Friday, October 21, 2011

Rocket Mass Heater

It's that time of year again... Hope you have plenty of wood split for the winter, I know I do. At least some of you must burn wood for heat in the winter. If so, good for you! Heating with wood is a lot of work and mess, but it's much cheaper than your other options, namely gas, oil, or electric heat.

It does seem to take a tremendous amount of wood. Unfortunately a lot of that wood goes right up the chimney, as heat and smoke. Extra wood to burn means more cost, extra splitting, extra storage, and possibly running out. What if there were a small cheap stove that got better "mileage" and was DIY friendly?

Let me introduce you to a true econobox, the Geo Metro of the stove world: the Rocket Mass Heater. It's dirt-cheap and super efficient.

A Rocket Mass Heater (RMH) uses 80% less wood than a standard stove - AND - you only need to fire it once or twice a day. The large heatsink stores the heat from an small intense fire in the morning, and radiates this heat all day. The exhaust is only about 100 deg F. No smoke, just steam and CO2.

Here's what you're seeing in the picture: The wood and air go in through that little "bucket" in the foreground. The heat radiates from the 55 gallon drum, enough to heat a kettle of water on the top. And the large masonry bench is warmed by the exhaust, capturing all the energy before it goes outside. Comfy bench, hot barrel, wood in a hole. Got it.

The wood actually sticks out of the hole, but only the inserted end is burning. Flames from the end are sucked sideways into the heat riser. The heat riser is an insulated pipe where all the smoke burns at high temperature - this is the "rocket" part. The extreme heat creates a powerful draft which drives the system. This flows into a barrel which surrounds the heat riser, and back down into the base. The barrel serves to radiate some of the heat, and cools the gases, improving the draft. The exhaust gas enters a large thermal mass which stores all the heat remaining in the gas, and slowly releases it to the room.

Video tour of several installations:

One being built at a workshop:

The RMH is by far the cheapest stove you can build. You will need a 55 gallon drum, stovepipe, perlite, some firebrick and plenty of "cob" (clay + sand + straw). This is a triumph of DIY design over mass-production. Anyone can build one of these, operate it and get "extreme MPGs", so to speak. Read more about these at, get the book at, or just watch the videos on Youtube.

I plan to build one, it looks fun and very easy. Stay tuned...