Pros and Cons of Earthen Masonry Heating Systems
As an alternative to straight up passive solar design, super-efficient masonry heaters may make sense.
For the past decade, I've been focused on sustainable heating in the home. Average North American households use 30-70% of energy for heating and cooling. This is a direct cost of maintaining our lifestyles, both in dollars spent and impacts on communities and global resources.
In this context, the “greenest” way to heat and cool a home would be:
- Life-on-earth friendly: healthy, non-toxic, renewable
- Efficient: minimize wasted resources, pollution, and time and money
- Useful: effective, physically comfortable, and affordable
- Lifestyle friendly: attractive, culturally compatible, perhaps even fashionable.
Passive Solar's Limitations
For all these reasons, I embrace passive solar design. It uses every trick in the book – sun angles, radiant heat and shade, thermal mass storage, insulation, air sealing and venting—to transform seasonal weather extremes into year-round comfort.
However, passive solar design has limited application in many existing buildings or those in deeply shaded locations or in high and cold latitudes with short winter days. Fortunately, however, shady slopes and temperate latitudes are great places for growing trees and other biomass fuels. Enter rocket mass heaters—super efficient, hot burning masonry heaters.
Facts about Mass Heaters
Rocket mass heaters use the same tricks as passive solar design and can be integrated into most homes. However, instead of capturing solar energy, rocket mass heaters capture heat from biomass.
Compared with wood stoves or conventional fireplaces, a rocket mass heater’s clean fire sends little heat, smoke, and creosote up a hot chimney. Instead, heat from the clean exhaust is captured and stored in thermal mass for gradual release throughout the day or overnight.
For fuel, rocket mass heaters can use local “waste” materials including sticks, carpentry scraps, cord wood and even dried garden waste. Heat storage mass can be packed earth, stone, brick, tile, reclaimed concrete, or a clay-sand-fiber mix we call “cob.”
The firebox itself needs heat-tolerant materials, usually firebrick, backed with perlite or ceramic wool. A “heat riser” or insulated draft chimney keeps the flames burning the right way. A steel cylinder, often a recycled barrel, brings the exhaust gas back down to a convenient entry for the heat-exchange channels in the masonry mass.
Traditional masonry heaters, widely used in Northern Europe, were often designed as a big column or oversized chimney. Those shapes work well but it's hard to curl up on them for a nap. Contemporary rocket mass heater builders tend to opt for designs that double as built-in furniture such as heated benches, couches, bed platforms, or warm floors.
Pros of rocket mass heaters:
Clean burn: First-time observers can scarcely believe that the clear, damp breeze coming from a rocket mass heater exhaust pipe is really the end product of wood combustion. When wood is burning efficiently, there is no smoke, just steam, CO2, heat, and a little pale ash.
Convenience and efficiency: Compared with a typical woodstove, you might tend the fire less than half the time and burn only 10-25% of the wood.
Direct contact warmth: Be it full-body heating pad or personal sauna. Cold toes, meet your new friend!
Healthy and non-toxic: Raw-clay masonry is highly heat-tolerant, durable, low or zero-carbon and comfier and easier to refinish or remodel than concrete. Clay regulates moisture as well as temperature for a surprisingly complete climate-control solution with no moving parts.
Cons of rocket mass heaters:
Labor: It may take a few tons of masonry to store enough heat for a modest home so this is a big DIY project, or an expensive hired one.
Space: The most effective long-term location for your rocket-heated divan is, of course, right in the middle of your living quarters, which is rarely a convenient place for a remodeling crew.
Recycle Chic: Recycled parts are a big draw for some people, but not for others. A steel drum in my living room? Are you kidding? Don't worry, there are options.
Compliance: Not all building inspectors will be familiar or accepting at first glance; read the code notes in our book The Rocket Mass Heater Builder’s Guide and check your local regulations.
Manual Performance: This is not your old woodstove. It won't burn the same way and there is a learning curve to operate it, but you won't need to burn as much, as long, or as often. Once mastered, the fancy little firebox amply rewards effort and attention.
The Bottom Line
Highly efficient wood heating is low emission, potentially carbon neutral, provides resiliency and local sourcing, and offers high user comfort, ambiance, and the potential for beautiful design.
If you’re thinking of sustainable wood heat, it’s well-worth investigating the potential of rocket mass heaters for your next project.
Erica and Ernie have built over 700 super-efficient, clean-burning masonry stoves and led over 50 workshops across North America. They are the authors of The Rocket Mass Heater Builder's Guide.