Kitchen hob ventilation

[Tony K]

Afternoon all.

My SB is single storey and has a flat roof. I will be using a rubber roof finish. We have a fairly settled layout plan for the kitchen including the size and position of the oven but have not yet chosen any particular cooker hood, other than we know we want it to be an extract model rather than recirculation.

 

As I am about to fit the rubber roof I need to make provision for the kitchen extract vent now. The oven will be about 900 - 1000mm wide, so the extract will be 1000mm or more. 

 

From what I can tell, I seem to be able to leave the exact choice of cooker hood until later so long as I....

 

1. Provide ducting and the external flat roof vent now (based on 150mm diameter vent pipe as opposed to 120mm).

2. Presume that the hood will have its extract connection in the centre of the unit and close to the back edge. They all seem to, but is this actually the case?

3. Opt as far as possible for rigid ducting over flexible to avoid any slight restriction in the airflow. 

 

Am I missing anything / about to make a problem for myself later?

 

Cheers

 

 

 

 

 

 

[Conor]

You're pretty much spot on. That's what I did. The solid PVC ducts have a bit of wiggle room, so if you are 25mm or so off centre, it won't matter. I used rectangular ducting to fit between joists and circular adapter at the top of the hood. Don't forget to tape the joints.

 

 

I assume you've not got an MVHR in the house?

 

Re extractor hoods, I HIGHLY reccomend an externally mounded fan unit rather than one in a hood. But by the sounds of it that's not an option for you as you're venting to a flat roof?

Edited March 31, 2022 by Conor
Speeling

[Marvin]

Sounds good, but don't forget a cowl of some sort to stop the rain from entering..

Check that the flat roof joists are not in the way,

Watch out that service runs avoid the pipe area ( cable runs for instance) 

 

Good luck

 

M

[Beckside]

If you are building with high insulation levels and good air tightness then a re circulation extraction system would be better.  As you are extracting warm air from your house. We fitted a Bora hob and downdraught extract on re circulation and it works a treat. Also you do not need to make a hole in your mew roof membrane

[Tony K]

Thanks all.

 

@Conor I am using MHVR in the house, but the heating engineer advises that cooking extract should be a separate entity due to grease. 

@Marvin I plan to assess any conflict between the joists (which are bloody big thick posi joists 122mm wide) and the vent pipe. Gien the joist sizes I may not have much wriggle room if there is a conflict. 

@Beckside What makes you suggest recirculation? One article on the comparative merits said that 'You may find that some steam is returned to the room via this [recirculation] method. In general, recirculation models offer a reduced extraction rate at a higher noise level than ducted models — it is usually more efficient to port the extracted air directly outside than to filter it inside the unit.'

 

Cheers

Edited March 31, 2022 by Tony K

[JohnMo]

You are right not to incorporate the cooker hood with MVHR.  But you still need to include a kitchen extract terminal within the kitchen to the MVHR.  When using cooker extract with MVHR it is recommended to open a window to keep the MVHR balanced.

 

Recirculation is pretty normal when using MVHR.  MVHR takes away the steam, cooker hood manages grease. Carbon filter manages smells.

 

I have never had to use the cooker hood above its first setting, using recirculation, as the flow rates are huge.  The important thing is to get the height above the hob correct.  The closer you get the better the performance.

[Beckside]

As Johnmo says, re circulation is generally the way when using MVHR. As the volume of air an extractor can move is way more than the mvhr, so upsets the balance. Plus you are removing warmed air. Any top end extraction system will have a grease trap or filter incorporated, re circulation or direct extract. I was sceptical about the re circulation, but with a years experience I can say it does work. We have an open plan kitchen, dining and living room, the living room is a vaulted ceiling. We have virtually no smells in the room, no matter what we cook or how we cook. I have seen in most European countries, if they fit direct extract with mvhr, they have a motorised valve to close off the extract duct when not in use. Also as I said it is one less penetration of your roof membrane.As I said we fitted a Bora downdraft extractor, as we did not want anything above the hob, as it sits on an island in the middle of the room. 

Edited March 31, 2022 by Beckside

[Conor]

@Tony K in that case make your life simpler by going for a recirculating hood. The MVHR will pick up anything that the hood doesn't deal with. We were going to duct out but glad we didn't as recirculating is fine.

[dpmiller]

I'll offer the contrary view that a short-term imbalance won't harm anything, and cooking may well be excess heat that then causes the rest of the house to overwarm.

 

We've two normal extract points at the "cooking" end of the kitchen area and these deal with day-to-day moisture and odours. But I also have a sizeable extractor over the cooker which can separately deal with the big heat, smell, and grease from rapid wok cooking. At other times it's sealed off by a gravity flap in the duct.

[Tony K]

Thanks all. I am leaning towards the circulation model based on the comments. Like @Beckside ours will be an open plan kitchen, dining living area and so I am interested to read that you didn't find smell and steam will be invasive, and that @Conor has a similar experience. 

 

My instinct was that chucking out all the heat through an external vent seemed illogical when the principle of MHVR is to reuse it, though I note the comments about balance. 

 

Cheers

[Beckside]

Hi Tony K 

We fitted a Bora hob in the end, but nearly bought a Novy Panorama, the Novy range is good quality, not cheap though. 
if we have 3 pans simmering and a fry pan going then we have to run the extract on its highest setting to capture probably 90% of steam. The MVHR picks up the rest. We have found that the ovens can create more odours as they vent to the front. On the whole though we don’t seem to suffer from odours. One mildly annoying side effect is the Bora vents out into the void in the centre of the kitchen island, and then grilles around the plinths, so you can feel the draft on your legs, but only on the highest setting, which to be fair is not used that frequently. Anything that prevents more penetrations in the airtight layer, especially to outside has to be first consideration. Good luck with your build.

[7dayworker]

On 01/04/2022 at 14:47, Beckside said:

Hi Tony K 

We fitted a Bora hob in the end, but nearly bought a Novy Panorama, the Novy range is good quality, not cheap though. 
if we have 3 pans simmering and a fry pan going then we have to run the extract on its highest setting to capture probably 90% of steam. The MVHR picks up the rest. We have found that the ovens can create more odours as they vent to the front. On the whole though we don’t seem to suffer from odours. One mildly annoying side effect is the Bora vents out into the void in the centre of the kitchen island, and then grilles around the plinths, so you can feel the draft on your legs, but only on the highest setting, which to be fair is not used that frequently. Anything that prevents more penetrations in the airtight layer, especially to outside has to be first consideration. Good luck with your build.

Good to know .  I think the idea of a recirculating extraction hood is good with mvhr. Just not sure if effective at removing spices and fat smells in open plan settings .

[Tony K]

For those to whom this thread may be of use, I received the following advice from the heating engineer I have hired to design my MVHR, ASHP and UFH set up:

 

'Having a cooker hood that vents to the outside can cause issues with an MVHR system as the fans are quite powerful and can overpower the MVHR extract, so we always recommend that hoods are the recirculation-only type.

 

The MVHR unit has a humidity sensor that automatically boosts it when it detects moist air. It also comes with a single gang switch that can be used to manually boost it; some folks put this in the kitchen or utility area so that they can boost the system when cooking particularly smelly/smoky food.'

[Temp]

Ours hob extracts to the outside and is pretty powerful. If it over powers our MVHR it hasn't complained  🙂 It clearly bypasses the heat exchanger in the MVHR so must reduce efficiency but its probably only used a few hours a week so the effect would be small. 

[George]

+1 for team re-circulation. 

 

The grease filters get pretty grotty so it does work as intended (but are easy to wash). I did add carbon filters too but haven't concerned myself with them too much - the MVHR will draw away smells as well as moisture. 

 

I also have wood burners left insitu so had an additional reason to not want to create any negative pressure situations.

[Adsibob]

We’ve just installed our Novy with re circulation and I’m disappointed with how noisy it is. Novy is very expensive, and I only bought it because it is one of the quietest hoods on the market, but I’ve now discovered that that is only for venting outwards. With recirculating, unless you have an even more expensive downdrafter one, it seems to be very noisy. As we have MVHR, I’m starting to wish I’d gone down @Temp’s route and just vented outside. I guess it’s not too late to just drill a hole in the wall and vent outwards, as our hob is on external wall, so solid be easy enough. Just frustrating, as the recirculating kit alone cost me £90.

[Russdl]

7 hours ago, Adsibob said:

With recirculating, unless you have an even more expensive downdrafter one, it seems to be very noisy.

Our down draft recirculating hob can be reassuringly noisy on its higher settings. It doesn’t stay on the high settings for long and soon just becomes a part of the general kitchen hubbub. 
 

Are you listening to your noisy hood in an otherwise silent house?

 

Now you’ve got the recirculating kit you may as well live with it for a while and hold off drilling that big old hole through the wall until the noise really does become an issue. I’d hazard that it won’t be. 

[Adsibob]

Yeah, you are probably right. Other option I’d be interested to learn about is attenuators or dampeners. Anyone use one of those to reduce noise with much success?

[JohnMo]

Not sure there is a quite kitchen extractor, in recirculation or outside venting.  Something that shifts between 250 and 800m3/h of air will not be quite when you are stiff besides it.

 

You should never need a speed above the first setting, I would only use it if you are frying.  Steam will go in and basically come right back out and be taken away by the MVHR.

 

Being negative, I doubt you will have much success with attenuators.

[DragsterDriver]

On 14/04/2022 at 00:10, Temp said:

Ours hob extracts to the outside and is pretty powerful. If it over powers our MVHR it hasn't complained  🙂 It clearly bypasses the heat exchanger in the MVHR so must reduce efficiency but its probably only used a few hours a week so the effect would be small. 

The extract fan scenario is driving me a bit around the bend- I have mvhr and airtight house but the kids use a hob like a bbq with stir fry’s etc. I’m wondering just how bad venting outside would be and if there’s a decent closing vent available. 

[JohnMo]

Change the locks, or send them to the grand parents.

[Faz]

On 13/04/2022 at 11:49, Tony K said:

For those to whom this thread may be of use, I received the following advice from the heating engineer I have hired to design my MVHR, ASHP and UFH set up:

 

'Having a cooker hood that vents to the outside can cause issues with an MVHR system as the fans are quite powerful and can overpower the MVHR extract, so we always recommend that hoods are the recirculation-only type.

 

The MVHR unit has a humidity sensor that automatically boosts it when it detects moist air. It also comes with a single gang switch that can be used to manually boost it; some folks put this in the kitchen or utility area so that they can boost the system when cooking particularly smelly/smoky food.'

That was my thinking - I have a meaty  Airflow MVHR system and 2.6m floor to ceiling heights - 2 extracts on the MVHR in the kitchen area and recirculating hood. 2 months in and I haven't even used it since moving in.  Same with the tumble dryer - another thing yet to be used.  The MVHR is like being inside a tumble dryer without the dizziness! 

 

The only downside is the accelerated drying time on the timber and associated shrinkage having moved in mid Feb.  Oh well - kept a stock of caulk in anticipation.

[DragsterDriver]

On 17/04/2022 at 18:42, JohnMo said:

Change the locks, or send them to the grand parents.

If that was a possibility I’d endorse it heavily.