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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorcjard
    • CommentTimeJun 30th 2016
    I asked the granite counter maker about the possibility of insetting my induction hob rather than having it sat on the surface. He said "the manufacturers require a 3 mm gap all the way round, which can only really be filled with silicone, and it'll look awful, but i'll do it if you want"

    3mm seems awfully big. Is it reasonable to assert that a 900mm hob might really grow to 906.1mm during normal cooking and smash itself to pieces?

    Engineering toolbox gave me the linear coeffiecient of expansion for pyrex (assuming this is what the hob top is made of) as 0.000004. For a 0 degree start (let's assume it's installed in the depths of winter) and a 300 degree final temperature (let's assume I've been boiling oil on all burners for 10 hours in the event that I'm under seige and need something to throw out of a top storey window), the hob will apparently grow to 901.1 mm

    Are my calcs right? Anyone else have a take on what a reasonable tolerance all round would be? I think 1mm would be fine for an induction hob - they're marketed as something you can touch right after you finish cooking and not end up in hospital with third degree burns, so how hot is it going to get, really?
    • CommentTimeJun 30th 2016
    A 1 mm gap will be too narrow to fill with silicone and over the years will collect crumbs, splashes of fat, etc. The exercise feels to me like it would be more trouble than it's worth but then I'm generally for a simple life.
    • CommentAuthorEd Davies
    • CommentTimeJun 30th 2016
    Isn't it the coefficient of expansion of metal that matters? What are induction hobs made of - something non-ferro-magnetic, I assume? Don't hobs designed to be inset have an overlap round the edge?
    • CommentAuthortony
    • CommentTimeJun 30th 2016
    Usually the lip of an inset hob sit on top of a worktop and the rest of it sits within the depth of the worktop so the size of the hole is almost irrelevant so long as it is not too big. There is sealant between the very thin stainless steel flange and the worktop the gap can be 0 to 1mm so barely visible. The hob should not in my opinion sit under the worktop (nor should sinks)
    • CommentTimeJun 30th 2016
    The hob that Caius is describing sounds like ours, in which the surface is made of glass. There may be others that are surfaced with stainless or something. The guts of the hob include many materials of course but it is designed so the guts are well clear of the cutout in the worktop. The glass normally sits on top of the worktop and is a few mm thick. So I believe it is the expansion of the glass that would be relevant to setting the top of the glass level with the top of the worktop. The hob remains cool enough to touch comfortably at all times, except in the vicinity of the heated areas.
    • CommentAuthorJSC
    • CommentTimeJun 30th 2016
    TBH, I would have thought that having a recessed hob with silicone around the side would look no worse or better than having the hob sat on top of the worktop (with visible edges), probably with a bit of silicone on the underside to seal it to the worktop in any event...

    Assuming you have a black hob, just get a black silicone. The ‘cleanliness’ of the solution would win for me!
    Hmmm - obviously the recessing will require some really high precision work, especially with regard to getting it square! What some have missed is that it will be a rebate, and I would go for 2 or 3 mm deeper than you need and use a higher viscosity sealant. There will be no need to fill the 1mm gap around the perimeter as the sealant will be applied to excess and squige out as the glass is pushed down into the rebate - any low points in the silicone will be filled during clear up. You will need good access to the underside and a very delicate touch with some wedges - anything more than 1/10 of a mm to high or too low be obvious you will also need (initially) very fine wedges to square the glass within the rebate, as soon as it starts to get down to the right hight remove the upper wedges. You'll only get one chance, you'll have to be calm, steady, slow but quick...if you know what I mean - sounds great though, could look just divine, could....
    • CommentAuthorcjard
    • CommentTimeJul 1st 2016 edited
    Will have to see what the granite boys can do then...good call on the deeper rebate.. It might be possible to temporarily glue some arms to the surface of the glass topside, projecting over the hob edge, to prevent it dropping too low as it is pressed into the silicone.. If the job is beveled ( haven't vhecked) wedges could be placed under the arm ends.
    A similar idea is used for jointing Corian; temporarily glue blocks of Corian to the work surface, then use a g clamp to clamp the blocks towards each other, holding the joint while the adhesive sets. Superglue should scrape off a polished surface (Corian, hob surface etf) quite easily with a new Stanley blade..
    • CommentTimeJul 1st 2016
    Some of the hobs come with a preformed gasket that fits underneath the glass to seal it to the worktop. I suppose you would have to discard that before using silicone as a bedding material.

    The other thing I noticed last night looking at our hob is that the top edges of the glass are bevelled down to reduce the thickness at the edges. That would look rather strange if we inset it I think, so worth confirming exactly what yours looks like before cutting the worktop.

    Our glass is labelled Schott Ceran by the way.
    Posted By: cjardIt might be possible to temporarily glue some arms to the surface of the glass topside, projecting over the hob edge, to prevent it dropping too low as it is pressed into the silicone
    KISS my friend - bet you don't get the arms accurate enough, plus 2 or 3 other reasons why this complexity might not pay off. Nothing better than an eye and a finger. With wedges underneath you should be able to recover a tiny bit too deep.

    Did you ever manage this in the end?

    I'm considering it for our induction hob and it would be good to know how it worked out...
    • CommentAuthorDarylP
    • CommentTimeSep 14th 2018
    I sunk a Neff induction hob into oak worktop, routered 3mm-ish margin, filled/sealed with black hybrid polymer sealer.
    Works fine... :-)
    • CommentAuthorowlman
    • CommentTimeSep 14th 2018
    I fitted an AEG low profile hob with a low bevelled SS surround that sits on top of the worktop to eliminate this problem. I don't find the 3 mm projection above the worksurface a problem, and the glass edge is protected from knocks.
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