The most effective masonry heater I've experienced was at my aunt & uncle's house in Minnesota, and I was kind of amazed by it as a kid. It actually drew cold air from outside the house rather than warm air from inside. As my uncle explained, 'if you let it draw from inside the house, the house will suck in the cold outside air anyway to replace that, so you may as well draw it directly from outside." I never knew exactly how the inside of it worked, but I remember it taking an armload or two of wood in the bottom chamber and a few sticks in a tiny upper chamber. You had to light the upper chamber first and let it burn for fifteen minutes before lighting the bottom chamber. It didn't *seem* to be getting hot at all - the outside of the heater was only a couple dozen degrees fahrenheit warmer than the house. But two armloads of wood (and a handful of sticks) kept it warm for four or five days. The chimney didn't seem to smoke at all, and there was hardly any ash left when it was time to light the next fire.

Great video, love the side by side test, and would love to see a comparison between new and an old sand trapped stove!

Thanks - really interesting ;-) How I see it and think it is/was: The reason for the older model's different design is that previously they did not use the type of ceramic insulation that we have and can use today. In order to achieve some form of insulation between the channels, the wall thickness was therefore increased in some places in the older model. The outer metal barrel set a limitation, so therefore the wall thickness of the internal channels had to be set according to the best possible judgement. I therefore believe that we must take this into account when we want to develop a new model based on old experiences. In the new improved model, we should take into account both the physical "laws" and properties, and recent experience for each individual component. We want to achieve as good a draft in the oven as possible and at the same time as good a utilization of the heat energy as possible. Design principles can therefore be: Every time the flue gas rises, we regard it as a chimney where we want as high a speed as possible and with as little heat loss as possible, because a heat loss in an upward flue gas will slow down the flue gas because the density increases, i.e. we insulate the chimney. Because we work with a closed system, each section, i.e. an upward or a downward section, should be seen separately. Every time the flue gas is cooled, we want this to take place when the flue gas is in a downward direction, because the cooling increases the density and therefore cooling in a downward direction helps to increase the draft in the furnace. (Large cross-sectional area and large volume in the downward channel is desirable, and relatively large wall surface with good thermal conductivity and large thermal mass. The large volume gives the flue gas time to release heat. Think: bell). There may (probably) be a need to use a shunt damper when the oven is started. And perhaps also one or more other dampers if the stove is built in an area with very low winter temperatures, because when it is extra cold outside you can actually allow the flue gas to be cooled a little more and still have a heat surplus (differential temperature, i.e. temperature difference between bottom and top of the chimney ) to drive the flue gas up through the chimney. On the other hand, this will also complicate the use of the stove, so it is probably worth considering how much it should be optimized because if you do not understand how to use the stove, you will end up with smoke in the house. And thank you for the description of why you remove the glowing coals from the stove. I'm thinking whether you could make a system where you can push the glowing coals into a small, separate mini-furnace, which is airtight (i.e. carbon monoxygen-tight) in relation to the house. But if it can't be made carbon monoxide safe, it's probably better to carry the glowing coals outside as you showed. You showed in a previous video "Finnish masonry heaters. -The baking oven. Part 2." , that this oven has a secondary combustion chamber, and now I understand even better the significance of this.

If I had the financial ability I would fund your research. Thanks for the useful info. God bless

very helpful. Thank you.

I'm curious whether the exhaust is as clean on the 2nd burn as the first, what the temperatures are on the exhaust and whether they're using the same amount of wood (I apologize if you said this last and I just didn't catch it). Part of the point of the rocket mass heater is to have a very clean burn. It would also be interesting seeing a head to head of a this against the same amount of wood in a well designed rocket mass heater (could be someone else's) as a comparison. How heavy is this? The small footprint is appealing for certain situations. I suspect like other masonry heaters it would require some remodeling for support to add to an existing home.

I’ve seen some ones who had the ash collection tube in the basement to keep that one defrosted as well

Very nice, thank you. The Pönttöuuni reminds me of the Utermark stove, which was developed in the Russian empire in the 1820s (ru.wikipedia.org/wiki/Печь_Утермарка ). I think that's where the idea of the metal sheet came from, as Finland was part of the Russian empire at the time. The Utermark stove had several up and down channels to maximise thermal efficiency by getting the last bit of heat out of the exhaust gases.

Very interesting, thank you. I particularly appreciate your diagrams. Perhaps the chief lesson here is that excess air might drive an impressive blaze but is also pushing heat up the chimney (not to mention pulling cold air into the house to replace it). I have a question about construction - how do you get the metal case over the brick lining - have you very high ceilings, or is it in sections that can be dismantled? I struggled to even get an oil drum over a tall rocket riser because of the ceiling and would appreciate any tips. Thanks again 👏💯

Your infrared themometer is seeing a reflective metal surface, which includes cooler images lowering your overall reading. The wall is much less reflective surfac e giving a more accurate reading

If I may I'm operating 2 coal stoves a stoker and a hand fired. 3rd and 5th year. To burn efficiently I've been a gusting them like a carburetor . Could you make an adjustment on the wide open stove buy closing one door and adjusting the other door . The other stove seal the door tite and leave the ashes.

Yes please & thanks

Very interesting but how well does it heat the space and how large of space does it heat? Just seems like those temps aren’t very hot?

Per my comment below,.... the draft is very impressive, but that's because of the extremely long riser in that giant metal tube. A "J" rocket will draft like that on a small, 3 foot (or less) riser. Please see my comment below on the website you must see.