The reason it takes less wood to heat with a rocket mass heater is simple, and you circled it several times in your video. The reason is that the RMH is very efficient, and a lot more EFFECTIVE.
The RMH is very EFFICIENT, because it burns 100% of the fuel. Efficiency is a measure of how much of the chemical energy in the fuel is converted to heat. Effectiveness is a measure of how well that heat accomplishes its intended purpose (most likely keeping the people in the building comfortable). So if you can be just as comfortable with 1/10 the amount of wood, the correct terminology would be to say that the RMH is 10 times more EFFECTIVE at maintaining a comfortable environment than the wood stove.
I have a RMH in my uninsulated shop in Northern Canada. It is uninsulated because I don't heat it often enough for insulation to be needed AND the RMH generates radiant heat, and it stores heat in the mass. When I am four feet from the heater I will be quite warm, but I can still see my breath. The radiant heat is not heating the air, it is heating me directly. Radiant heat only heats mass that it cannot go through, e.g., people, floors, walls, etc. so heat is not wasted heating stuff that does not need to be heated.
Another factor is that the mass absorbs almost all of the heat not converted directly to radiant heat at the barrel, and slowly radiates it back into the work area. Hydronic radiant floor heat works in a similar way. At the exhaust pipe, there is almost no heat left in the exhaust stream because all of the heat is being converted to radiant heat or it is stored in the mass and radiated back slowly.
In any radiant heating situation, you can typically drop the air temp significantly and still be comfortable.
As you discussed, if you are sitting on, or working near the mass or the barrel, the heat is used much more effectively than if you were to have to heat the mass of air and blow it around. Also as you discussed, if you open the door, you are not trying to heat all of northern Canada as you would be with a forced air unit.
An electric baseboard heater can theoretically be 100% efficient as all of the electrical energy that it uses is converted to heat (ignoring the minute inductive and capacitive losses in an an AC system), but it is not at all effective because most of the heat is wasted heating the air to create convective currents. As you mentioned, heated air can escape.....ergo, not effective. As you also mentioned, that hot air rises and sits near the ceiling, so unless you are a bat hanging from the rafters......again not effective.
A heat pump is not really 130% efficient, it just moves heat from one place to another, so while you do get more heat energy inside than you put into the system to move the heat, you are lowering the temperature outside, so thermodynamically, it is not more than 100% efficient. It is, however, more effectively using the energy input because the great outdoors is a huge heat sink compared to the indoor area being heated so we don't notice a change in outside temperature.....we are only concerned with the inside temperature. No system can be more than 100% thermodynamically efficient. If they could, we could build a perpetual motion machine and put all those oil companies out of business for good
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Darryl C.
P. Eng
