The Rocket Mass Heater is an updated masonry heater with a rocket stove made of inexpensive materials that requires moderate skills to build.
A masonry heater is a wood burning heater, made of brick or stone, that stores heat in the masonry structure, which continues to release heat gradually, even after the fire is no longer burning. They have been used for thousands of years by various cultures around the world. The RMH was invented in the 1980’s.
A rocket stove is an ultra-efficient, wood-burning stove utilizing a thermal siphon.
Traditionally masonry heaters have had high up-front costs: such as the cost of materials, specialized foundations to support their weight, and highly-skilled masons to build.
Our farm house is made of stone. There are stone walls around the perimeter of the building as well as internal stone walls that partition the spaces. Because the building is large, we will build two rocket mass heaters located next to internal walls, thus incorporating the stone walls into the “mass” of the heater.
The rocket stove has numerous benefits: 1) It is easily built from common materials. 2) It offers close to complete combustion of the wood, with just water vapor as residue. 3) It can use wood typically considered too small to call firewood.
The basic rocket stove has three components: Feed tube, Burn tunnel, and Heat riser.
Connected to the rocket stove is a stratification chamber. The hot air from the heat riser enters the upper part of stratification chamber. As the air cools, it sinks to the bottom, where there is an exit to the chimney. The size of the stratification chamber (specifically the internal surface area of the chamber) is important to allow as much heat to absorb into the surrounding masonry as possible, but keep enough heat to expel the water vapor from the chimney. Details on how to calculate the stratification chamber size for a batch rocket stove are here. The rocket stove produces half the heat of the batch stove, so the stratification chamber needs to be half that size.
I’ve made some notes about the dimensions of our heater to determine the quantity of materials I will need. The rocket mass heater consists of a rocket stove about 20″W x 39″D x 59″H with a stratification chamber of about 42″W x 22″D x 59″H. The rocket stove will need to be made of high-temperature brick since the burn tunnel can reach up to 2000o F. The rest will be made of earthen bricks (earth + sand + straw). This summer I will make test bricks to determine the optimal mix of earth + sand + straw, using earth from the property.
