A rocket mass heater is a rocket stove firing horizontally into a long duct buried in a heavy bench of cob, brick or stone. The exhaust gives up almost all its heat to the mass on the way through, and the mass then radiates that heat back into the room over the following day.
The performance is genuinely remarkable: a two-hour burn of small wood heating a space for twenty hours or more, on a fraction of the fuel a conventional stove needs.
It is also the largest, heaviest, most permanent and most consequential thing in this domain, and this page spends its first section on why rather than how.
Read this before you build one
None of that makes it a bad idea. It makes it a project to do properly, on purpose, in advance, rather than something to improvise in a crisis. This is a summer build for a winter you can see coming.
How it works
Three stages, and the second is the clever part.
1. Combustion. A J-shaped rocket core: a vertical feed tube, a horizontal burn tunnel, and an insulated vertical riser. Exactly the geometry on the rocket stove page, and exactly the same two rules apply: constant cross-sectional area throughout, and insulate the riser. Combustion happens at 900°C or more, cleanly, with secondary burning of the wood gas.
2. Heat extraction. The riser exhausts into a sealed steel drum, which turns the gas flow through 180 degrees and sends it downward around the outside of the riser. The drum surface gives immediate radiant heat, and the gas leaving the drum base is already much cooler.
3. Thermal mass. From the drum base, the exhaust runs horizontally through a long duct buried in a bench of cob or masonry. It gives up the remainder of its heat along that run, and exits the far end at close to room temperature before going up a short chimney.
The reason it works so well: a conventional stove sends 70 to 80% of its heat up the flue. A rocket mass heater extracts nearly all of it into a mass that then releases it slowly. The exhaust at the far end of a well-built one is barely warm and nearly clear.
The bench is the battery. Two hours of burning charges it; it discharges over the next day.
The build in outline
This is an outline, deliberately. A full construction guide is a book, and this domain's job is to tell you what matters and where it goes wrong.
- Foundation first. A tonne and a half of cob on a domestic timber floor is a structural question, not a detail. On a shed floor, build on the slab or on a proper footing.
- Build the core. Fire brick for the burn tunnel, ceramic fibre or perlite-clay for the riser insulation. This is the part that must be right and the part that is most work.
- Set the barrel over the riser with the correct gap at the top. Too little and it chokes; too much and it does not scrub the heat off.
- Seal the barrel to the core with clay. Everything must be gas tight.
- Run the bench duct from the barrel base, with a very slight rise along its length toward the chimney.
- Build the mass around the duct, in layers, keeping the duct clear.
- Fit clean-outs at every bend. You will need them.
- Build the chimney. Short is fine, because the system's draught comes from the riser, but it must vent outside and above any opening.
- Cure it slowly. Small fires for a week or two before the first real burn. A cob mass fired hard while still wet cracks badly, and can spall.
The failure modes that matter
Other things that go wrong, in rough order of likelihood:
| Symptom | Cause |
|---|---|
| Smoke back out of the feed | Cold chimney, or not primed before lighting |
| Weak burn, no roar | CSA restricted somewhere, or riser not insulated |
| Smoke back after weeks of good running | Ash accumulation in the bench duct |
| Cracks in the bench | Fired too hard before it cured, or no expansion allowance |
| Barrel warping or thinning | Normal over years, plan to replace it |
| Poor heat from the bench | Duct run too short, or mass too thin over the duct |
| Bench slow to warm | Normal on the first burns of the season, and after a long idle period |
Ash accumulation is the maintenance job people forget. Fine ash settles along the horizontal duct and slowly restricts it, which slowly reduces the draught, which is the safety margin. Clean-outs at every bend, inspected annually, are not optional.
Running it
- Burn small wood. Same as the rocket stove: thumb-thick sticks fed continuously, not logs. A rocket mass heater eats coppice, hedge trimmings, offcuts and brash.
- Burn once or twice a day, hard, for one to three hours. You are charging the mass, not maintaining a fire. Long slow smouldering is the wrong pattern entirely and it fouls the system.
- Feed it while you are there and let it go out when you leave. The bench keeps working for the next twenty hours.
- Expect the first burn of the season to be poor. A cold, slightly damp mass absorbs a great deal before it gives anything back. Two or three burns in, it comes right.
- Sit on it. A warm bench at body temperature is a very effective way to heat a person rather than a room, which is a far cheaper thing to do. This is the same principle as the single-warm-room strategy in the scenario cards, taken further.
For most households the honest recommendation is: build a rocket stove for cooking this month, and consider a mass heater as a serious summer project for an outbuilding, with Building Control involved if it is ever going anywhere near the house.