Heat Storage Hypocausts: Air Heating in the Middle Ages (2017) | Hacker News Reader

Heat Storage Hypocausts: Air Heating in the Middle Ages (2017)

lowtechmagazine.com

75 pointsKirinDave39 comments

If you want to learn about an ancient solution to the opposite problem, air conditioning, I recommend this:

https://en.wikipedia.org/wiki/Windcatcher

Thanks for posting this, the gallery section left me speechless. I hope tensions die down so I can go visit Iran one day; i'm always impressed by the historical architecture that is present in the Middle East.

You can go there now as an American, and it may be even safer than going there as a non-American, because you will have a government minder spend every waking moment with you.

I don't have references - but I remember reading that the gently sloped roofs of desert buildings would channel breezes, and that would be combined with narrow passages between buildings to create a more shaded and cool environment. I also remember reading that when Rome extended its control to warmer regions the buildings they built always included a basement for use during the warmer months. I sometimes wonder if a lack of inspired building codes has undermined our entire society. In my mind I imagine buildings created to foster community and encourage good use of resources such as heating, cooling, and food production.

In some rural areas of Spain where access to wood used to be limited hypocausts still exist, work great and are heavily used because they are very efficient. I've seen lots of them and their construction is practically the same described by Romans

The podcast 99% Invisible had an episode[1] earlier this year that featured some of these passive innovations like India's stepwells[2]

[1] https://99percentinvisible.org/episode/thermal-delight/ [2] https://en.wikipedia.org/wiki/Stepwell

If you're in the UK or visiting the UK the Roman Baths in Bath, UK offer an amazing experience to see saunas heated with hypocausts.

https://www.romanbaths.co.uk/

>A furnace's dimensions would depend on the size of the room to be heated. In private homes, where only the bedroom was heated, it would be one to two metres long, a little more than a metre wide and 50 to 60 cm high. In public buildings and monasteries, where large halls and rooms had to be heated, the furnaces would be much larger.

This is massive compared to the size of a wood stove. It's amazing what a thousand years of incremental refinement will do.

These systems do predate mass production of steel, so no surprise there.

It is interesting to contemplate what solar or geothermal variants of this system could do. Nearly every home has a crawlspace of those dimensions underneath the ground floor.

Nitpick:

I think you mean cast iron. Steel is kind of tricky to cast and we've only been casting it for ~100yr and even then not for large things in high volume. The largest high volume production (i.e. consumer item sold by the thousands) cast steel thing I know of is steering knuckles on the newest Ford trucks. In contrast we've been casting things of all sizes and shapes our of iron since the 1800s. Considering the mechanical requirements of a wood stove it doesn't make sense to cast it out of steel.

You know, I thought that specific parts being switched to steel was a reason for the uptake of popularity in a wood burning stove, but I see this is only a case in a very specific market of wood burning stoves. Thanks for the correction!

Which parts switched to steel? Other than fasteners and misc. hardware that has to be strong and fairly malleable to withstand owner abuse (hinges, handles, latches, etc) I don't see much of a need for steel in a wood stove.

The modern wood stove is probably less energy efficient. With chain saws and wood splitters being affordable people don't care nearly as much about efficiency. Also most people are using their fireplace as decoration: when in use they are actually negative efficiency: more heat is going out the chimney than they are getting from the wood.

Of course there are people who actually care about efficiency (wood is actually used for heat, not decoration). Most of them are running some form of masonry heater very similar to these designs.

I wonder what the efficiency is compared to a modern "rocket stove" build [1]

Mass rocket stoves are _incredibly_ efficient. So much so that I've always wondered why more similar designs weren't used in times when the primary heat sources were wood, etc.

1 - https://www.youtube.com/watch?v=fwCz8Ris79g (skip to 1:50 if you want to see a full one and not the ms paint diagrams)

You still need similar amount of of mass and volume and if you want to store the energy and not to tender the fire all the time.

If your sole source of heating is wood, you want a massive brig stove that stores the energy. You heat it maybe once a day.

These are a fore-bearer of the masonry heater. Very similar to what some off-grid types have built and called a rocket mass heater. Very interesting principle but unfortunately (or fortunately if you are so inclined) they will never compete with the ease of operation of a gas (or oil) central heating system.

... as much as there are issues w/oil & gas heat, reverting with wood would for the current population would quite quickly deforest the planet, not to mention be more polluting (especially particulates). Design for efficiency, solar collection into a thermal mass, etc...

Coppicing and pollarding are sustainable methods of producing wood for rocket mass heaters, which operate best with kindling-sized sticks of wood.

Rocket mass heaters are extremely efficient. The wood undergoes almost complete combustion and particulates are not a concern.

It's worth noting a good rocket heater will have similar properties to a wood gassification system, which people may recognize as powerful enough and light enough to run converted service vehicles in the tail end of WW2.

Kindling is the very smallest pieces you start a fire with - are you really supposed to use mm sized pieces? Coppicing would not be useful for that, unless you're heavily processing the coppiced wood first?

We use a lot of energy. Last time I ran the numbers I estimated that if the Drax wood-fired plant used UK wood rather than Canadian wood it would consume every tree in the country within a year.

750k tonnes/year: https://www.drax.com/sustainability/one-company-helped-trans...

Drax looks like it generates electric power, not heat.

Well, what's an interesting question about these systems is, "In a modern context where we understand science much better and have much more sophisticated materials available, could we use these principles?"

The real trick here is using available materials for heat storage, using air convection to circulate heat but not waste gas, and tightly limiting oxygen flow to make the most of minimal materials.

It's definitely not bad, from an efficiency standpoint, to be compared to rocket heaters. They're still some of the most efficient systems we have.

We could probably improve the dimensions quite a bit if we used something other than granite for heat storage. Something that changes phase, like in those pocket warmers, would probably be more effective.

There is fireclay [1], and I have fireplace with several bricks made from it. When fired in the evening for 2-3h, will keep warm until morning.

[1] https://en.wikipedia.org/wiki/Fire_clay

“High efficiency” water heaters and furnaces use similar principles to this (and the glazed tile oven mentioned). A winding network of passageways to get maximum contact with the flue glasses and thus extract as much heat as possible.

By contrast a traditional straight chimney wastes a lot of heat by dumping it outside.

That can be a feature in the summer when you wanted to cook inside.

I cook on a wood-burning stove year round. Here in west-Sweden it can get quite warm in summer (with last summer being an extreme example with a maximum of 37°C and a period of around a month with daily temperatures between 25°C and 30°C, we did most of our cooking outside on an electric plate as open fire was prohibited due to the extreme drought). When firing the stove just for cooking I use quick-burning wood (spruce, fir, etc) and it doesn't really add that much heat to the kitchen, the thing is quite efficient. Come the colder months I light the stove a bit earlier, using heavier types of wood (birch, elm, oak, etc). I have yet to fell a living tree for either the stove or the heating stove (which, between the two of them, heat the whole house) as every year there is enough windfall to keep up with the demand (between 3 and 5 cubic meters of split wood per year). Felling, hauling, cutting and splitting wood is about 2 weeks of exercise per year, it is the fuel which keeps you warm twice - once while harvesting, the second time while burning it.

Yes, this might be more work than just flicking a switch, touching a zone on an induction cooker (which I used before I moved to Sweden) or turning a knob but it is more reliable, more sustainable, more useful and more fun than the alternative (electricity).

37C is really not that hot. 40C with 90% humidity is IMO where things start to get unpleasant.

Growing up I had access to basically free wood and we used it regularly for heating and occasionally cooking. However, at scale it takes fewer resources to use solar power than a wood stove. It just does not seem that way when you have ready access to plentiful trees.

In west-Sweden 37°C is hot, this region normally sees temperatures around 20°C in summer, -15° to -20° wintertime. The highest temperature ever measured in Sweden is a paltry 38°C, lowest -52°C [1] so that 37°C was... hot.

[1] https://www.smhi.se/kunskapsbanken/meteorologi/svenska-tempe...

I understand when you're not used to it that might have felt bad. But, most places regularly hit 40+C and some hit 50+C.

So, going back to my point if expect to want to cook when it's 40+C, being able to open some windows and get a real draft going with a fireplace is a nice feature.

The whole discussion on whether 37°C or 40°C is hot or not rather misses the point which was the use of a wood-burning stove when there is no need to heat the environment. One of the ways this can be done is by using fuel that burns fast and hot so the amount of heat can be regulated more easily, another way is to simply use a smaller stove [1] which is placed on top of the existing one, in Swedish this is called a 'sparspis' (spar = to save, spis = cooker). The exhaust for such a stove sits on the bottom, it connects to the chimney by removing as many rings as needed to get a tight fit. These come in single (like the one pictured in the link) and double-ring versions. The smaller stove uses less wood - it is fired with sticks and branches picked from the forest - and spreads less heat.

[1] https://gamlavikmanshyttan.se/annat-se-lasvart/sparspisen/

>it is more reliable, more sustainable, more useful and more fun than the alternative (electricity).

It will also lower the life expectancy of whoever has to breathe the smoke [0] so I hope you don't have neighbors.

[0]: https://samharris.org/the-fireplace-delusion/

I do, the closest lives a few hundred meters from me. Wood burning is not a problem in a sparsely populated area like the Swedish countryside. If you live in an urban environment things look different, both for the reasons you state - wood smoke being noxious - as well as for the simple fact that you need a few hectares of forest per house to keep it sustainable (about 4 ha. for heating a normal-size house, I need much less but have much more so I can get by with using windfall).

Do keep in mind that I'm not talking about burning wood in a fireplace like Harris' article talks about. A fireplace is an inefficient and wasteful way to burn good wood, about 85% of the generated heat disappears through the chimney. The heating stove I use is rated at 12KW nominal, uses secondary combustion (using fresh air injected into the top of the combustion chamber, you can see the 'smoke' burn) and achieves an efficiency of 87%. The smoke then enters a maze built into the back wall (which is part of the chimney, a massive - about 3 by 3 meters at the bottom - stone construction built into the centre of the house) where more heat is deposited, leading to a higher total efficiency. The temperature of the smoke gases coming out of the flue is around 70°C, going lower than that would require forced draft. The cooking stove is less efficient (around 60%) but also vents into a maze where most of the remaining heat is captured.

Interestingly the rocket stoves I mentioned are probably much better in that regard. They use an insulated firebox which means the gas is burned at higher temperatures which causes cracking of the longer chain organic compounds. The output is much closer to the ideal CO2 and H2O (which fairly self evidently is more efficient).

More info on existing and still in use hypocausts in Spain https://en.m.wikipedia.org/wiki/Gloria_(heating_system)

Fun fact: even modern Korean condominiums have a modern equivalent of an ondol.

I wish more housing in cold-winter North America (like here in Minneapolis) had something like that.

primitive technology dude built himself a hut with some form of hypocaust (fire tunnel beneath clay tiles)

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