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

[HappyHippyDad]

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.

 

[HandySparks]

Would be an excellent idea but I would need a vac too Handy!

I've used mine with a Henry. Works pretty well.

 

[Dave OCD]

Deleted double post.

 

[Dave OCD]

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!

£160-£170, a good bit of cleaning up too.

 

[HappyHippyDad]

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! ) and it looks much better

Decision made for the job!

Cheers everyone.

 

[johnduffell]

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.

 

[Midwest]

Well there's going to be lots of kitchens & bathrooms that don't comply then. I'm off to purchase an air flow meter for £400

 

[johnduffell]

Agreed there's a lot of incorrect work in this country, due to lack of knowledge, but that's just life.
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

 

[Midwest]

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.

 

[johnduffell]

Ah bathroom is different, usually they only have one speed and are just designed to provide a weak draft. But good point about power consumption not mattering on something that's not on 24/7. Cheers

 

[HappyHippyDad]

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.

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.

 

[johnduffell]

I only know physics I don't have a lifetime experience of what happens in the real world of people. If you do something and never know if it was the right decision, it was the right decision!

 

[NDG Elecs]

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..

 

[Midwest]

Domestic kitchen extractors generally are noisy so and so's..

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

 

[happysteve]

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?

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.

 

[NDG Elecs]

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

Stuff that Mid. I would get a £80 extractor and spend the rest on eating out more often...!