Rocket Mass Heaters: Efficient Home Heating With Wood

A living room heater
A living room heater
a J-shaped Rocket Stove combustion core
a J-shaped Rocket Stove combustion core
A big heated bench in a converted garage
A big heated bench in a converted garage
A campfire demonstration: Rocket stove core, and Pocket Rocket
A campfire demonstration: Rocket stove core, and Pocket Rocket
Design drawing for a 6" rocket heated bench
Design drawing for a 6" rocket heated bench

Building a Rocket Mass Heater

Layout
Layout
Mortar
Mortar
Footing & thermal cob
Footing & thermal cob
Insulation
Insulation
Thermal core
Thermal core
Earthen Plaster
Earthen Plaster
Done
Done

Is It a Woodstove or a Fireplace?

A Rocket Mass Heater is a wood-burning device. Beyond that, it gets weird. Technically neither a woodstove nor a fireplace, the rocket mass heater is related to masonry heaters, rocket stoves, and traditional earthen buildings. In terms of efficient, primeval comfort, it's in a class by itself.

The magic starts with the 'combustion unit.' Wood feeds in downward, a small batch at a time. The fire burns sideways and then up inside a super-insulated burn chamber. This is the "rocket stove" part: all those angles and insulation create superior mixing and draft for a good, hot burn. Complete combustion doesn't leave any unburned fuel as smoke or creosote; in fact, the exhaust is primarily CO2 and water. It's more like 'flue gas' from a modern furnace than smoke from a woodstove.

The flue gas is channeled through a massive heat-exchanger. The mass is earthen masonry: 'cob' or monolithic adobe, which forms a comfortable, dense heat-sink. Metal ducting carries the gases back and forth within this mass, before exiting the building. The heat-exchange unit might be a horizontal bench, floor, or bed, or a vertical column or wall. Its surface reaches a comfortable 100-120 degrees F, perfect for using it as a full-body heating pad.

Instead of smoking out a chimney at 400 F, the rocket mass heater's exhaust wafts out of the building around 90-120 F, leaving most of the heat trapped inside. A conventional chimney is optional: some owners choose a dryer-vent style exhaust instead.

What Makes It So Efficient?

The Rocket Mass Heater is one of the most efficient wood-burning heaters around.

  • Less fuel: less than 1 cord of wood compared to 3-5 cords for most woodstoves
  • Complete combustion: almost no smoke, no creosote, no pollution, and no wasted fuel
  • Captures More Heat: heat-sink traps 90% of the heat indoors, instead of sending it up the chimney
  • Quick radiant heat: during the burn, the stove's metal barrel radiates like a woodstove
  • Lasting Comfort: after the fire is out, the masonry mass gives gentle warmth for 12 to 48 hours
  • Heating People, Not Air: Cuddling up to the thermal bench warms people directly, instead of losing heat to air or turning in front of the fire like a rotisserie chicken.
  • Convenience and Safety: a 2-4 hour daily fire gives all-night warmth; no need to bank a smoldering, smoky fire that could harm you while you sleep.
  • Low cost and carbon-neutral: a small suburban lot can supply its own fuel from yard prunings, eliminating the cost and transportation of firewood, and ensuring sustainable heat.
  • No waste: the fuel is burned completely, leaving white ash rather than charcoal and creosote.
  • Natural and Recycled Materials: Rocket Mass Heaters can be built from recycled and repurposed materials, local subsoils, and non-toxic amendments like sand and perlite.

So what's the catch?

Every home heating system is a compromise between cost, effort, durability, and desired effects. Only the owner can determine if it's the right one.

Occasional Use: Spaces that are only occasionally used, such as vacation cabins, churches, or 'tinkering shops,' get no benefit from storing heat for slow release. Good insulation and quick radiant heat are the best options for occasional-use spaces. Think certified woodstove or infrared heaters.

DIY Heating: Building your own heating device is not for everyone. There's a temptation to change things you don't understand, and it's definitely work. You need a foundation that can support 3-5 tons of masonry (140 lbs/sf), clearances from combustibles, etc. It doesn't always fit under your homeowner's insurance policy, either. Masonry contractors may be able to install a rocket-like masonry heater, but expect a hefty price tag for their time and expertise.

Hands-On Wood Heat: Like any wood-fueled heater, a rocket stove involves a certain amount of wood cutting, splitting, drying, stoking, fiddling, ash cleaning, and annual maintenance. There's less work, because you need less wood, and you can use skinny sticks that break over your knee. But you still have to keep the wood dry and tend the fire while it's lit, and learn the nuances of sideways fire.

Hidden Fire, Visible Barrel: Some people miss seeing the flames or smelling the woodsmoke. Radiant heat means bare metal in your living space; covering the barrel's surface with pretty soapstone tiles or cob will also slow its heat.

Central Space Heating: Like other masonry stoves, a rocket mass heater only warms what it can 'see.' If your house has several stories or wings, you may need multiple heaters (or a back-up system like a furnace or heat pump) to evenly heat all the rooms.

Thermal Mass Options: Thermal mass can help heat and cool a home, stabilizing temperatures of day and night, and providing year-round comfort. Passive solar heating is even more efficient than rocket heaters, and may be all you need depending on location & exposure.

More Research Recommended: To learn more about Rocket Mass Heaters, try www.Rocketstoves.com, or the alternative energy forums at www.permies.com. Workshops are conducted by Cob Cottage Company (www.cobcottage.com) and others. The most complete resource is the 2005 edition of Rocket Mass Heaters, by Ianto Evans and Leslie Jackson.

Paul Wheaton has a great collection of Rocket Mass Heater videos, interviews, and an animated explanation at his rocket stove article on Richsoil.com.

For inside information about other wood heating devices, try www.woodheat.org, the Masonry Heater's Association (http://mha-net.org/), or the Chimney Sweep (www.chimneysweeponline.com). Entertaining and informative.

About the Author:

Erica and Ernie Wisner teach workshops on Rocket Stoves and other topics. The City of Portland, Oregon is currently reviewing a proposal to permit rocket mass heaters under local building codes. Find out more at http://www.ErnieAndErica.info.

Video clip from a Rocket Mass Heater workshop

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Comments 11 comments

justinskier profile image

justinskier 6 years ago

Check out my passive solar heater.


solar.power profile image

solar.power 6 years ago from Brisbane

Passive solar heating is the most efficient type of heating, however what you have shown is a great alternative.


MinMan 5 years ago

Solar works great where you have sun, try passive solar heating in Northern Minnesota when it's -35 Deg F outside...


Motocalmi 5 years ago

Does anyone have any experience adding waterpipe through the heat sink and circulating through an existing baseboard heat plumbed throughout the house? Would thewater be hot enough to benefit?


Farmsteader 5 years ago

Motocalmi, you could do this with those pex piping , but in the floor thru a mud floor-,(like for tile) with pex would conduct better,just do your homework 1st, and it is even worth it to pay an expert to run a formula for your specific application, or contact a College in Green Science to get some assistance. One of my Neighbors ran a metal pipe thru a brick oven he built for hot water use, has not finished all yet. Best Regards


MinMan2 4 years ago

There are passive houses in northern MN, WI, and Scandinavia. We just have to try harder at 45 - 60N


Erica K Wisner profile image

Erica K Wisner 4 years ago from Oregon Author

There have been some attempts, some people have done hot water and the heat sink is the place to do it (not inside the burn area!)... Another thing we've done is build the heater itself under the floor, and skip the water-circulation problems. Continuous pex should work, but we are leery of any material that would have joints in the earthen material as even a small leak could become a problem. I would put joints and valves in a separate box, for easy maintenance.


Erica K Wisner profile image

Erica K Wisner 4 years ago from Oregon Author

Exactly. Passive solar, or 'annualized thermal inertia' (Paul Wheaton's wofati) is the most efficient as it takes no fuel and almost no maintenance.

This is an option for areas where cloud cover, or tree canopy that isn't going away, makes supplemental heat necessary. It can also be a great demonstration of the awesome advantages of thermal mass, and feasibility of earthen building, that might allow a seasoned wood-burner to re-think their home heating strategy and go for a more passive technology.


Erica K Wisner profile image

Erica K Wisner 4 years ago from Oregon Author

Absolutely - I prefer passive solar myself, but I've lived in a number of houses now that couldn't have had worse heating design if it was intentional. Like an H-shaped floor plan with all the windows facing north. These can be a low-cost fix that allows a poorly designed or located house to be much more earth-friendly and economical than it would be if heated and cooled with electricity or a woodstove.


Oystein Tandberg 4 years ago

Hi!

I have a question:-) I'm about to start designing my rocket mass heater, and there are basically two things or three things.. First of all I got some pipes that are 16 cm in diameter, and some that are 20 - could I use the ones that are 16cm to lead the gasses away from the heatriser, and then connect them to the pipes that are 20cm?

Also, will it work to install the heat duct from the bench and down under the floor, and then out of the house, and up a small chimney?

I'm reading the book, (Rocket Mass Heaters: Superefficient Woodstoves YOU Can Build) - been collecting most of what I need by now.. I want to try to use an old wood stove as burn a chamber..

best regards


PhoenixV profile image

PhoenixV 2 years ago from USA

I am seriously considering building one of these in the future.

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