Now that we’ve the outside of the build done, (regular readers will realise that there is something of a gap here, but don’t worry its coming soon), 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.

Next step is to hard-connect all the flue parts with screws and then heatproof tape to keep it airtight, before it’s all buried in cob, and of course to build the burner itself. Watch this space