Now that we’ve the outside of the build done, we are turning our attention to the rocket mass heater that is going to keep the space warm. For those who haven’t encountered one before, the rocket mass heater (RMH) is a well proven though not widely used way to burn wood very efficiently, and then capture all the heat produced, in a mass – normally a bench or bed – by passing the flue horizontally through it. Rather than explain all the intricacies, please do visit this excellent site to find out more.
Anyhow, above you can see the outer brick surround of our heater – the bricks were unwanted surplus, and we’ve used lime mortar to build with. This wall contains the flue, which will be encased in a large amount of cob, and the brick will be clay plastered too. The burner will live at the near end. We’re building it before the floor goes in to make life easier.
Just to make sure that the thing really will work, I fired it up ‘dry’ – that is with the burner built up out of firebrick, but with no cement in the joints, or around the flue. Sure enough it burned happily once I got it lit, and seems to draw well enough. Having done that, the burner has been dismantled, and the flue set up now that we know what goes where.
Here’s the far end of the flue, where it turns through 180 degrees and heads back again where it will exit vertically. These two caps will be removable to allow the flue to be cleaned, and will be hidden in the end of the bench.
The flue is 8″ and there’s a lot of it…
So at the back, you can see two openings – the left one will be a cleanout with a cap on it, the next one is where the vertical triple wall flue comes off and exits through the roof. The one in the middle is where the exhaust from the burner enters the flue, and the one on the right is yet another cleanout.
Having tried (and failed) to get clay soil cleaned up enough to make smooth clay mortar, I just went out and bought some clean clay, which was thirty quid well spent, and allowed the burner for the rocket mass heater to get built. Several of you will be wondering what on earth we are conjuring up here, and once again, to save lots of explanation, have a look here.
The ducting is all properly jointed and sealed now, and here you can see the burner built up as far as the top of the fuel feed (right) and the bottom of the riser (left). Building with firebrick and very smooth clay mortar means the joints are very small – down near 1/16″ to keep as much heat in as possible and the fire super efficient.
Here’s the heat riser built up completely, and although the outside looks rough, the inside is all cleaned flush, and super smooth. The next job is to cobble together a metal cylinder to surround the riser and contain some insulation, and mix a load of cob to fill in the void around the burner, and give the outer barrel something to sit on.
Today we pushed on, built a brick support around the base of the firebrick burner, – above, and then backfilled all the gaps with perlite and a clay mix to thoroughly insulate the fire itself, which we want to burn as hot as possible. The clay mix is a little hit and miss, but is roughly 1 part clay to 4 parts sand. For areas around the burner we’ve stuck to using the clean clay bought from a pottery supplier. For everything else, it will be the locally dug clay, at around 1 sand: 3 or 4 clay. No straw in the bits immediately adjacent to the pipework, apparently, though its a bit late for that in one or two spots.
The next step was to surround the firebrick riser with a metal cylinder (an old water tank in this case) which was again filled with perlite insulation and capped with clay. Over the top of this goes the 50 gallon drum, leaving a 2 inch gap around between it and the inner riser. This is then connected to the horizontal flue by means of a variety of bits of firebrick and more cob.
Having got all this together, and dropped on the top part of the insulated flue, it was time for the moment of truth. Rocket Mass Heaters aren’t known for easy lighting, as until they get warm, there’s a tendency for smoke to blow back up the fuel feed. Other than a bit of initial smoke, it went really well, especially considering that it is trying to dry out a load of clay mortar along its way. The barrel got nice and hot, while the insulated flue stayed cool, and the exhaust really was a little smoke, and mostly warm steam, just like I’ve seen in countless videos. I don’t know why, but I’m still surprised it actually works..
In this short video you can hear the distinct ‘rockety’ sound that gives these stoves their name.
