i am not about to comb the hour long thing but is there are reason it has to have metal parts instead of all brick/mud?
You talking about the metal drum? Seems like that's necessary. He explains why in the video, tldw: it radiates heat which cools the exhaust, causing it to sink, and this starts pulling air through the combustion chamber. He calls it an air siphon at one point. Yeah you lose some heat from this, but without that siphon it won't create the jet furnace in the firebox. Maybe there's a way to engineer that without metal, I have no idea.
The heat is all being dumped into the living space either way, so you aren't losing anything from the metal. It takes away from how much heat will be available in the earthen part after the stove is turned off, but it still warms the room. Plus I imagine it's nice to have some heat immediately instead of having to start the fire hours before you actually want it.
You could adjust the design if you wanted to change the ratio of fast heat and slow heat, but you would always need to have some part with a high thermal conductivity.
Good point!
The goal is to have the fire drive a high airflow, both to give itself oxygen, and to pass the exhaust through a long path of heat absorbing dirt. The metal drum does this by passing the hot exhaust up through its center, then rapidly cooling it as it falls back down along the metal surface. The difference in density drives the airflow. The drum needs a high thermal conductivity to be able to maintain this temperature and density difference, and this is also the primary way the heat is transferred to the living area. The earthen part gathers up the remaining heat to increase efficiency and retain heat for when the stove is off, but most of the heating is done immediately through the metal.
Metal has better thermal conduction (transferring heat from one end to the other) the brick/clay/mud is there for thermal insulation to trap more heat and keep the metal conducting even after the fire has gone out
so the rationale is that it warms up the whole thing faster? but that's at the cost of insulation & materials requirements. someone should compare a full brick version to this
It's not really a matter of heating it up faster, tho I guess it would, it's more about fuel efficiency i.e. how long the heating can last on the same amount of fuel
With the clay acting as an insulator, heat that would usually escape from a naked metal pipe would be retained and used to keep the metal conducting heat for a longer period of time (so more bang for your buck in terms of fuel)
I suppose you could use clay piping, but I doubt it could match aluminum piping in thermal conduction, it would require several experiments using different materials both metal and ceramic to find the sweet spot
the whole thing kind works on old principles so i was thinking how antecedents would work without high quality steel. involving ceramic is probably correct
Rocket stoves are very efficient for longer term heating applications like heating a home.
If you just want to quickly warm yourself up after getting out of a shower then look elsewhere.
Also note that a lot of people build rocket stoves without a solid understanding of the design of the stove itself, so I've seen things that are just metal stoves with a J shape for gravity freeding wood into being labelled as a rocket stove (it isn't) and I've seen people put heat-exchange shrouds over the recombustion chamber for heating water which, in theory, may still work as a rocket stove but you'd hamstring the initial heating up stage and you'd be prolonging that phase of inefficient burning by a lot and then depending on other variables the heat exchanger may be sapping enough energy from the stove consistently to maintain it below the right temps for recombustion or it may be significantly impacting on the efficiency of recombustion. Any built-in inefficiency like this is going to lead to more build-up of gunk like creosote in the flue which itself can cause more inefficiency, it requires additional cleaning, and it can eventually cause a fire risk (which is pretty minimal under this sort of design but idk if I would want my rocket stove flue shooting out flames from the side of my house like it's the exhaust of a high-performance car tbh.
On matters of cleaning and maintenance, you absolutely want to keep the flue straight. Any curves or bends makes cleaning and maintenance into a nightmare. Better to build a straight one that is longer and throw curved cob architectural design around it as thermal mass. Trust me on this.
Last of all, if you want quick heat, you're looking to heat water, or you want something small/portable then I'd definitely opt for a regasification stove than a rocket stove - this design is similar in efficiency but it heats up in seconds, it doesn't require being a permanent architectural design, and with the right configuration heating up water can be done without compromising the efficiency. (Truth be told it's possible to do the same with a rocket stove but you'd want to run a thermal exchange around the exhaust, I guess, and that would mean it would be embedded into cob and it wouldn't necessarily draw huge amounts of heat, you'd need the thermal mass of the cob to reach a high internal temperature before it stops robbing heat from the exchange rather that contributing to the heating etc. - not impossible but it would take careful consideration of the design and I wouldn't expect to get hot water in under a few hours from this sort of configuration, at the very least, which is fine if you have a hot water tank and/or you are running the stove basically 24/7 but, again, you'd want to really carefully consider a lot of factors before you go all-in on this and that's way beyond the scope of this comment.)
Idk what things are like at the moment but it was extremely difficult, if not downright impossible, to get approval for rocket stoves under building codes. Things have been shifting in a positive direction gradually but you would probably want to keep things on the down-low if you were building one of these and it would be likely to have implications for insurance too so yeah.
If I was going to tell anyone about rocket stoves I'd tell them that it's really important to spend a good deal of time studying up so you have a solid understanding of the principles about how a rocket stove functions before you design or build one; it's easy enough to slap some cob together but it's more complicated matter to build a well-designed rocket stove.
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