Cooker Extractor for a 3.5 Metre Duct length

[Jupiter01]

Hi All,

I have an open plan kitchen and hence, I require an efficient and effective extractor above my gas hob. Additionally, the wall that the extractor is mounted to is not the external wall and therefore, I have a 3 metre duct which terminates outside.

My first extractor has now failed on three occasions and upon closer inspection, the manufacturer inst so sure about the 3 metre duct length and whether this is powerful enough to extract over this distance. My gas hob is 700 and that's the space I have for the extractor mounting.

Can anyone please advise on the above. Thanks in advance.

 

[John D v2.0]

The problem isn't specifically the length, it's actually the resistance of the duct. What has the hood failed on? It could be water falling beck down the pipe?
If it is the resistance, the easiest way is to increase the diameter of the pipe and remove any bends or terminals that restrict the flow. Round pipe is better than rectangular.

 

[Jupiter01]

I've got a round pipe and I think it's around 100mm in diameter. I've got the flaps on the external vent which should prevent rain from coming through if that's what you mean.

the duct cannot be replaced as its in between the joists and above plasterboard. There is only one 90'ish degree bend where the duct curves into ceiling after connecting to extractor.

 

[John D v2.0]

OK 100 is no where near enough especially with a bend. You need at least 125, ideally 150 to deal with the air flow and keep noise down.
Can you knock out the wall bricks and slot in a new pipe from outside?

 

[bernardgreen]

I've got the flaps on the external vent which should prevent rain

The water if it is a problem is likely to be steam condensing in the duct.

As mentioned a larger duct would help if it can be installed.

Another option is a more powerful weather proof extractor mounted externally at the end of the duct.

 

[The fan man]

Hi Jupiter,

1 x bend and 3m of duct is not a big issue, the size of the ducting may need to be increased depending on how air you need to move, for the extractor to be effective. Do you have any details on the fan currently installed?

For the fan to be effective you want to generate at least 20 Air changes per hour for the Kitchen Area. For example a Kitchen area of 5m x 4m x 2.8m = 56m3 with 20 air changers per hour would give an extract rate of 1120m3/hr (this would be to much air to fit down a 100mm pipe).

If you can calculate the required extract rate the right size duct can be sized and a suitable fan selected.

 

[Jupiter01]

Thanks so much for your help guys.

This is what I have currently. I can confirm that the duct is 100mm round and travels over a 3.2 metre distance.
http://www.spareszone.co.uk/BOX70-ART10303-box-hood

My kitchen size is 3.3 x 3 metres.

I could start to knock out bricks from outside and try to squeeze in another duct but something tells me that this hood is inadequate...

I will await your advice.

Thanks again

 

[The fan man]

If you ceiling height is 2.4m (standard ceiling height for a commercial building) the amount of air you would need to extract for the fan to be effective would be 476m3/hr (132 l/s). The duct size you have would be to small for moving this amount of air.

You would ideally need a minimum of 150mm. With an inline fan 150mm, as the fan is quite big it will be noisy and we would recommend fitting silencers before and after the fan, and wrapping the fan with acoustic material to reduce the noise break out from the casing.

Please see link to Vent-Axia's website below for a suitable fan.

http://ventaxia.fanselector.co.uk/export/fmcm55ah40xltpdwymdm1tip3.pdf

If you don't have an openable window you would need to fit a run on timer set to a minimum of 15 mins.

 

[Jupiter01]

OKAY...
Any tips on how I replace the current flexible ducting and replace it with a new 150mm duct? I am thinking that removing the bricks from the outside is my best option but I will then need a solid duct as oppose to a flexible one to push through across the 3 metre run.

 

[John D v2.0]

Oh if it's flexible ducting that's even worse, you really need smooth bore to let the air travel quickly.
I think bricks o out then slot in lengths of duct from the end. If you needed a join then seal with acrylic sealant and duct tape then carry on. You may need to make a hole in the ceiling to get it out of a tight spot.
You can get 150 rectangular duct which is about 220x90. Not as good as round but it can fit in more places.
Good luck!

 

[Jupiter01]

At the first instance, I will remove the vent on the external wall and shine a light through there which I am hoping will reveal the challenges that lie ahead. If there are no obstacles, I can remove the current duct and start to slide in the smooth bore duct. I can also foresee another issue and this is that these ducts (particularly the round ones) come in very small pieces which means that I've got lots of joints across 3 metres and also when I push this through, its probably going to disturb the integrity of the joints.

 

[bernardgreen]

Put a draw cord in when you pull out the old duct. Then you can use the cord to pull / guide the new duct into place.

 

[John D v2.0]

At the first instance, I will remove the vent on the external wall and shine a light through there which I am hoping will reveal the challenges that lie ahead. If there are no obstacles, I can remove the current duct and start to slide in the smooth bore duct. I can also foresee another issue and this is that these ducts (particularly the round ones) come in very small pieces which means that I've got lots of joints across 3 metres and also when I push this through, its probably going to disturb the integrity of the joints.

That's why I suggested acrylic sealant and duct tape to make a longer one. But don't buy the 350mm ones, go to ducting online or bpc and get a longer one delivered. I think even TLC do some reasonable ones

 

[bernardgreen]

160 mm plain drainage pipe

http://www.jewson.co.uk/building-ma...roducts/DRPDN005/pipe-plain-ended-3m-x-160mm/

 

[John D v2.0]

160 mm plain drainage pipe

http://www.jewson.co.uk/building-ma...roducts/DRPDN005/pipe-plain-ended-3m-x-160mm/

I like your thinking! Very heavy weight, but if you can collect it may be cheaper