Once the sticks at the front of the Feed Tube have burned to just below the height of the Burn Tunnel, I’ll usually start adding new sticks to the Feed Tube. To do that, I feed from “back” to “front” — again, by that I mean furthest away from the mouth of the Burn Tunnel towards closest to the mouth of the Burn Tunnel. By “feed”, I mean shove the thinest end downwards wherever I can make it fit, so that it presses the rest of the wood forwards.
Essentially, you are shimming the wood from back to front into the steadily growing pile of embers in the first ⅓ of the Burn Tunnel. These embers are essentially burning charcoal, adding heaps of heat to the processes at both ends of the Burn Tunnel. It is at this point where my Rocket Mass Heater really starts to perform, and my Barrel Cover temperatures will jump upwards by 56° C / 100° F or more in a few minutes.
Obviously, there is a real advantage to working with tall and thin feedstock. First, there is more time between needing to reload with taller pieces. Secondly, thin feedstock makes it easier to shim the existing bundle of feedstock forwards.
Fire is capricious. While it obeys pretty clear laws of thermal and fluid dynamics, there are times it’s interpretation of those laws in the face of current circumstances can result in “surprises”. As a result, we need a way of shutting a Rocket Mass Heater down at the end of a burn that does not require us to sit there and stare at the last dying embers.
Do Not Use Water — unless it is either an emergency or you feel like building a new Core
Thermal shock of vaporizing even a pop can’s worth of water can and will shatter fire brick. That is “bad”.
The best way I have found of winding down an established burn is to start by knocking the remaining feedstock over with a metal poker so that the feedstock is no longer standing up and down. Instead, it is laying in a heap with the burning tips closest to the mouth of the Burn Tunnel. Next, using the poker, I shove most of the wood into the Burn Tunnel.
This process disrupts the fuel air mixture of the entire system. Too much cold air is now entering the system and the bundle of burning wood introduces turbulence into that airflow too soon. The temperature of the system beings to drop and the draft begins to slow.
Next, I put my two “safety bricks” over the mouth of the Feed Tube. The brick closest to the front of the Feed Tube is an actual firebrick, of the same kind as I built the rest of the system with. The one at the back of the Feed Tube is a standard “red masonry” brick with holes cast in it from top to bottom.
This adds a bit of length to the Burn Tunnel and continues to let air flow downwards into the Feed Tube and into the Burn Tunnel. Since I have greatly restricted the flow of air now to just what is getting pulled down the holes in the brick, the speed of airflow through those holes to meet demand is pretty swift. It’s pretty difficult for anything to come up out of those holes.
Enough air is present to keep burning the feedstock left in the system to fine ash, which means it keeps adding heat to the Thermal Mass Storage for a while. However, the entire system is functionally starving to death at this point and will quietly run down and “shut off” by itself.
Reminder – the inside of your Rocket Mass Heater basks in temperatures that would melt aluminum pots to puddles. Do not try and clean one out that has been lit any time in the past twelve (12) hours.
Do not be fastidious about cleaning the ash out of the Burn Tunnel of your Rocket Mass Heater. A layer about 12mm / 0.5in deep in a 200mm / 8in system actually protects the firebrick floor of the Burn Tunnel, and helps insulate the Tunnel even further.
On the other hand, much more than about 12mm / 0.5in deep and you start restricting the air flow through the tunnel. This, in turn, degrades the performance of the system.
For me, about once every fourth or fifth burn, I scoop out the equivalent of two soup cans from the Burn Tunnel. Otherwise, I do not worry about it.
I dump any ash that comes out of my Rocket Mass Heater into our compost pile — wood ash is loaded with stuff that our garden needs. One or two soup cans of wood ash sprinkled over an area 1.2m / 4ft to a side is hardly close to saturation.
Ash is good. Chill about the ash. Accept the ash.