Downdraft hob ducting - incorrect size

[DavC]

Having a kitchen extension built with downdraft induction hob on island (Elica Switch model), which ducts out the building. The spec of the duct should be 6" round or equivalent rectangular ducting. I was told by Elica this must not be reduced in size. The builder was given this information and kindly sourced some ducting and installed it at build stage. The duct runs in between the concrete slab and wooden floor and out the side of the house. The floor structure was sort of designed to accommodate this. The duct run is around 5m in total with 2x 90 degree bends.

The extension is now built with kitchen install almost complete and it's been brought to my attention that the rectangular duct used is smaller than spec. It's 200x60 (5" equivalent), but should be 220x90. According to data I've found online, it seems the flow rate is around 25-30% less in the smaller duct. The builder has apologised for the error but said it's too late to do anything at this stage. Note: There was nothing on the drawings about this, it was an "extra". What should I do? Should I push for the duct to be removed and replaced with the correct size? This would cause significant disruption and extra work including partial rip out of the new kitchen, etc. Or should I just live with it?

The hob was only just connected up yesterday and I'm not that impressed with the extraction performance to be honest. I tested it with a small pot of simmering water. The extractor had to be switched to one of the highest 2 settings in order to pull all the steam downwards. Set it to any of the slower settings (1-7) and some of the steam floats up to the windows on the ceiling. The "automatic extraction" setting on the hob seems ineffective too. This is supposed to monitor the hob settings and use readings from an air sensor to adjust the extraction rate. It simply doesn't pull enough on this setting.

 

I then tested the extraction with the duct pipe disconnected from the back of the hob. Extraction rate seemed noticeably better and the "automatic extraction" worked as expected, pulling just the right amount of air when required. 

Extraction was an important element of the design for us. There is a glass roof lantern directly above the hob and no opening windows nearby, so we wanted the downdraft to be as effective as possible. We purposely chose a hob with a high extraction rate.

Builder is a great guy and I don't want to fall out with anyone over this, however I'm feeling very disappointed at this stage and don't know where to go with this....
For the record, the kitchen fitter (independent from the builder) thinks the ducting with be sufficient.

[Pocster]

I have the same hob . Sounds a bit tricky to change the ducting now .

Im guessing ducted version and recirculating version are different models entirely also !

Very least some compensation from builder for his error 

[Kelvin]

Compensation and live with it vs major disruption to make good. We are fitting a Neff recirculating hob but also have MVHR. 

[AartWessels]

Regardless of the duct size, with such a long duct with 2 angles of 90 degrees, it will not perform as well as when you disconnect the duct. 

 

For the best performance you'd probably have to set it up for recirculation, so blow it out from under the kitchen cabinet. That's assuming it can be set up that way. Pulls a lot less cold air into the house in winter as well that way. 

[Pocster]

1 hour ago, AartWessels said:

Regardless of the duct size, with such a long duct with 2 angles of 90 degrees, it will not perform as well as when you disconnect the duct. 

I did think from memory the manufacturers max length of duct was 5m . You loose 0.5m per 90 elbow …

[DavC]

From memory I think the manufacturer said it’s 5m with up to 4 bends.

 

To be more accurate, my duct run is probably around 4.5m inc the downward part from the hob. 
 

I’ve included the 90 degree piece coming out the back of the hob and down. Then there is one more 90 degree turn at floor level. So 2 in total. 
 

I don’t think that duct run is excessive at all. 
 

it changes from rectangle to a round 6” flexi as it comes out the back of the hob. The round flexi runs down inside a cabinet. Then it changes to the smaller rectangle (5” equivalent) at floor level as it turns, then goes straight out (about 3.5m this part) 

[DavC]

6 hours ago, Pocster said:

I have the same hob . Sounds a bit tricky to change the ducting now .

Im guessing ducted version and recirculating version are different models entirely also !

Very least some compensation from builder for his error 

 

Since you have the same hob, I hope you don't mind me asking a question regarding performance. When I simmer a very small pot of water, I have to have the extractor set to 9 in order for 100% of the visible steam to go downwards. If I turn the extraction any lower, only a percentage of the steam will go down and some will rise towards the ceiling. 

How does this compare with your extraction performance and how long is your duct run?

Edited November 19, 2023 by DavC

[Pocster]

32 minutes ago, DavC said:

 

Since you have the same hob, I hope you don't mind me asking a question regarding performance. When I simmer a very small pot of water, I have to have the extractor set to 9 in order for 100% of the visible steam to go downwards. If I turn the extraction any lower, only a percentage of the steam will go down and some will rise towards the ceiling. 

How does this compare with your extraction performance and how long is your duct run?

The same tbh 

I viewed a bora and it was similar 

To get steam to go horizontal out of saucepan required a 9 or 10 on extraction 

[markc]

In order to pull the steam downwards you need to move a lot of air. It’s similar to dust extraction, getting 30% (ish) of it is quite easy, getting upto 80% takes a big fan and power, to get the last 20% or so needs a very big extract rate and a very big fan and motor. 
if you are pulling all the steam from across a pan make sure you don’t have any loose kitchen roll, onion leaves or similar lying around or then will be next.

[Conor]

7 hours ago, Pocster said:

I have the same hob . Sounds a bit tricky to change the ducting now .

Im guessing ducted version and recirculating version are different models entirely also !

Very least some compensation from builder for his error 

 

It's a £350 extra to convert if you have bought the ducted version. We bought 5he extract version but didn't install the duct work as was almost impossible to get a good run that would have worked.. ducts too big for either burying in floor or in ceiling void below. 

 

46 minutes ago, DavC said:

 

Since you have the same hob, I hope you don't mind me asking a question regarding performance. When I simmer a very small pot of water, I have to have the extractor set to 9 in order for 100% of the visible steam to go downwards. If I turn the extraction any lower, only a percentage of the steam will go down and some will rise towards the ceiling. 

How does this compare with your extraction performance and how long is your duct run?

 

Tbh, if ours is on its either 9 or P. I don't think any downdraft works on low speed, you need high velocity for the effect to work.

 

Rest of the time it's off and the MVHR copes well with general steam and light odours. In your case, either open the nearest window (I assume you have at least one openable window in the kitchen for BC compliance) or install a in-wall extractor fan.

Edited November 19, 2023 by Conor

[DavC]

Thanks for the replies folks, that does put my mind at ease a bit. 
 

I have read various information saying that if the duct width is reduced it will seriously affect performance. Our has only been reduced from 6” to 5”. 

 

However it sounds like ours isn’t working too badly after all, if you guys also need to have yours on 9 to suck all the steam downwards. 
 

This is our first downdraft, so had no experience of the performance. 

[AartWessels]

I wasn't saying your duct length is excessive by the way, as that's almost impossible to have an opinion about. It's just a fact that any length of ducting has a significant impact on the performance of a fan. 

I have a Samsung induction hob with extractor fan in storage for ouw new kitchen and I'm quite curious as well to see how it performs. My expectations are honestly not very high. We used to have a massive overhead extractor in our previous kitchen, and even that would have issues catching all the steam of a 110cm hob. I always blamed that on the impact of the closed panel in the center of it, so it would only extract round the edges. Looks pretty, but such a waste of power 😂