Wednesday, 8 January 2014

A Japanese-style kotatsu made in Scotland

Traditional Japanese wooden houses don't have central heating - instead some rooms have air conditioning (イアコン), the toilet seat is usually heated, while the living room often has a kotatsu heated table (炬燵, こたつ). Modern kotatsu heaters are compact electrical units mounted on the underside of a low table, which is then covered with a quilt hanging down to the floor to trap the heat. Family life in winter is centred on the kotatsu, for watching TV, eating dinner, etc - with everyone sat round the table with their legs and lower body nicely warm.

A kotatsu heater is probably my best souvenir purchase from Japan to date. The heaters are small enough to bring back in a suitcase (unlike a complete kotatsu table unit), maybe even in hand luggage? To create a kotatsu here in Scotland I've attached the heater to an Ikea LACK coffee table, and added a large duvet and voltage transformer:

The kotatsu is a welcome addition to the traditional stone-built Scottish cottage where we live, which is hard to get warm in winter - despite retro-fitted central heating with radiators. If this winter gets really cold, we'll move the kotatsu in front of the open fire place...

Electric Kotatsu Heaters

Nowadays kotatsu heaters are electric units, usually mounted under the table surface within a frame which is covered by the quilt which the table top rests on top of. The main manufacturer selling replacement heaters seems to be Metro Denki Kougyou (メトロ電気工業, or Metro Electrical Manufacturers) whose products also seem to be available as OEM rebadged versions like Yamazen (山善). Metro have a nice English page about their heating elements and their kotatsu heaters.

The current Metro kotatsu heater range are 600W units: MSU-600E(K) with a quartz heater, MHU-600E(K) with a halogen heater, and the MQU-600E(K) with a red quartz heater. These all have U-shaped heating elements with a fan, and have temperature control on the cable itself which is very convenient and an improvement on the older 400W and 500W models like the MS-400HS(K) and MSF-500H(K), where this was on the heater itself - hard to reach under the table.

There are also variants of the current 600W Metro/Yamazen units with more advanced controllers featuring a timer, but I didn't see any of these in stock at the shops I visited.

I bought the MQU-600E(K) with a red quartz heater while in Japan, and brought it back in my suitcase. The unit is designed to fit in a 29x29cm frame, and comes with spacer bars for a larger 31cm frame, and four thumb screws to attach it. According to the label on the back, it was made in Malaysia.
Metro MQU-600E(K) red quartz kotatsu heater kitSmall 'wart' on the red quartz heating element
I was a little worried that there was a defect on the red quartz heating coil - there is a quite visible 'wart' about 2mm in size on the right hand side of the U-shaped coil, especially noticeable once glowing, but apparently this is just a manufacturing artefact.

Note the Metro kotatsu heaters expect the Japanese standard of 100V. Most of the links below are to people running them in USA or Canada (120V) without a transformer, but the heaters probably require a step-down transformer to run in the UK with 230 to 240V. Better safe than sorry.

Other DIY Kotatsu Builds

Super Sailor Mars' kotatsu used a small Ikea Lacks table with the legs cut down (finished version). Similarly Silver Skeeter's kotatsu and Annie's kotatsu project used a large Ikea Lacks table.

Chronicles of a Sleepy Rabite's kotatsu used two Ikea Hemnes tables, 90cm by 90 cm, legs cut down from table being 45cm high to 40cm high. I liked how they made an actual frame for holding the heater - and wondered if this would be possible using the shelf parts from a single Hemnes table instead?

As a final example, Count-Down-Zero built his own kotatsu table from scratch!

Table & Quilt Sizes

Although you can get round kotatsu tables, browsing in-store and online most are 40cm high and either square (75x75cm to 80x80cm) or rectangular (75x105cm to 80x120cm). Ready made kotatsu quilt/futon sets assume this (up to 190x190cm for the square tables, 190x240cm for the rectangular tables), although international postage costs makes buying them online unattractive (and slow).
Cover for square kotatsu tableCover for rectangular kotatsu table

Based on these sizes, the Ikea Lacks square 78x78cm coffee tables should work well with a standard double bed duvet (200x200cm), even if kept at 45cm high. Going for this smaller square table would therefore have made things quite a lot cheaper as we could have reused existing bed linen.

Instead we opted for the larger Ikea Lacks rectangular 78x118cm coffee table, and therefore needed something bigger for the quilt. Sadly even a UK King sized duvet isn't big enough. However, at 220x240cm the European King sized duvet works - and conveniently Ikea sell these and duvet covers too. I found with the table at its original 45cm height the duvet was only just long enough to touch the ground along the short edges. In addition, I found the table uncomfortably high, so cut the legs by 6cm bringing the effective table height in line with the expected Japanese norm - and ensuring the duvet fits much better.

My Kotatsu

Parts list:

As in the Ikea Lack examples above, rather than having a removable tabletop above a frame holding the heater, this approach mounts the heater directly to the original thick (5cm) table top - and uses what would normally be the coffee table's shelf as a thin (1cm) second table top above the quilt. This works, but to make it more stable a heavier tabletop could be used - or a thinner quilt?

Following the other blogged examples linked to above, I looked for suitable right angle brackets in our local DIY stores. For a flush mounting I wanted a hole about 12mm up, and managed to find a set of 4cm zinc corner brackets which worked perfectly - any larger and they would have stuck out above the heater.

Thumb screw provided with heater, added corner brace Kotatsu heater attached with four corner braces
Testing the Metro Kotatsu heater, mounted in place Metro kotatsu heater mounted under Ikea Lacks coffee table
Without the shelf in place to prevent this, the Ikea Lacks table legs are prone to rotate. To prevent this I reused the little right angle brackets Ikea include to attach the shelf to instead lock the table legs in place (see photo below).
Ikea Lack shelf support reused to prevent table legs rotating6cm offcuts from the hollow Ikea Lack table legs
Note if you do cut the Ikea Lacks table legs, on mine the lower parts are actually hollow with only a thin capping piece inside (see photo above). Perhaps it would have been better to cut off the top, and re-drill the screw hole... hard to say. By cutting the table legs from 40cm to 34cm, this made the final table height 39cm, or including the former-shelf table top about 40cm (plus the quilt), matching most ready made tables on sale in Japan. This also ensured the Euro King duvet touched the ground easily on all sides.

View from under the kotatsu blanket at full powerThe bulky step down transformer to run the heater in the UK
The weather so far this January has been quite mild, but we should be ready when it gets colder :)

Update (Feb 2015)


One year later the kotatsu is still going strong, and I've improvised some Ikea ZAISU to go with it.

Update (Feb 2016)


Two years on, we moved house. We don't yet have a sofa and we're still using the kotatsu and zaisu. The new place has a wooden floor with under-floor heating. We're using the same rug with a couple of blankets underneath making it comfortable to sit on, but have added a few cushions too. The heater is still plugged in, but is now only used occasionally rather than daily in winter.

Update (June 2017)


Since moving out of the charming traditional stone cottage to more modern well insulated accommodation, we only used the kotatsu heater a few times. In fact, last winter we never plugged in the heater at all. The low table with duvet has found another role - it is padded and a nice height for a crawling baby to practice standing. 

15 comments:

  1. Dear Peter, thanks for very interesting blog. I'm wondering what is the experience with this setup so far? What kind of floor chairs or seats do you prefer? Are both your heater and step down transformer working correctly? And have you considered to upgrade for that LCD and timer equipped "PC-Kel32" handheld controler for the Metro heater, it's only +10quid in Japan or so? All best Natasha

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    1. It's been working fine with near daily use, no problems with the transformer or heater (but not had a new electricity bill yet). We don't have any floor chairs (not easy to find in the UK) and just use cushions and sometimes lean on the sofa for back support.

      I have wondered about upgrading the controls, but never saw any of the more advanced Metro controls on sale in Japan in stores. The basic controller has both constant heating (numbers 1 to 9) which can get a bit too hot, or thermostatic control (red dots of differing sizes, next to the off position) which we usually use.

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    2. Thanks for the update. I've found EU made kotatsu heater in stainless steel for 230vac, but it's only 300w and the description and pictures don't show any signs of functioning as proper "infra" quartz tube heater system, perhaps only a resistive heater with fan? And it's bloody expensive on top of that even in comparison to individual Metro heater import + transformer combo. http://www.levraimeublejaponais.com/authentic-japanese-furniture-en/catalog/nagomi-line/furniture-list.htm
      Regards N.

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    3. That heater you linked does look very overpriced. I suggest you check on eBay for imported kotatsu heaters - currently I can see several of the older/cheaper Metro models like the MS-504HS(K) or MSF-500H(K) - see the links on the Metro page I linked to for full details. It will take a bit longer to warm up from cold, but otherwise the older style heating elements or lower power versions should work just fine. One benefit of a lower power heater is you could get a cheaper transformer (e.g. 500VA rather than 750VA version that I bought).

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    4. It took a while, but we now have some Japanese style floor chairs (zaisu), again sourced from Ikea. See link at the end of the post.

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  2. I am a Japanese living in Surrey. I am thinking of doing what you are doing. I am having trouble convincing my Japanese wife that we need one!! :( I have couple of questions. Do you have kids? If so do they like it and use it? Also, how did you accurately cut the table legs? Lastly, do you think you save money on the electricity bill? Thanks

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    1. We've not tested our kotatsu with children. I cut the table legs with a hand saw, then sanded the ends for smoothness and to match the lengths better. We're paying more electricity but saving on kerosene heating oil - it is hard to say if this saves us money overall (but Scotland is colder than Surrey).

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  3. Hi there, as people in US are not using a transformer, do you think it would be OK to use a US/UK stepdown convertor in the UK with the kotatsu heater? Asking because the Airlink one you bought is now £102 inc. postage, whereas you can get a decent US one for £60.
    thanks!

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    1. That would probably be fine (although given the time you'll probably be using the kotatsu the extra money isn't much). Alternatively, since we don't seem to use the heater at full strength, a 500VA rated transformer ought to be enough and also costs less.

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  4. Hi Peter,

    Awesome. I actually kind copied your design but I am finding it a little wobbly. Do the L brackets help to stop it wobbling that much? I intend on using it at my pc desk as want to sit on the floor and work.

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    1. If you find the table comfortable, it certainly is a cozy work space - but I don't find it ideal for long stretches of time. The L brackets Ikea include with this table are tiny, only 1cm in length - the way I attached them, they do nothing for stability other than stopping the leg rotation. As an aside, try to screw the legs on with the veneer seam at the back, otherwise it will get rubbed by the duvet.

      Our kotatsu table is also a little wobbly, partly from the second table top resting on the duvet, but also from the underlying table (even with the shortened legs). The original Ikea LACK design has the shelf attached underneath which acts as a brace, making the table more rigid, but we have to remove the shelf (to put our legs under).

      I would suggest buying some larger L braces from a DIY store, perhaps even two per leg. Hopefully you can make it more stable without restricting leg access or leaving any sticky out sharp bits to bang your knees on.

      Even the authentic kotatsu tables that I've used in Japan are a bit wobbly - it hasn't really bothered me.

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  5. Hi Peter,

    I thought the same thing. The reason it is wobbly is because the shelf isn't bracing it. I ordered some 40mm L brackets. I'm hoping to get the screwed on. My only hope there is enough good material to screw into as the legs felt like they are made of chip board.

    I don't currently have the cover on it as I've not had time to get one on, plus I wanted to fix the wobble first but I have mine so I lean against the wall and I can sit at it for hours. I have a small cushion to sit on and another to lean against. It's not as comfortable as my office chair at work but good enough for a few hours and it forces me to get up, stretch and have a walk around which is good. But I left the legs at the normal length it's perfect for using a pc.

    Thanks again for the great idea.

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    1. I'd have to turn our table upside down to check exactly where the legs are hollow from, but I think you'll be fine attaching 40mm L braces.

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  6. Brainstorming #1
    Hello This is my first time commenting on here but would like to add something I feel that this would work with a few hot water bottles instead of an electrical appliance? Im going to try it out at a work desk that my daughter uses Im a big fan of hot water bottles and the theory that we should heat the person rather than the whole room I make garments which have large pockets into which the hot water bottles can be placed. It doesnt take much heat to keep a person warm in the coldest of climates Used in conjunction with the kotatsu it would prevent the bottle cooling too quickly. By the time you would need to replenish the bottles you would probably need to stretch your legs anyway

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  7. This looks very nice, you can be proud of the result! I love kotatsu a lot as well, unfortunately there are not so many available online where I live, which makes for a perfect DIY project! Smart of you to buy the heater in Japan, saves you a lot of trouble finding a suitable one!

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