Some advice regarding 125mm hole for cooker hood fan please?

HappyHippyDad

HappyHippyDad

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Morning all..

I am about to quote for a cooker hood installation including core drilling for the vent. The fan is 125mm.

I have checked upon the sizes of core drill bits and 127mm is the closest fit. I cant see the 125mm ducting getting through that due to the outer diameter of the ducting being greater than 127mm. Does it fit through?

How much of an impact would it have on the motor if I used a 125mm-100mm reducer for the ducting?

Could I cut a 152mm hole and use a larger vent on the outside wall, also using a 150mm-125mm reducer for the ducting?

Cheers.
 
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I shall be adding a FCU next to a kitchen socket then chasing perhaps 1.5m (no filling), fitting the cooker hood, core drilling and fitting ducting and outside vent.

I've estimated 4 hours but am thinking it may be closer to 5? Just trying to work out the quote!
 
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happyhippydad said:
I'm guessing drilling all the way from one side is better if using solid ducting?
Click to expand...

Yeah defo. The actual shape of the core drill helps to keep the hole straight. If you core from both sides it is easy to deviate off from a nice straight line. Don't forget to angle it slightly to prevent water any water from coming in from the outside.

I think four hours sounds about right. 30-90mins on the core itself. 60-90mins chasing, fcu etc. Remainder for testing and tidying up.
 
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I've actually just had a look under my cooker hood chimney. It has a 125mm ducting vent coming out of the top of the cooker hood, followed by 100mm flexible ducting siliconed inside it! NOT the way I am going to do it.

I've just this second fitted the reducer on (had one hanging around, as you do. What else are you going to do on a Tuesday night! :D) and it looks much better :)

Decision made for the job!

Cheers everyone.
 
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Reducing the duct size has a ridiculous amount of effect compared with what you'd think on the air flow. Look up the pressure loss. And to illustrate:
100mm duct its 7850mm2 Free area and 125 is 12265mm2 free area so almost double.
The resistance (pressure drop) is also related to the diameter, so you can see the noise level due to pressure and the flow rate will be compromised. And the power usage isn't going to meet part L if you're interested in regs.
Flexi duct is a disaster and you won't find many manufacturers recommending it. It's the ducting equivalent of those canoe rapids you see people paddling down with water sloshing everywhere.
 
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Agreed there's a lot of incorrect work in this country, due to lack of knowledge, but that's just life.:eek:
No need for an air flow meter, just never use a reducer and never use flexi, then you won't go far wrong.
Most cooker hoods can remove 300+m³/h with an open outlet on Max, so I'm guessing they'll manage something flow over the regs regardless, just they'll be wasting power and noisy.
I took the fan out of a cooker hood that had been chucked due to being too noisy to use even on the lowest setting. With no duct attached it is super quiet. Find me someone who doesn't ask their cooker hood to be quieter and I'll find you someone with adequate size ducting;)
 
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I can see your point when designing or installing a heat recovery system, but not with a simple cooker hood mounted on an outside wall, or a bathroom fan with less than 3m of ducting. I've never had issues with reducers or flexi duct. Last few bathroom fan kits I've installed, came supplied with flexi duct, by the manufacturer.
 
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johnduffell said:
Reducing the duct size has a ridiculous amount of effect compared with what you'd think on the air flow. Look up the pressure loss. And to illustrate:
100mm duct its 7850mm2 Free area and 125 is 12265mm2 free area so almost double.
The resistance (pressure drop) is also related to the diameter, so you can see the noise level due to pressure and the flow rate will be compromised. And the power usage isn't going to meet part L if you're interested in regs.
Flexi duct is a disaster and you won't find many manufacturers recommending it. It's the ducting equivalent of those canoe rapids you see people paddling down with water sloshing everywhere.
Click to expand...
You have to go and ruin the entire thread don't you John! 'Decision has been made' was a lovely ending and I was happy. Now, not only will I not be complying with the regs but I may very damage the customers ear drums and possibly even kill them due to Legionnairres. :eek:
 
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John, I will bow to some of your possibly superior knowledge on this. I agree with Mid though, a bit of 100mm flexi running for 30cm through an external wall is not going to disrupt things much.
Also, I have never noticed a massive difference in noise levels with the ducting reduced in size to 100mm. Domestic kitchen extractors generally are noisy so and so's..
 
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NDG Elecs said:
Domestic kitchen extractors generally are noisy so and so's..
Click to expand...

Was having a debate with some chaps about kitchens, one recommended Gaggenau appliances (German of course). Do you think this would be worth the extra, save on noise & that;

Gaggenau Aw 442 160 - Serie 400 Hood Wall Cm. 160 - Stainless Steel | Vieffetrade - https://www.vieffetrade.eu/sale/cooker-hoods/gaggenau/hood-wall-cm-160-stainless-steel/aw-442-160-serie-400/113116?s=v&valuta=GBP&gclid=CjwKCAjwxJnNBRAMEiwA8X_-QQ37XvmCSLLEKiUWvZb-uHmxrIeZXBgFWG20W8rpnaRvEoJbqcRirxoCXJgQAvD_BwE
 
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happyhippydad said:
Morning all..

I am about to quote for a cooker hood installation including core drilling for the vent. The fan is 125mm.

I have checked upon the sizes of core drill bits and 127mm is the closest fit. I cant see the 125mm ducting getting through that due to the outer diameter of the ducting being greater than 127mm. Does it fit through?
Click to expand...

I've used a 152mm core drill for 150mm ducting. Was a snug fit, but was fine. :) There's a reason they make the core sizes they do... the 117mm core drills that you usually get in sets are for soil pipes (110mm) though will serve for a 100mm fan duct, with ample wiggle room. 127mm for 125mm ducting should be fine.
 
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Midwest said:
Was having a debate with some chaps about kitchens, one recommended Gaggenau appliances (German of course). Do you think this would be worth the extra, save on noise & that;

Gaggenau Aw 442 160 - Serie 400 Hood Wall Cm. 160 - Stainless Steel | Vieffetrade - https://www.vieffetrade.eu/sale/cooker-hoods/gaggenau/hood-wall-cm-160-stainless-steel/aw-442-160-serie-400/113116?s=v&valuta=GBP&gclid=CjwKCAjwxJnNBRAMEiwA8X_-QQ37XvmCSLLEKiUWvZb-uHmxrIeZXBgFWG20W8rpnaRvEoJbqcRirxoCXJgQAvD_BwE
Click to expand...

Stuff that Mid. I would get a £80 extractor and spend the rest on eating out more often...!
 
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NDG Elecs said:
John, I will bow to some of your possibly superior knowledge on this. I agree with Mid though, a bit of 100mm flexi running for 30cm through an external wall is not going to disrupt things much.
Also, I have never noticed a massive difference in noise levels with the ducting reduced in size to 100mm. Domestic kitchen extractors generally are noisy so and so's..
Click to expand...
Don't bow to my knowledge, it's more theory than experience, I'm learning a few things here myself :) I just have a personal preference for quiet extractors so it's my personal crusade!
 
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happysteve said:
I've used a 152mm core drill for 150mm ducting. Was a snug fit, but was fine. :) There's a reason they make the core sizes they do... the 117mm core drills that you usually get in sets are for soil pipes (110mm) though will serve for a 100mm fan duct, with ample wiggle room. 127mm for 125mm ducting should be fine.
Click to expand...
Was that flexible ducting Steve?
 
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How can drilling from one side be classed as the wrong way of coring the hole? That video shows some DIY numty having a go.I've never had a problem with bricks blowing. I know where I would rather be stood whilst drilling the hole and that's not at the top of a ladder on a gable end with a drill and core bit in the hand.
 
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Leesparkykent said:
How can drilling from one side be classed as the wrong way of coring the hole? That video shows some DIY numty having a go.I've never had a problem with bricks blowing. I know where I would rather be stood whilst drilling the hole and that's not at the top of a ladder on a gable end with a drill and core bit in the hand.
Click to expand...
Wouldn't to be up a pair of steps using this blighter
 
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Dont reduce the size of duct as it will reduce performance of fan and ability of cooker hood to perform. Use solid steel duct rather than flexi plastic out of choice for a better install or solid plastic duct. Plus suggest on penetrations fill the gap arround with intumescent foam rather than standard builders foam. It is assumed that this on a outside facing wall rather than running through the ceiling void. Remember flexi-duct pipe only works well if it is pulled taught when it is left loose the air movement is reduced and becomes a greater risk of fire due to grease being caught plus from memory the BS indicates that flex tube is max of 1.5m long from fan without compromising install.
 
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Personally I would never run flexible ducting through a wall especially the wall of a kit house or cavity wall ! Solid 4" soil pipe is what I use for two reasons (1) with the movement of air in the flexible stuff it can soon rub holes in it causing dampness the only part normally run in flexible normally is from the hood to the roof space (2) if there was a fire the grey soil pipe collapses on its self and helps stop the spread of fire ! The orange soil pipe doesn't have the same caricatureisticts hence why it's gray used above ground and orange below.
 
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vjsmarwick said:
Personally I would never run flexible ducting through a wall especially the wall of a kit house or cavity wall ! Solid 4" soil pipe is what I use for two reasons (1) with the movement of air in the flexible stuff it can soon rub holes in it causing dampness the only part normally run in flexible normally is from the hood to the roof space (2) if there was a fire the grey soil pipe collapses on its self and helps stop the spread of fire ! The orange soil pipe doesn't have the same caricatureisticts hence why it's gray used above ground and orange below.
Click to expand...

I tend to use either manufactures wall kits or with cooker hoods, metal flexible ducting.

Not sure that grey soil pipe has any intumescent properties, by design or accident. Manufactories supply provide fire collars or wraps for that purpose.

Grey soil pipe is generally used above ground as its is uv stabilised, whereas brown is not. It also has slightly different construction, to withstand additional forces, or so Google tells me. But I'm not a plumber :)
 
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HappyHippyDad

HappyHippyDad

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Some advice regarding 125mm hole for cooker hood fan please?
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