Here is our rocket mass heater: A classic j-shaped rocket stove. It is made of firebricks and housebricks, cob and ducting. It burns very cleanly, and stores the heat tremendously. It was built in our woodland cabin, a suspended wooden structure, which has very little thermal mass.
Installing this mass heater means that the cabin now can regulate its temperature – it will release heat over time after a burn session, and also keep the cabin cool on hot days as the mass absorbs the heat from the air. It is clear that this is working well. The air in the cabin has completely changed. It feels much more stable and comfortable rather than stratified icy cold at the floor and stuffy at the top. It is summer and dry now, so the main function of the heater at the moment is to keep the cabin from over heating. However I did light it a few times to help it dry out. It contains a lot of moisture from the cob and mortar, and the fire mortar that we used for the burn tunnel needs heat to cure properly.
The rocket mass heater follows a 6 inch duct system. The feed tube and burn tunnel has an approximately cross sectional area to that of 6 inches (our firebrick dimension determined the exact size) and the heat riser (an insulated stainless steel flue pipe) and the pipe in the bench (one piece of flexi flue) is all 6 inch pipe. The feed tube and burn tunnel is made of firebrick as is the heat riser casing. The bench ducting is laid in a shell of ordinary house brick filled in with cob. The mortar is all clay mortar, except in the case of the feed tube and burn tunnel where we used a commercial fire mortar for a stronger result. The plate at the top of the heat riser is a thick cast iron side plate off an old Jotul wood burner. I originally had a mild steel plate cut for it but when we lit the heater it warped so badly that it became apparent that it would need cast iron instead as it hardly moves when subjected to heat.
Last time I fired the mass heater, I measured the top plate temperature to 179 degrees Celsius after a 1 hour burn, this is with 350 degrees Celsius in the firebox. The exhaust is quite cool, and it is very clear which indicates a clean burn.
The mass heater is relatively easy to light. Rocket mass heaters can be temperamental to light and may kick out quite a bit of smoke when lit from cold. But as long as it is primed well (bit of lit newspaper stuffed into the burn tunnel) and I prepare the fire properly (dry kindling, firewood etc) I find I get it going with as little effort as our wood burner. I will try and retrofit the Jotul’s door which has the air vents on it as a cover for the heater so while lighting it the smoke, that may go upwards rather than into the tunnel, will be caught. As for most solid fuel heaters it takes a few goes to know how your heater is lit the best.
Many rockets are made of metal barrels. Although this gives a good immediate heat output it is by no means as durable, wont store as much heat, and it is not as child friendly. My son (see picture) can stand and watch the fire, resting on the brick without getting burned. The fire is vertical so does not reach him. The upright heat riser part is not dangerous to touch either. Having the cast iron top on the heat riser will give an immediate heat output still and so I really recommend investing in firebricks rather than in barrels (new barrels may cost as much as £100 and will not last as long as brick).
I chose to cover the bench with brick rather than a plastered cob bench because it is more durable and it gives a more conventional aesthetic which people may be able to relate to better than cob.