Searching for 600 – My Experiences With A Rocket Mass Heater
In my last blog post, I talked at length (3 pages!) about the fundamental concepts behind Rocket Mass Heater systems as well as the physical parts of the system. I also talked about safety issues as well as ways to better incorporate them into your living and working spaces.
In this post, I will talk about how I run my Rocket Mass Heater system in my greenhouse, including lighting, feeding and cleaning.
After that, I’ll talk about my first and subsequent testing burns that I have done with the “Version 2” system and a few surprises I have learned along the way.
“Normal” Operating Temperatures
As a quick review, when you are working with or around the Barrel Cover of a Rocket Mass Heater, please keep in mind a few important numbers:
- The exhaust plume coming out of the muzzle of the Heat Riser, inside the Barrel Cover, can be around 480° C / 900° F.
- During peak heating, the top of the Barrel Cover can be 260° C / 500° F without difficulty.
- During peak heating, the sides of the Barrel Cover can be 175° C / 350° F without difficulty.
The top of my own Rocket Mass Heater in my greenhouse has hit barrel-side temperatures of 450° F (230° C) and barrel-top temperatures of 600° F (315° C) for a period of an hour or more.
Word Of Warning: Do not allow pets or children, or anything else you do not wish to cook, to come in contact with the Barrel Cover … ever.
Learn To Burn
Rocket Mass Heaters do not behave as either camp fires or regular wood-block heating stoves. As such, it takes a bit of trial and error to get used to “normal” with them. To complicate matters, since these systems are all home-brew jobs, no two of them are identical. They all have their own quirks.
The general behaviours are the same. The details can vary wildly, in my experience. The only way to know how yours is going to run, is to spend time with it while it is running. At worst, it’s an excellent chance to catch up on that book you have been meaning to read.
How much wood?
My own system does not run well with less than a ¾ load of wood in Feed Tube for the first 30 minutes. It is prone to whisping smoke out of the Feed Tube area and not burning as evenly as it should. Thus, the minimum amount of wood I will ever run the system on is a bundle around 15cm in diameter and cut about 30cm long (6in x 12in).
That will generate some real heat out of the system, but only takes about 45 minutes before I can put the “Safety Cover” over top of the Feed Tube area and walk away from it. As a reminder, my own system wound up 210mm / 8.4in Rocket Mass Heater, so its got a fairly large Feed Tube capacity. The smaller the cross section of your Rocket Mass Heater, the smaller a bundle you can run on.
Of course the flip side to that is also true; the smaller your system is, the more manual reloading of the Feed Tube has to be done to sustain a longer burn.
A “full burn” for my system will consume a 19L / 5gal bucket filled with kindling sized dry wood. Generally, I am trying to be burning the wood I split yesterday while the system was running. It gives me something to do while the system is coming up to “full power” as it were, which can take ten to fifteen minutes.
Lighting my Rocket Mass Heater is still a bit of voodoo for me, but I can usually get it to light up on the first try. Here is how I do it.
First, I take a single sheet of newspaper and tear it into three pieces. Those pieces are then crumpled up, and put in the Feed Tube and then pushed into the mouth of the Burn Tunnel.
Next, I have a double-handful of sweepings off the woodshed floor in a tin can. This is wood shavings, bits of bark, sawdust, pine cones, dried leaves, and whatever else came in with the wood. I dump this into the Feed Tube, and then push it into the mouth of the Burn Tunnel. This, of course, pushes the paper ahead of it.
I then take a half dozen of the smallest sticks of wood from my “to burn” pile, and snap them half their length and lay them in an “X” pattern at the bottom of the Feed Tube until the pile is about 2/3 the height of the Burn Tunnel. Then, this also gets pushed into the mouth of the Burn Tunnel.
At this point, we are ready to light up. Now, keep in mind that the flame in a Rocket Mass Heater burns away from the Feed Tube through the Burn Tunnel and towards the Heat Riser. However, your feedstock is lit by contact with that flame as well as radiant heat effect.
If you light the end of the starter fuel closest to you, it would burn the starter fuel away from the Feed Tube, and there wouldn’t be anything to light the feedstock. So, instead, we need to light the end furthest from the Feed Tube and let it burn back towards us.
I take another ½ sheet of newspaper and twist it on the diagonal to form a “paper rope”. I hold one end and put the other end over the Feed Tube opening. With a Mk.I standard Zippo lighter, I light the end I am not holding. That then gets put down the Feed Tube and pushed, burning end first, along the roof of the Burn Tunnel. Once my hand is at the mouth of the Burn Tunnel, I just let go and let it start to generate the crucial heat required for the whole thing to run.
After about a minute, I can usually hear the sound of the fire burning. If I am uncertain, I just put my bare hand down the Feed Tube, and I can usually see the light of the fire on my hand, if not feel the heat.
If the fire has failed to catch, I repeat the “paper rope” ignition process. It has yet to fail to light on the second try.
Now, I start filling the Feed Tube with feedstock. I fill from back to front — that is, furthest away from the mouth of the Burn Tunnel towards closest to the mouth of the Burn Tunnel.
The tallest sticks, regardless of diameter, go at the “back” two rows deep across the entire width of the Feed Tube. They are stood almost straight up and down; no more than their own diameter off pure vertical. This forms a “guard rail” that the rest of the wood being added can lean against.
When placing the sticks in the Feed Tube, ensure that the narrowest end goes down and the thickest end goes up. The improves the gravity feed of the wood down into the Feed Tube as it burns.
Now I continue loading from back to front, this time in descending order of diameter or thickness. As before, the narrowest end goes down and the thickest end goes up.
When I am done, the thinest pieces of wood are closest to the fire that has been steadily burning backwards towards the union between the Burn Tunnel and the Feed Tube. Pretty soon, those very thin pieces of wood will have caught fire at the bottom tips and shortly thereafter I can start to hear the characteristic sound of my Rocket Mass Heater powering up.
At this point, there is ten to fifteen minutes of waiting time as the bottom of the entire wood bundle catches, a bed of coals begins to form in the mouth of the Burn Tunnel, and the temperatures within the Burn Tunnel spiral upwards to the point where the “afterburner” lights. It is a good time to split some wood for tomorrow.