Mass Heater Comparisons

Mass Heater Comparisons

As a builder of both traditional masonry heaters and rocket cookers/heaters I am often hear the question: ‘What is the difference?’

My response is: ‘Not much, and lately less so.’

To start with, lets look at the commonalities between the two.  Both are time-honoured techniques of firing biomass (usually wood) at very high temperatures (up to 1300c in bigger rocket cores).  Both have specially designed fire chambers made of high-heat-duty refractory materials.  Both offer highly efficient combustion of gasses (smoke).  Both  then effectively harvest and utilize the rapidly produced heat in various ways.

For gradual heat delivery into the dwelling the gasses can be directed through flues and bell chambers comprised of dense (high mass) earthen material (cob, brick, stone… all much the same).  This mass can be shaped a variety of ways, ranging from vertical contra-flow, to a long comfortable bench, to an in-floor hypocaust.  The intensity and duration of the fire, thickness of mass, and heat losses of the dwelling determine the delay and rate at which the heat is radiated from the mass.  This time lag between combustion and heat release is known as the ‘flywheel’.

For more immediate (short flywheel) heating applications the gasses can be directed into thinner masonry or hollow metal chambers.  For example, when a 55 gallon metal barrel is placed around the heat riser of a rocket core the system gains the element of a (fast) radiant heater, much like a metal bodied wood stove.   Flue gasses can also be directed to heat up a griddle, oven, or open burner (so long as the space is well ventilated) and can then continue on through a body of dense mass (long flywheel) to ‘harvest’ most all of the remaining heat.

Until recently, the main distinctions between masonry and rocket heaters have been the shape of their fuel feed chambers, and where the secondary combustion takes place.  Traditional rockets have an open feed-chamber shaped as a horizontal or downward tunnel.  Combustion takes place in a sharp elbow and up a ‘heat riser’ (vertical tunnel) creating a very concentrated zone of heat at the exit.  The rate of combustion  in L and J feeds is controlled by rate of fuel delivery, which is ideal for most cooking applications.  A rocket-fired oven can offer both quick heat via convection (which is great for pizzas), and gradual heat from the inch or two of mass in the dome and deck (which is fine for baking and slow roasting.

Masonry heaters, by contrast, have a relatively large enclosed fuel-feed chamber that is loaded once and burns for an hour or two.  This chamber is like a fire-box but usually taller with a specially designed exit throat and air port up top where secondary combustion occurs.  These bigger firing systems are particularly good at heating large areas of mass, including high mass ovens (ideal for extensive baking and roasting) by sending the flue gasses either through or around them.  They require no tending aside from the initial loading and (in some designs) adjusting of a start up bypass damper.  A large (but not too large) glass door also makes for great viewing of the primordial television.

As a sweeping generalization, the advantage a rocket with open-feed chamber has for cooking is a disadvantage for mass heating, in that frequent reloading with thinner-than-usual firewood can become a chore.  By the same token, masonry heaters with large feed chambers involve less user input to heat a large area, but with the large body of fuel combusting all in one blast they are not as well suited for on-demand cooking applications during which one wants to concentrate the heat into small areas and/or modulate the length and intensity of the burn.

With these pros and cons in mind, much effort has gone into creating hybrids that solve every shortcoming.  The advent of open source sharing via internet has made possible an international collaboration of stove/heater builders and researchers, resulting in a cross-pollination of building traditions from around the world.  The results have been very fruitful.  As we share our experiments and findings, we are evolving new features and combinations of techniques to achieve unprecedented combustion and harvesting efficiency.

To view working examples of traditional rocket cookers, check out the L-feeds I built in Centennial Park.  For examples of traditional masonry heaters go to link: ‘A Piece of the Sun’.  For an example of one of my more intriguing hybrid heater builds, check out: