rocket mass kotatsu?

a kotatsu came up in paul's electric heat thread.  we built one last year that works pretty well, but I'm not comfortable for very long sitting on the floor.  my roommate really likes it, though, so I'm looking for a way to make it work for both of us.

I've heard of a kotatsu that is placed over a pit sunk into the floor.  I've sat at an unheated table like that, and it was comfortable and pleasant.  so I would like to build one in a structure I'm planning that will have an earthen floor.

my next gripe with our current kotatsu is that I don't like to heat with electricity.  the kotatsu we built uses an inexpensive infrared heater designed for this purpose that my roommate brought home from a trip to Japan.  they're not too difficult to find in the U.S., but they're very expensive.  this one works, but I would prefer an alternative.  I believe charcoal heaters are sometimes used, but I'm clumsy enough that I would probably set myself on fire with charcoal burning under a table.  so, what about a rocket mass heater?

I was thinking that the exhaust duct could travel around the circumference of the pit, so it would warm up the posterior and legs of whoever used it.  my roommate likes to take naps under a kotatsu, so maybe I would build the pit in such a way that there's room to lay down under the table at ground level.  the combustion unit could be close to the table and used for making tea or coffee.  I would probably sink that part into the floor as well so that it could be operated and used for tea by a person sitting under the kotatsu.

I guess this isn't such a revolutionary idea, but I thought I would put it out for public comment well ahead of when I'll actually build it in case anybody has suggestions or sees potential problems.  and maybe it will stoke some other creative fires, or inspire someone to build something similar.

Are you making it round or square? Have you thought of making it also be able to cook food, heat water or some other double duty, besides heat the butts?  Will the feed tube be off to the side somewhere or perhaps right in the center to be able to roast hot dogs and marshmallows?

Got a diagram of some kind you can take a photo of and post?

shape:
my working plan is for a rectangle.  I think it would be much more difficult to plumb a circular route for the exhaust.  building a rectangular table also seems easier.

multi-use:
I was thinking of a pretty standard exposed barrel for the combustion unit, so the top of that could be used for cooking and heating water.  I'm also considering using one combustion unit for several heating routes and using a switch to choose between them like Erica demonstrated in one of paul's videos.  a couple of bathtubs, a cob bed, and an earthen floor are the other things I'm wanting to heat with one or more rocket stoves.  the floor and kotatsu could probably be combined into one route more easily.

diagram/photo:
nope.  but I made a quick MSPaint drawing to demonstrate the main idea.  not to scale or a final design or anything but a demonstration of what I'm thinking about.

I like it. I've been thinking of an in ground RMH to heat with.

I don't know about splitting the exhaust...I wonder about just running the exhaust all the way around and back towards the barrel then out.

My thinking is that you could sit over the first part of the exhaust at first, then move around to cooler parts of the bench if you wanted different temps. Plus it might be easier to run it straight. I guess that would work best for me.. You may have different reasons for wanting both sides to be heated equally.

I was planning one loop around the perimeter instead of splitting the exhaust.

sort of thinking that gradually bringing the exhaust toward the surface as it gets further from the burn.  could provide a more even temperature all around the table while it's burning.  the parts toward the end wouldn't hold their heat as long in that configuration, though.  and on the other hand, it might be nice to have different temperatures at different spots.  something to think about and talk to my roommate about.

Well I guess when you get the sauna rocket finished you can post the progress on the Kotatsu rocket on here>?

ronie wrote:
Well I guess when you get the sauna rocket finished you can post the progress on the Kotatsu rocket on here>?

that project is dragging on and on, but I'm getting there.  most of the challenges for that stove are about the small space.  and my inexperience.  I'm optimistic that future stoves will go more smoothly and quickly.

the building this kotatsu is destined for isn't exactly a sure thing just yet.  supposing it happens, though, I'll certainly post about it here.

I LOVE this idea!

I find myself with FREEZING feet as I sit and type at my computer desk in the winter. Something like a Kotatsu would be idea.

I have had the same idea, and think it is a winner.  It will be a while before it happens for us, so I will probably steal some ideas from your design.    My biggest concern is that the kotatsu may be too warm.

yukkuri_kame wrote:
My biggest concern is that the kotatsu may be too warm.

I bet the key to avoiding that problem is to bury the pipes deeper.  that would add more inertia to the whole thing, so it would both heat up and cool down more slowly.  just have to find the sweet spot, which could be challenging where actual tons of rubble and mud are involved.

Thank you for this thread.
Very interesting discussion.

I have been thinking about the same thing, but to be used outside under a roof for outdoor dining and entertaining. My thought is that having a heated bench and table area would allow us to be outside earlier and later in the season. I hate having to give up the outside dinners as the weather gets colder, and the idea of using one of those propane heaters that sits above you just does not appeal to me.

I'm wondering about using a Y split to create two separate benches. That way there is an easy way to get onto the bench even if you are older or find it difficult to get your legs over the bench and under the table. Has anyone ever tried to split the exhaust? Successfully?
    ________
-<________ 

Sort of looking like this. The end close to the infeed would have the Y, then the opposite end of the table would be clear of the bench and only have the table covering hanging to the floor. It would hold the heat in but allow easy access.

Any thoughts?

I believe folks have tried split exhausts before, and I think they work better if they come back together at some point.  leads to more equal flow between both branches.  haven't gotten there myself yet.

your outdoor idea is similar to what I had planned.  the building in my imagination that would house this kotatsu is sunk below grade a couple of feet, but the walls would be something like hazel hurdles that could be removed during warm weather.

You could increase the draft in the exhaust tubes, if you used a small rocket stove on the exhaust end, and have a double walled riser tube on the small rocket stove.  Feed the kotatsu exhaust into the outer wall of the heat riser near the base, and allow the hot gases of the rocket stove to accelerate them by thermal induction.  By small, I am talking one made out of a one gallon paint bucket and some 3 inch tubing.