TL;DR: filling a hole with fire and sleeping on it works way better than you'd think.
Last night I spent my first full night out in full kit. I'll be posting a video of the outing, but one thing I wanted to share was a way of using a Dakota Firehole that I haven't seen before...maybe for good reason.
I've seen lots of designs using fires to keep warm through the night: long fires, self-feeding fires, and some very labor-intensive seeming methods in which you bury the fire or stones heated in the fire and sleep on top of that to keep warm.
I'm not saying no one could pull these off, but they all seem so...finnicky.
Well, after last year's trip to Japan I've been pondering a better way, and can now confirm that it works...exactly how I envisioned, which is kind of wild.
Basically, this takes the concept of the Dakota Firehole and merges it with the concept of a traditional Japanese kotatsu. A kotatsu is a low table with wide insulating skirt running all the way around it--basically, a coffee table with a blanket for a tablecloth. Modern kotatsus have an electric heating element on the underside of the table which warms the air under the blanket, along with the legs of anyone sitting at the table. You can even scooch further under and warm your whole body, depending on the size of the table.
Traditional kotatsus didn't have electric heaters, though--instead, they had a pit set into the floor under the table that held burning charcoal.
That is...kind of a wild concept. Just sleeping with a pit of burning coals IN your bedding? That sounds crazy. I had to try it.
I've previously mentioned that a Dakota Firehole is very cozy to tent your cloak up over. Well, what if you just...slept on top of that bad boy?
So, on day 1 of my outing I dug a firehole and just kept feeding that sucker the biggest branches I could, kept it roaring at full bore for hours, with the express intent of building up as deep a bed of coals as I could in the bottom of it. As night fell, I cooked dinner , then went back to burning that sucker like crazy. I probably had a 6" (15cm) deep bed of coals in the bottom of my 8"-ish (20cm) wide hole. That may not sound like much for hours of burning, but remember that this thing burns WAY more efficiently than an open fire, and basically just eats twigs. I then slept over the hole with it right between my thighs and my wool blanket over the top of me.
It worked GREAT. It was downright hot at first, and despite the sub-freezing temperatures I was warm and happy right off the bat. As the coals were blanketed with ash, they cooled from lack of airflow, and I was able to close my legs over the hole without burning the backs of my thighs. Eventually, the ash got thick enough that heat production slowed down and I started feeling chilly. I sat up, stirred the hole with a stick to bring fresh coals to the fresh air at the surface, and the cycle repeated. I had to stir the hole about once every 3 hours, an operation which took 30 seconds, max, and unlike stoking a fire it took no finesse or concentration. I was warm until morning, sleeping on the ground in the open with just a wool blanket and cloak in sub-freezing temps. I was blown away by how well it worked and highly recommend it.