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22 minutes ago, JSHarris said:

 

 

The dampers provide near-zero insulation, they just stop air flowing through the duct.  The duct will therefore act as a thermal bridge, as it's effectively a hole in the insulation layer.  Without filling that hole with something to provide insulation, it will always be a thermal bridge.

 

But a trapped layer of air the thickness of the wall (say 350mm in my case) isn't that bad? The fact that it would be trapped and not replaced with cold external air would make a huge difference.

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13 minutes ago, Conor said:

 

But a trapped layer of air the thickness of the wall (say 350mm in my case) isn't that bad? The fact that it would be trapped and not replaced with cold external air would make a huge difference.

 

 

Once a layer of air gets thicker than about 30mm, convection currents start to flow, so heat will flow across that air gap from the air circulating within it (from the warm side to the cold side).  Insulation usually works by making trapped air spaces small, so restricting the ability of air to move around within it.  It's the reason than the spacing between multiple pane glazing never usually exceeds about 20mm, as if the gap gets bigger than this the insulation value starts to drop off. 

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13 hours ago, JSHarris said:

 The incoming air flow will increase, and over power the ability of the heat exchanger to work efficiently.

 

It occurs to me this might not actually be a problem?

 

Even in our old not-that-insulated house any cooking tends to heat up the kitchen quite a lot. Given I've read various people here say that even visitors coming into their low-energy houses has a noticeable effect, sometimes triggering cooling systems, I'd think cooking has the potential to create a great deal of excess heat?

 

In that case it's maybe not the end of the world if, during cooking, most of that heat gets dumped outside rather than being efficiently captured by the MVHR?

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10 minutes ago, andyscotland said:

 

It occurs to me this might not actually be a problem?

 

Even in our old not-that-insulated house any cooking tends to heat up the kitchen quite a lot. Given I've read various people here say that even visitors coming into their low-energy houses has a noticeable effect, sometimes triggering cooling systems, I'd think cooking has the potential to create a great deal of excess heat?

 

In that case it's maybe not the end of the world if, during cooking, most of that heat gets dumped outside rather than being efficiently captured by the MVHR?

 

 

The real issue is getting enough air in to the house, as the MVHR will be so imbalanced that it probably won't be able to deal with the additional fresh air supply requirement.  If a hob extract is sucking air out of the house at around 50 to 150l/s, then that additional flow has to try and over power the fans in the MVHR and try and get loads more air in through all the MVHR ducts (they are the only route for air to come in).  Room supply ducts rarely flow at more than about 5 or 6 l/s normally, and so the terminals and ductwork on the supply side won't be sized to allow the big additional inrush of air.  The result will probably be that the extract rate from the hob drops, due to the fan depressurising the house (it's way more powerful than a blower test fan, for example).

 

If fitting an extractor plumbed to the outside of the house, then I think the best bet is to try and arrange for some way to supply enough air to make up for the high flow rate that's being sucked out.  Depressurising the house is not great, as the chances are that the extract side of the MVHR may well end up reversing because of the pressure change.  This might not be great if it pulls smells and condensation from the bathrooms, WC etc into the house (plus if it did this there would technically be a problem with building regs, as these rooms are required to be extracts, with a minimum extract rate.

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51 minutes ago, JSHarris said:

If fitting an extractor plumbed to the outside of the house, then I think the best bet is to try and arrange for some way to supply enough air to make up for the high flow rate that's being sucked out. 

Open the kitchen window. 

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@JSHarris yes, I see that would be an issue. So I guess the solution would be a standalone high-flow inlet duct coming in by the cooker somewhere, with a damper linked to the fan/extract damper?

 

Then you'd avoid causing problems elsewhere in the house, keep the cooking smells/fat etc in a fairly tight "channel" between inlet and extractor, and minimise impact on air temperature away from the cooker?

 

Would still give you the issue of how to avoid those ducts being thermal bridges when not in use. Given convection only happens in larger cavities, would it be insane to make the wall ducts something like a honeycomb of say 22mm pipes?

 

 

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The easy way out is to just use a recirculating extractor with filters, one for grease and an activated carbon one for smells.  Saves having holes through the walls and seems to work well.  The filters on some are designed so they can just be stuck in the dishwasher to clean them, I believe.

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WRT the position of the extraction, what is trying to be managed is in hot air, which rises. Therefore the optimal position for the extraction is directly over the pan, the next best one is to the side and the worst at hob level. 

I visited a Neff dealer to have a look at the in-hob method, it didn’t impress me and the dealer didn’t rave about it either - apparently the claimed efficiency is achieved by having the extractor on for 30 mins before starting cooking and with no order air movement, hardly a real world environment. 

In our particular case the island isn’t centrally located in the room, which is on the first floor with the ceiling following the roofline, so we don’t want a ceiling hung version. The.Novy sounds like a potentially good compromise, as does a Gaggenau lifting extractor (£££😳). @vivienz are you pleased with it? It’s a lot of money, but potentially justifiable if it does 2 jobs well. 

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Just been doing some rough estimates as to how much effect having a fairly powerful  external extract fan has, in terms of heat loss when turned on.  Air from outside has to come in to replace the air being extracted, so if it's 10°C outside (it was colder than that here this morning) and the house is at 21°C inside, then a 500m³/h extract is going to pump just over 1.8 kW of heat out of the house all the time it's running.  If the outside air temperature drops down to 0°C, then that increases to a bit over 3.5 kW. 

 

Some of that will be made up by the heat from cooking, but almost certainly not all of it, as the heat from an induction hob mainly heats the pan, so it's easy to estimate the heat loss.  A big pan, with an external surface area of about 0.25m² sat at 100°C in a room at 21°C will lose heat at a rate of about 265 W, so even two or three such large pans sat at 100°C aren't enough to come close to making up for the heat lost through the extract fan.  The same goes for the heat from an oven.  Heat will be lost at a lower rate than from a pan, as the surface temperature of an oven is a lot lower.  There's also the issue that heat is being extracted from the kitchen, and unless there is a way to deliver cool air from outside directly to the kitchen, the cool air being drawn in via, say, the unbalanced MVHR, will flow into the living areas of the house, so cooling them down.

 

Using our house as an example, the worst case heat loss rate in very cold weather (-10°C outside), is about 1.6 kW.  An extract fan rated at 500m³/h, running on such a cold day, would increase the house heat loss rate to just over 6.8 kW.  To keep up with this heat loss rate increase, the heating system would need to be increased in capacity by a factor of about 4.

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8 hours ago, JSHarris said:

 

 

The dampers provide near-zero insulation, they just stop air flowing through the duct.  The duct will therefore act as a thermal bridge, as it's effectively a hole in the insulation layer.  Without filling that hole with something to provide insulation, it will always be a thermal bridge.

 

To be clear, I'm mostly convinced about the recirculation being a better option, probably cheaper too.. but interesting discussion so..

 

I suppose it depends on what you consider "inside the house", then. (obv not talking about when air is flowing). 

If I encase a bit of pipe in the perfect insulation on all sides (except the house) then wouldn't one have simply a slightly strangely shaped house, where effectively the insides of the pipe is still outside, for the purposes of insulation? 

 

Of course once you want to do something with the cold air you'd still run into trouble, so I'm not really considering using external air for extraction. 

 

Alternative thought - is it an idea to put an extractor away from the hob, e.g. 'in the garage' and do the filtering, recirculation there?

Most recirculators are reasonably quiet so perhaps too much overeengineering..

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Here's a rubbish picture of the novy in situ with the extractor on the lowest height position. Each hotspot is in front of the slider control, i.e. parallel with the line of the extractor.

 

I have found it to be very effective; my MVHR extracts for this room are also located about 1.5 metres back from the hob so rising steam isn't a problem.

 

I'm very pleased with it.

20190914_194635.jpg

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4 hours ago, vivienz said:

Here's a rubbish picture of the novy in situ with the extractor on the lowest height position. Each hotspot is in front of the slider control, i.e. parallel with the line of the extractor.

 

I have found it to be very effective; my MVHR extracts for this room are also located about 1.5 metres back from the hob so rising steam isn't a problem.

 

I'm very pleased with it.

20190914_194635.jpg

 

Weird, I've been looking at Novys for a little while and indeed only your picture made me realise how it actually is oriented. Okay, that looks quite sensible. That also explains the 'Panorama' name... Ha.

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We have an AEG induction hob (model HK854320FB) which is mostly very good but does have a few issues which are annoying me. I don't know if it's a faulty model or if this is behaving as standard and we are misusing it. It often beeps and deactivates itself.

 

Frequently this coincides with the surface getting too hot, mostly when we are using heavy cast iron pans so I'm curious to know if @JSHarris has the same problem when using Le Creuset pans.

 

Often it deactivates when the surface is not hot. Reading the manual suggests this may be due to something being put on the 'sensor field' (the control panel?), which I'm not aware of having done. Has anyone else had this problem or have any idea if the relevant sensor can be replaced?

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Ours beeps sometimes when you're wiping it down, due to the "something being on the sensor field" issue.  It's a minor nuisance, but I'd guess that any touch sensitive hob is probably going to be similar.  It doesn't deactivate when in use, though, and it also doesn't seem to get that hot at the surface when in use. 

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