This is a description of my recent experiments with body temperature control. (shortened)
Excuse me, I'm not a scientist though I'm interested in science. I tend to use a verbatim, imaginative descriptions of the phenomena. There are usually no professional meanings from physics or biology, a words resembling them are usually meant laically, imaginatively, or utterly wrongly.
And before you begin to read, I'm deadly serious with every word I wrote. I have no desire to waste my and your time with made up stories. All what I describe entered my brain through a sensitive network of my nerves. It entered my subjective brain, not yours, I can't guarantee that you can have such a subjective experience. I had it, you may or may not, and there is only one way how to find it out. By practice, not belief. Furthermore, I only rarely drink alcohol, (not much in recent years) and I don't take any drugs nor psychopharmacology medicines. I'm also not diagnosed that I should. So we've got that clear, let's continue.
It is one of my hobbies to explore the power of the self and the so-called paranormal abilities. It is very diffcult to explain them scientifically, but I believe that thanks to a serious work and cooperation of scientists and practitioners it will be happening in next decades. Some signs even suggests, that this is already happening.
Today, I will present to you my own investigation of the ancient Tibetyan technique called Tummo. However, I did not follow any of the Tibetyan methodology, it's mostly an improvisation of my own.
If you are interested, sit comfortably and prepare for a complex and long reading, which will be hopefully also interesting.

I work in a big factory hall with big doors which are being opened every once a while, into a snowy, windy and rainy day. The work is usually not very active. People on most of the workplaces doesn't usually get warmed by moving. It doesn't involve a lot of movement, but rather a standing, slow walking, bending, opening and closing stuff. The main priority is to be systematic, watchful and to save strength to do the same, slow and seemingly easy movements for 8 hours.
Thus it's often rather cold in there, and we, poor exploited workers usually wear a jackets or sweatshirts with long sleeves, scarves, and so on, but everyone feels cold, anyway.

Well, not quite so with me. I experiment with controlling my metabolism and body temperature - by the power of my mind. I only wear a t-shirt and a light jacket with removed sleeves. Most of my arms remains bare. (except of hands, on which we must wear a white cloth gloves, but these gets often wet)
How do I feel? Warm, dear people, overwhelmingly, pleasantly warm, even hot.
My co-workers keep asking me if I'm not freezing, or that I should wear something more. But I actually need to keep my arms bare to cool down somehow. If I feel my temperature dropping, I only keep the zipper up to raise the collar of jacket, and that's all isolation I need. (besides that, I've got a dense hair which also helps a bit)

So the actual process is like that... I imagine an energy field, I visualize it wrapping the body parts I need. (body and torso usually) I visualize a fire, embers, heat waves, a white-hot inside of a furnace, and so on, and at the same time, I focus on the areas of body which should be heated.
An important part of the visualization is breathing. (through nose) It's regular, steady, not very deep, and it's visualized as to directly blow up the fire. (this may actually be  borrowed from the Prana exercises) In the first moment it does nothing, but a few moments later the mind focusing goes on and I feel a real warmth spreading through my heat-yearning tissues and staying there. The inner heat awakens and I feel some kind of energy streaming. A further mental effort makes the heat stronger, more persistent and better placed. When the inner fire is well founded, only a small amount of thought effort is needed to support it, if the cold draught ocassionally blows more. A small mental order - and an intense heat waves comes immediately, warming me again on a pleasant temperature. It's a wonderful thing!

Does it have any drawbacks? Yes, it has. My control over the heat production is not perfect. It requires some time, effort and mental focus to start, which I usually do in the first half an hour of work. The preservation of it also requires a mental focus. It's like an inner fire, which must be carefully sustained. Next, it doesn't automatically heat up all the body, only a chosen regions, like forearms or torso. I must use the mental focus to extend the energy field on my hands, for example, and then start 'pumping' the heat energy there mentally. The heat doesn't spread regularly, like I can warm my foot, but the whole leg before it can stay cold. Fortunately, it doesn't require a continual focusing later, just an ocassional check.
It probably requires to eat something in the morning, though I suspect it's only a feeling of hunger which hinders the necessary mental concentration. Furthermore, the whole mental heating is not a protection from a direct freezing wind and it requires some insulation against the cold.. (at least not with my present skills) And finally, it's diffcult for me to turn it off. This day was freezing and I had to heat up a lot, more than usually. My forearms still feels like burning, while I with the rest of body actually shivers slightly, because the house heating is not on.

One day at work a month or two ago I was outside and I had to stand and wait in the frost and wind. As always, in such a boring whiles, I entertain myself with mental exercises. 'What these exercises are good for, if I'm freezing?' I thought. So I started to focus on an imagination of fire, and with a partial success. (mainly thanks to a heavy jacket) In next days and weeks of an awful, cold weather I kept practicing and training myself, so I got much better in it. It's still far from perfection, but very useful. I didn't get sick or cold at all, though the warmly clothed people around aren't so lucky.

Is it a thing unheard of? Actually, not. As I mentioned, this may be my personal version of the tibetyan Tummo technique. However, Tummo is done by sitting in deep meditation, but I need to keep myself warm, while working.
There are some hypotheses on how it might work.
Firstly, I have a theory on this which drives Nordmann crazy.

Secondly, it may be a classical physiologic process. A layers of skin or muscles can relax themselves, to allow the blood flow into a surfacial capilars and to heat the body better. At the same time, a metabolic burning of fats and sugars may occur to produce the heat.
It is however unclear, how this can be so precisely and quickly controlled solely by mental imagination. I observed no increased hunger or exhaustion when heating myself like that. Also, no typical pointy pain in liver, when it lacks a sugar, as it feels after a running. Also, the plentiful heat couldn't be generated by movement, as I wrote, the work we do is usually rather slow. No shivering, no big strain on muscles at one moment, which could generate such a heat. No heavy breathing.
If this hypothesis is true, I hope that it metabolizes my fat, and not my muscles, that my liver and other metabollic organs will not get damaged somehow. Furthermore, the psychosomatic mechanism allowing that to happen is unknown to me. Some expert is welcomed to clarify that.

So what do you guys think? I tried to be as comprehensible and serious, as possible. I even had cut a lot of crap which you don't need to read. Is there any peer among you who could do a peer-review? Who could devote a few days or weeks to replicate the results? Any questions or comments? Is a typical rationalist able to work meaningfully in frontiers of human experience with his/her own physical and mental tools, (like I try) or are they dependent on a scientific authorities with often unaffordable mechanical detectors?
Maybe this will ruin my reputation even more, but fortune favors the brave. Before you try to criticize this all, imagine how would you handle such a situation by yourself. Would you rather rattle your teeth in the cold, because the science says that you have to? I just hope that the response will be more rational than from a Christian, who would be scared by the Hell and witchcraft resembling mental invocation of the fiery heat