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Changing gas hob to induction

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We have just (finally!) moved into a new-build house, which has a gas hob fitted and we want to get this changed to an induction hob (we didn't have any say in the matter, as I think it was already fitted when we first viewed the house).

The current gas hob is 75x52cm with a 56x49cm hole in the worktop and the one my wife wants is 80x52 and requires a 75x49 hole, so we will have to get a GasSafe registered person to remove it and then get the hole trimmed out to fit, although those aren't really a big deal.

However, the bit that's bothering me is the wiring.

The electric double oven has it's own isolator switch in the kitchen and fuse in the consumer unit and will be using suitably beefy cabling to cope with the power demands.

The hob, on the other hand, currently only requires enough power to run the ignitors on the burners and is currently connected via an "ordinary" cable and plug to a socket behind the drawers. This has it's own isolator switch on the panel above the worktop and is on a ring labelled "kitchen sockets" in the consumer unit, which includes the fridge/freezer, fume hood, dish washer and the visible sockets for the toaster and kettle, etc.

My concern is that this ring won't cope with the power output of the induction hob, which seems to be 7400W, according to the manufacturer.

So, can I wire the new hob into the existing cooker ring, or do I need to get another circuit installed just for the induction hob?

And please let's not descend into a debate about gas -v- induction

Thanks in advance.

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 henwardian 05:20 Mon
In reply to LastBoyScout:

The wiring is always designed to safely carry a current greater than the fuse/circuit breaker so all your calculations are based on the latter (which is fortunate because they have ratings written on them).

The calculation you need to do is to P = I * V with mains electricity V = 230 to 240 but to be safe, use 230 in your calculation.

When you do this for your hob, it's going to be drawing about 32A max and the plugs ring circuit probably has a circuit breaker rated at 30A so clearly that is not a good idea (even if the ring circuit breaker is rated to 40A it's still a bad plan because you have lots of other sockets in the circuit which might be being used at the same time).

My guess when you look at the cooker circuit and do the calculations with the power of the cooker is that what you will find is that there isn't an extra 32+A of rating available to add the hob on the same circuit (overengineering the circuit is a waste of money and putting in a much higher rated circuit breaker than needed risks a fire or other damage in the appliance being supplied before the circuit breaker cuts out and finally, I would strongly advise against assuming the cooker circuit wiring is thick enough to just beef up the circuit breaker and add the hob as this is just asking to start a house fire).

So what you probably already suspected is what you are going to have to do: Wire in another circuit.

You can do this yourself pretty easily but if you are going by the book you need to get an electrician to check your work (and honestly I suspect the price to get an electrician to do it completely themselves would be little different) because it isn't a minor enough change that you are allowed to just DIY it.

But then who is ever going to even know, let alone question you if you do DIY it? :D

Post edited at 05:23
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 kristian 09:12 Mon
In reply to LastBoyScout:

Simple quick answer is get an electrician to pull in a new circuit. The isolator will have to go in an easily accessible cupboard below or alongside if you don't want your tiles smashed up. This may not be so easy. I would not recommend it as a simple DIY job.

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In reply to LastBoyScout:

As they've said - this will definitely need a new circuit. There's no way you can safely or legally add a 7.4kW device to a regular ring main. If you don't want to invalidate your household insurance it'll need an electrician to do it (or at least sign it off).

Does your consumer unit have a spare circuit? If not then this just got more expensive 😣

I know you said you didn't want the gas vs induction debate - but I have to ask why you want to do this? Electricity is about three times the price of gas; even taking efficiency savings into account it'll cost about twice as much to run. And under almost all circumstances it's no greener; the saved CO2 from not burning it in your kitchen is just transferred to a power station somewhere. It'll only ever be a net CO2 saving at times when the UK grid is running completely on renewables, which isn't very often (admittedly becoming less rare these days).

I have an inductor and I love it - but I also have a massive PV array to drive it, and a couple of gas hobs to use when it's not sunny.

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 abcdef 11:14 Mon
In reply to LastBoyScout:

Another option would be an induction that just plugs into a normal socket via a 13amp. It would probably have to be a small size (60x51) but that may fit the worktop hole you have. They are pretty underperforming though, so if you use multiple rings at once some can be starved of power, boost function won't really work etc.

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In reply to all:

Thanks - I'd already done the maths and was essentially after confirmation I was right and not missing something.

The ring for the sockets is 32A, as is the cooker.

There are 2 unlabelled fuses on the consumer unit - they're both turned on, but I haven't worked out yet what goes off if I turn them off. I would have hoped one was for an induction hob, but the fact that I think they are 6 and 20 amps and there's no isolater in the kitchen suggests not. Putting one in won't be an issue, as the kitchen isn't yet tiled - that's a job for another day - and, happily, there is a spare slot in the consumer unit.

Why do I want to do it? Because that's what my wife wants! We had one at the old house and she wants the same. We might get solar panels here at some point - it's a nice, south-facing roof with clear view of the sky.

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In reply to LastBoyScout:

Applying diversity, I get a design current of 16.65A. Depending on the capacity of the oven circuit, it could share that. New house? You should have an Electrical Installation Certificate which will have all the information your (competent) electrician will need to advise you correctly.

Forget to say, the hob will have installation instructions which should be followed.

Post edited at 12:01
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In reply to Taylor's Landlord:

> Applying diversity, I get a design current of 16.65A. Depending on the capacity of the oven circuit, it could share that. New house? You should have an Electrical Installation Certificate which will have all the information your (competent) electrician will need to advise you correctly.

The oven circuit will be a spur rather than a ring though, surely?

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In reply to Jamie Wakeham:

> And under almost all circumstances it's no greener; the saved CO2 from not burning it in your kitchen is just transferred to a power station somewhere. It'll only ever be a net CO2 saving at times when the UK grid is running completely on renewables, which isn't very often (admittedly becoming less rare these days).

30+% of the time so far this year iirc?

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 marsbar 16:17 Mon
In reply to LastBoyScout:

I presumed it was your wife that wanted it, it is quite an illogical thing to do in my opinion.  

I suppose its better to do it now while the kitchen isn't tiled.  

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In reply to Toerag:

> The oven circuit will be a spur rather than a ring though, surely?


It could be a spur off the RFC, but I think you mean that it's a single point Radial cct, which is what LBS states in his OP.

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In reply to Toerag:

I'm not sure.  It's been coal-free for an impressive length of time this year, but I can't easily find out how much time has been without CCGTs running.

Simplifying somewhat, for the induction hob to be greener than gas, it needs to be a period when there is excess green energy being fed into the grid and your increased electricity load won't contribute to a gas turbine being switched on.  Obviously, unless you're generating your own PV or are on a variable rate tariff, it'll always be more expensive than gas.

It's been ridiculously sunny recently (my PV production for Mar/Apr/May was up 27% on the previous best ever for that period!) but I don't know if that means all the CCGTs were off.  Would love to know if there's a way to look at the historical mix.

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In reply to LastBoyScout:

Thanks, all.

Dug the wiring certificate out of the pack and the unlabelled 6 and 20 amp breakers are for the boiler and immersion heater, respectively - there's no other circuit mentioned.

To cut a long story short, we'd already asked about it a couple of times before (site manager and sales office) and had been assured there shouldn't be a problem, but having looked at the wiring and checked on here, it currently isn't.

The site manager popped by today to see how we were getting on, told him what we'd established and he's sending one of the site sparkies round in the morning to look at adding the necessary wiring, so fingers crossed...

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 Philip 19:06 Tue
In reply to LastBoyScout:

Depends how much the scrimped on cable.

I've run all my large devices in 10mm², but then fused them to the appropriate appliance size. If they've done that they upgrade the MCB to 60A or whatever is needed and out your hob and oven in together.

Essentially, if you can't do the calculation yourself don't attempt the work. The fuse protects the cable, the cable is sized for the appliance.

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In reply to marsbar:

> I presumed it was your wife that wanted it, it is quite an illogical thing to do in my opinion.  

> I suppose its better to do it now while the kitchen isn't tiled.  

Which aspect of this do you find illogical? Getting induction at all, or wanting it in a new build already fitted with gas? Just that I am planning to get a plug and play induction hob and wondered if that too is illogical (I am always interested in your input as it often seems well considered). I have got rid of a knackered gas cooker. 

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In reply to Taylor's Landlord:

Most people forget about diversity and just add the maximum ratings together.

Though to be fair if the OP doesn't know what diversity is, it's best to speak to someone who does (i.e. an electrician), as it's a *bit* complex.  You can of course not apply diversity and just add the maximum ratings up, but if you do you'll overspec (and so spend money) unnecessarily.

(Diversity, to summarise it, is to formally make use of the fact that you never actually have everything on full at once - even a double socket makes use of that, because they aren't rated to 2*13A=26A, they are rated below that, though I forget what!  There is a formal way to properly calculate it.)

For those talking of the socket circuit, I think the OP was talking about a dedicated 32A cooker circuit which currently feeds the oven only?  Most houses will have something like this, I never saw one that didn't.  If, applying diversity, the oven plus the hob don't come to 32A wiring both into that is fine, and indeed is normal practice.

Post edited at 20:23
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 marsbar 20:35 Tue
In reply to Blue Straggler:

Changing a perfectly good new hob for one that isn't that much different seemed illogical.  

If yours is broken then a replacement gas one will need a gas safe engineer so a plug and play makes sense, unless you end up needing a new circuit at which point you are deciding between electrician or gas(wo)man.  Not much in it at that point. 

I don't know much about efficiency but there was a post about this already. 

Having said that I didn't realise the builders are going to do it for the OP anyway, and its certainly better to do it now if that is what she wants.   

Post edited at 20:39
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In reply to Blue Straggler:

If you've got gas, it's an odd choice to fit an induction hob. 

From a cost point of view, electricity is more per kWh than gas. Even allowing for the fact that induction is less wasteful of heat, it'll be about twice as expensive.

From an eco point of view, there's not much in it, and won't be until the grid becomes quite a bit cleaner than it currently is.

From a cooking point of view, I'd pick gas. A good induction hob is nearly as good as gas, but not quite...

If you only have electricity, then induction is a no brainer. And there are other circumstances in which it might make sense - if, like me, you have a big solar PV system, for example. 

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 marsbar 22:33 Tue
In reply to Neil Williams:

Diversity or not you can’t put a large induction hob on a ring main.

With a double oven it’s unlikely to go with the oven.  

Diversity is all very well until Christmas for example where you do have all the rings and both ovens going.  

Post edited at 22:37
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In reply to Jamie Wakeham:

https://www.gridwatch.templar.co.uk/

has some historical data

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 Philip 23:12 Tue
In reply to Jamie Wakeham:

Induction over gas any day.

Induction, press 5A, walk away pan comes to boil and auto turns down to 5 (happens to be right number for my common pan). And spillage, wipe clean Schott glass top.

Gas, light it (hopefully), combustion products require ventilation, risk of fire or kids burning themselves, difficult to clean up.

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 Sir Chasm 23:22 Tue
In reply to Philip:

> Gas, light it (hopefully), combustion products require ventilation, risk of fire or kids burning themselves, difficult to clean up.

Induction does sound like a good idea for people who can't manage to use a gas hob without killing their kids or burning the house down. And slip on shoes for those who can't manage laces, those nice round ended scissors for those who can't cope with pointy things, a nice top rope. 

Post edited at 23:37
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 Philip 23:41 Tue
In reply to Sir Chasm:

Also easy to operate one handed - you might try it out.

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In reply to Dr.S at work:

http://grid.iamkate.com/ has now become my go to National Grid dashboard - runs off the same data but has clearer breakdown of the total generation into parts. The clear change over the past 5 years is that fossil fuel is now nearly always way under 50% in the UK and only going one way - so any appliances changes from gas to electric are only going to get greener.

The days with out coal has been a bit of a silly stat for some time, its clear there is no need for any coal on the grid and is just being used for anoth couple of winters as emergency backup (more for plant/ gas supply failure than lack of wind/solar). They clearly need to turn them on occasionally to keep them operational.

Demand is now pretty much met with simply topping up the wind/solar available with gas, allowing us to use our gas supplys only when needed and be far less reliant on imported fossil files than in recent years, and limited storage is allowing backup gas turbines to be left at a much more efficient lower tickover speed as they do not need to do instant starts in case of a grid problem. The future is at least in one way brighter 

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 jbrom 00:37 Wed
In reply to Sir Chasm:

No comment on gas vs induction hobs. 

But comparing induction vs gas to toproping vs leading is peak UKC.

Loving your work.

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In reply to Jamie Wakeham:

Are economy and the environment common misconceptions causing people to choose induction over gas and then find out they have been conned? 

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In reply to Blue Straggler:

> Are economy and the environment common misconceptions causing people to choose induction over gas and then find out they have been conned? 

Our kitchen is open plan to the lounge.  I just don't want stuff burning open-flame in living room.  Not just because it could put stuff on fire but because I don't want to breathe in gas or combustion products.  Electricity is cleaner.

Also induction hobs look modern and cool and are easy to wipe clean.

Post edited at 07:51
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In reply to marsbar:

> Diversity or not you can’t put a large induction hob on a ring main.

Indeed you can't, and I never said you could.  There are hobs where you can, though.  Basically if it was delivered with a 13A moulded plug on it, you can plug it in.  That type generally either are less powerful or have special software to limit what can be on at once (I don't mean turned on, I mean it manages how the thermostats or equivalent work).

> With a double oven it’s unlikely to go with the oven.  

It might or it might not.  There is a specific way to do the calculation - the answer, which is absolute, can be found that way.  The answer is simply to do the calculation (or pay someone to do so if you don't know how to do it) and that will inform what to do.

> Diversity is all very well until Christmas for example where you do have all the rings and both ovens going.  

It still works then, because everything has a thermostat (even a hob) so nothing actually is all on at once.

Not applying it is just throwing money away.

Post edited at 08:07
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In reply to tom_in_edinburgh:

If you're open plan, you want a proper extractor (not a recirculating filtered one).  That will solve that problem.

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In reply to Neil Williams:

> If you're open plan, you want a proper extractor (not a recirculating filtered one).  That will solve that problem.

I've got an induction hob.  I'm perfectly happy with it.

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In reply to Blue Straggler:

I mean, conned is a strong word, but I bet that a lot of people who put them in have no idea they cost more than twice as much as gas to run!

I do take the point about ease of cleaning, and perhaps the one about not having combustion outputs in your kitchen if it's poorly ventilated.  And I can also see that if you need to pay a gas engineer, but could simply plug in a 13A hob, that's going to be cheaper in the short and medium terms (perhaps even over the appliance lifetime).

Templar and iamkate are both brilliant, and I teach about them often.  It's clear that the grid is cleaning up, and rapidly, which is brilliant.  It's why I run an electric car.  But CCGT still make up a large portion of the supply at most times, and while that is the case there's no environmental gain to be made switching from gas to induction.  You're just changing the location the gas is burned in.  See also those 'solar diverters' that dump your PV panel output into your hot water tank - they're not green at all, because they're just causing the grid to have to work harder.  At times when coal is burning (thankfully not that often any more) they're significantly carbon positive.

One of the reasons I got an induction hob fitted was that this is, hopefully, my kitchen for life, and I very much hope that in ten or 20 years, the grid will be more or less carbon zero - in that situation the induction is a clear environmental win over domestic gas.

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In reply to Jamie Wakeham:

>  See also those 'solar diverters' that dump your PV panel output into your hot water tank - they're not green at all, because they're just causing the grid to have to work harder.

Can you explain this? Surely water heated with your solar panels is water the grid doesn't have to heat?

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In reply to Toerag:

If the default situation would be that your solar PV is exported to the grid, and your gas boiler heats your water, then the net result of using an Immersun type diverter is that you burn less gas in your boiler, but you also export less electrical energy to the grid. That means a power station somewhere has to do a bit more work to make up for it.

At most times, that extra grid load is provided by a CCGT, so it's burning approximately the same amount of gas that you just saved from your domestic boiler. Net change in CO2 is zero.

At really bad times, that extra power is provided by coal or small scale diesel: the net effect of the diverter at these times is actually to increase CO2 emissions.

Only when the extra load on the grid is met by low carbon intensity supplies (ie there's an excess of wind or solar, and all the fossil fuel generators are switched off) can these diverters cause a net decrease in CO2.  And that's still relatively rare.

PV owners love them, though, because of the peculiar way most systems are subsidised. You get paid for the power export not based on actual measured export, but on the assumption that 50% of your total generation is exported. So it doesn't actually matter how much energy they export, they get paid the same. 

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In reply to henwardian:

> But then who is ever going to even know, let alone question you if you do DIY it? :D

 I wonder if you would’ve had the same cavalier attitude about gas 30 years ago 

 You’re quite right it doesn’t fall and DIY it is a professional job  30 years ago it wasn’t  but today it is to answer the question who is going to know maybe the loss adjuster will find out if  it has a latent fault and causes an  Earth fault that an MCB  did not ( pick up )  yes it’s on in  RCD but how often will the function test be  tried  and how often will it be tested properly  all resulting in severe electrical burns on the paralysis of a limb for the remainder of one’s life 

 Yes go-ahead break the law but accept the consequences 

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 marsbar 16:37 Wed
In reply to Blue Straggler:

I've just remembered someone said part of the reason induction was good  for your kitchen was the damp issue.  

I'm not sure how much difference it makes but it may be relevant.  

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In reply to marsbar:

> I've just remembered someone said part of the reason induction was good  for your kitchen was the damp issue.  

> I'm not sure how much difference it makes but it may be relevant.  


Hahahaha, what about all the water that you boil off from your cooking? I suspect you've been bullshitted by an induction hob salesman 😂😂😂

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In reply to Neil Williams:

> Most people forget about diversity and just add the maximum ratings together.

> Though to be fair if the OP doesn't know what diversity is, it's best to speak to someone who does (i.e. an electrician), as it's a *bit* complex.  You can of course not apply diversity and just add the maximum ratings up, but if you do you'll overspec (and so spend money) unnecessarily.

> (Diversity, to summarise it, is to formally make use of the fact that you never actually have everything on full at once - even a double socket makes use of that, because they aren't rated to 2*13A=26A, they are rated below that, though I forget what!  There is a formal way to properly calculate it.)

Thank you for the explanation. I think I'd factored it in for the sockets without knowing it's proper name, as you'd be unlikely to be running the kettle, toaster, radio, coffee machine, extractor hood AND the hob at full power all at once. Once again, I've learnt something from UKC. Even so, it seems I was correct that just plugging in a big hob was too much for it.

> For those talking of the socket circuit, I think the OP was talking about a dedicated 32A cooker circuit which currently feeds the oven only?  Most houses will have something like this, I never saw one that didn't.  If, applying diversity, the oven plus the hob don't come to 32A wiring both into that is fine, and indeed is normal practice.

There is the socket circuit I mentioned, which runs the sockets and the current gas hob is wired into that.

There's also a dedicated 32A cooker circuit for the double oven. Our last house had both a double oven and an induction hob, but I didn't have anything to do with the installation of them and you're probably right that they're wired into the same 32A circuit, as I don't remember seeing a specific breaker for the hob and there certainly wasn't a separate master switch in the kitchen. We are looking at a slightly bigger hob here than we had at the old house, though, so hence my question, as there's no obvious way here of connecting it to that circuit without cutting holes in the back of the unit and putting in at least a bit of cable.

No sign as yet of the sparky, but it seems someone's been ill and the message hadn't got through - hopefully see them tomorrow.

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In reply to marsbar:

> I presumed it was your wife that wanted it, it is quite an illogical thing to do in my opinion.  

> I suppose its better to do it now while the kitchen isn't tiled.  

I'm quite happy cooking on gas, but, as Tom_in_Edinburgh said, it's a brand new modern kitchen and the monster gas hob in there looks incredibly out of place and positively archaic.

Induction is so easy to clean if anything spills and gives you more work surface when not in use. We've also noticed that on the same pans, the handles get really hot on the gas rings, as there's so much more wasted heat going up the sides of the pans.

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 Yanis Nayu 07:37 Thu
In reply to LastBoyScout:

We’ve got an induction hob - it’s brilliant. 

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 marsbar 08:42 Thu
In reply to LastBoyScout:

Fair enough.  It's your house. 

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In reply to Taylor's Landlord:

> Applying diversity, I get a design current of 16.65A. Depending on the capacity of the oven circuit, it could share that. New house? You should have an Electrical Installation Certificate which will have all the information your (competent) electrician will need to advise you correctly.

Thanks - that is useful. Seems the oven is rated as 4.8Kw, so what would that work out as and would the cooker circuit cope? Please could you show your working?

The site electrician popped in earlier and said he would put it on it's own circuit, due to the "Christmas Day" effect, but it would be a bitch to get the wiring in because of the way the joists run in the ceiling and the wall's already been plastered.

Wife is not happy!

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In reply to Taylor's Landlord:

> Hahahaha, what about all the water that you boil off from your cooking? I suspect you've been bullshitted by an induction hob salesman 😂😂😂

You still have the steam from water boiled when you cook with gas, so that argument is invalid. When you burn gas you're creating CO2 and water vapour. With induction you don't have that extra water vapour.

A quick google tells me a gas hob will consume up to 2m3 per hour of gas, which will create 2-4 times that volume in water vapour.

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In reply to Toerag:

I was interested to see if Taylor's Landlord's scorn was warranted here so I've just been playing on the back of a fag packet.

Assume we want to boil pasta for two.  That means I need to bring 2L of water to the boil, then simmer for eleven minutes.  Bringing 2kg water from 15C to 100C will take 714000J, or 0.7MJ.

The energy density of methane is 56MJ/kg, so it'll take 12.7g of methane.  Google suggests gas hobs are about 50% efficient so we'll have burned something like 25g of methane to boil this water. 

Total oxidation of 25g of methane produces about 56g of water vapour.

Then we have to simmer it.  Judging by how quickly my gas meter needle clicks around, I reckon it's at about 1/4 power when it's simmering, so let's call that 800W.  800W for 11 minutes means I've used a further 530000J of gas; that'll add another 42g of water vapour.  So the total vapour from combustion is about 100g.

Meanwhile we've just put 530000J into the pan.  Gas hobs are more efficient at low power (less of the burning gas is pushed away to the sides) so let's assume 75% efficiency for this process; about 400000J is available to turn water to steam.  L for water is 2260kJ/kg so about 175g of steam is produced.

Thus it looks to me like the steam from the boiled pasta water is around twice as significant as the vapour from the gas combustion (or to put it another way, if your gas hob makes your kitchen steam up, switching to induction will remove about a third of the problem).  I can se that, if this is a real problem in your kitchen, you might decide that reducing it by a third is a good thing to do, but it's by no means a magic bullet.

Can you tell I'm bored?

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In reply to Jamie Wakeham:

Probably

Realistically it's a bit moot, though, as if you have an open plan arrangement you want a proper extractor above any type of hob, otherwise your lounge will be damp and stink of food.

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