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My bureau has recently been provided with a city charter and the plans to develop the colonial territory of Endrast into an urban center befitting the dwarven people, but after reviewing the documents I am concerned by the possibility of suffocation.

Mr Grin, the lead architect on this project, has assured me that the subterranean city he envisions will be adequately ventilated by a novel (by which I understand he means 'untested') technique of his own design. I disagree. Alas, for reasons ostensibly of national security (but more honestly of bureaucratic red tape), I am forbidden from submitting the blueprints themselves for your review, but I shall attempt to summarize his plan as best I can.

He proposes, essentially, that from the walls and floors of the city should be carved a network of ventilation shafts converging on an isolated cavern wherein enormous fires shall burn at all times. Above these fires would be great chimneys cut to the open air at the mountain's peak. The premise is: the fire would draw from the city a constant stream of air, which in turn would draw fresh airflow from the surface downward into the city.

As I say, Mr Grin assures me this will provide the citizens of Endrast a constant supply of fresh, clean air. I remain unconvinced. All prior experience has led me to understand that burning a fire in an underground dwelling is an excellent way of suffocating everyone inside. I have never heard of it improving the flow of breathable oxygen.

Furthermore, these fires would surely burn only as long as there was a sufficient surplus of oxygen being drawn from the tunnels. In other words, the vacuum would need to be strong enough to draw enough air from the surface to exceed the city's need for fresh oxygen by a broad enough margin to fuel the fires by which the vacuum was generated. Is that even possible?

In short, I have come to ask: has our architect lost his mind, or am I mistaken? Could this principle actually work? If not, is there any way it might be salvaged? I must be absolutely sure of this - the last thing my bureau needs is another Inbad Incident.

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Kudos to you for focusing on infrastructure! (Something old JRRT gave short shrift to.) Assuming gravity and gasses work in your world as in ours (PV = nRT), heat-driven convection can cause the airflow you're after, though there are potential hazards, as you're clearly aware.

If you can arrange it, you'll probably be happier and safer with a non-combusting heat source heating your ventilation chimneys. A fire Elemental would work, if you can get one to cooperate. (Personally, I wouldn't trust a dragon farther than -- but I digress.)

If there's a dormant volcano in Endrast, that would be a good site (other things being equal) -- provided the volcano remains dormant!
I suggest you consult your local Mages on that. Chimneys above the living/working quarters. Anti-backflow valves (at least one per chimney) would be a great help, especially if you have to settle for a conventional (wood, coal) heat source.

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    $\begingroup$ You raise excellent points, and I may have just the thing to get away from combustion. Much obliged! $\endgroup$ – Bibliophael Nov 30 '16 at 17:56
  • $\begingroup$ keep in mind termite build self ventilating tunnels just by putting the parts of the colony that generate the most heat directly under the exhaust shaft, this has even be utilized in modern architecture. the differential does not need to be that large as long as it is consistent. sites.psu.edu/abcdesigns/2014/04/11/… and asknature.org/strategy/mound-facilitates-gas-exchange/… $\endgroup$ – John Dec 13 '17 at 21:47
  • $\begingroup$ I wouldn't recommend building near volcanoes. They volcanoes are there in the first place because they're right on the fault lines, near the worst Earthquakes of the area. Lava produces so much carbon that I don't know if it's any better than a wood-fire. $\endgroup$ – Johnny Dec 14 '17 at 13:27
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Yes it would work, with some care and protection measures as Catalyst says. Similar techniques were historically used to ventilate mines; see for example, "A Brief History of Mine Ventilation" by M.J. McPherson (in Subsurface Ventilation and Environmental Engineering).

On the other hand, it may be more energy efficient and much less dangerous to use traditional fans driving air into ventilation ducts. Mines and tunnels were and are ventilated by fans moving air in a carefully designed system. Even in the Antiquity they used (muscle powered) centrifugal fans to force fresh air into the ventilation ducts.

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This will only work if you have sufficient ventilation openings into the city to allow enough make-up air to reach the city from the surface.

If there are insufficient ventilation openings for make-up air, or the openings somehow get blocked, lighting the fires would cause a pressure imbalance lowering the air pressure in the city preventing the fire and smoke from being effectively drawn up the chimney and filling the city with smoke, and suffocating the fire as well as everyone in the city.

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