exits into the tool shed where the garden tools are kept which protects the exit even more, and the door is so ill fitting it may as well be outside. If there is space drilling a big hole and running the pipe out might be better than the faff of utilising the soil pipes and it keeps the run shorter.

 

[Craig89]

I propose to do something simular. I plan to have a gas range in the chimney breast and use the flue as the extract. I fitted a 150mm stainless steel liner down the flue and hopefully the natural draw will be sufficient, if not I will fit a fan to help get it going. Failing that I will fit a fan to lift it the entirety. Failing that I will put a hole through the back of the chimney :lol: . The height of the flue is around 8m

 

[Bogart]

I think you are being very optimistic regarding the natural draw. If it is a windless day there will be "buggar all" draw. If you fit a fan it would need to be accessible as it is first in line for any moisture and cooking oils that might get sucked in, these coarse filters one gets on a hob only remove so much of the oil vapour.

 

[Craig89]

We don't have an extractor - we have a range cooker set in the opening where the original range would have been (previous owner ripped it out :evil: ). The old flue directly above the hob provides ventilation and seems to do a good job of it - it's the unlined flue, it's not got greasy or damp.

This statement is very interesting to me, hopefully it pans out the same for me

 

[Feltwell]

Craig, the bottom of the chimney is blocked off with plasterboard mounted on a frame, with a 9" square plastic grille fitted in it with a removeable centre section. This does get greasy - I stick it in the dishwasher every now & then!

Above the grille is a bit of plywood mounted a few inches above it - a bit bigger than the grille - just to stop any bits of old soot or whatever that comes down the chimney from falling through onto the cooker.

This was done by the previous owners of the house, it's been like that for about 17 years with no problem.

The top of the chimney was capped off with a "pepperpot" insert, but I did change it for a mesh cowl when I did the chimney work recently - just to give a bit more draw - not that I'd noticed a problem.

Hope that helps.

I did think about putting a hole through the back of my chimney, but thought the neighbours might complain about it venting into their dining room :lol: