Rocket Mass Heaters: Efficient Home Heating With Wood
Building a Rocket Mass Heater
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.
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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