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Wednesday, October 30, 2013

How It Works: Masonry Heater

System Purpose.  Maximize wood as primary winter heat source.

How It Works Summary.  Wood energy efficiently converted to heat which is stored in masonry and slowly released into house via radiation and convection. 

System Cost.  $22,000 including masonry heater and costs associated with cutting, splitting and storing wood.   Since masonry heater is very reliable and splitting mauls are cheap, repair and replacement of chainsaw is main lifetime cost.   By spending money on a very efficient and effective heater we’ve reduced time and costs associated with wood:  moving, cutting, splitting, and storing.    The masonry construction virtually eliminates fire hazard, a serious and costly challenge for some wood burners.    Masonry heater also eliminates health risks and costs caused by ducting of heated air. 

System Components

Masonry Heater, the heart of our home, acts like a giant, super efficient fireplace.   Nine feet long, four feet wide, the heater’s exterior is lined with 8 inches of masonry:  firebrick inside, concrete block outside, all finished with beautiful  American Clay.  We light two fires a day December through February, and one fire a day otherwise.  To light a fire, cordwood is stacked in the 20”x20”x20” firebox, topped with kindling, and lit with a match.  Within 15 minutes, the fires burns from the top to the bottom,  igniting the entire stack.  Fed by a 4 inch pipe of outside air, the fire is now a raging inferno.   The smoke escapes via a contraflow system.  Smoke rises, then falls down the sides, slips under the bench, then rises and exhausts via the chimney.    Above the firebox,  the escaping smoke is momentarily trapped in a contraflow vortex and---like the catalytic converter in a car---reignites at a temperature of 1700F, converting smoke to heat while eliminating many pollutants.  Within 90 minutes, no matter how dense the wood, the entire flaming stack is reduced to glowing coals.    While a small amount of heat escapes out the firebox glass window, the vast majority is absorbed by the masonry and slowly released into the home.   As I type this, I can feel the gentle massage of radiant heat on the back of my neck.    Unlike homes with steel wood burners, the temperature inside our home varies only slightly, from a 73F peak 8 hours after lighting the fire to a low of 68F 24 hours later, just before lighting the next fire.   The 8 hours to reach peak is the time needed to conduct the heat through 8 inches of masonry.   The outside of the masonry becomes warm but never dangerously hot.    Laundry dries quickly when the drying rack is placed on the heated bench.  So too wet clothes.    The massive masonry heater requires deep footings and works best when centrally located in an open floor plan.   Even the most remote room in our home remains cozy warm all winter long.  Masonry heater does not work well during the shoulder seasons around winter.  The heater must be slowly broke-in each heating season, requiring several days to reach its full heating potential.  The combination of masonry heater  (for winter) and solar heating (for the shoulder season) is excellent. 

Cordwood---wood that is cut, split and well-dried---is stacked into masonry heater fire box and ignited with kindling.  Since the masonry stores the heat, fast-burning wood---like boxelder, willow and poplar--- works well in the masonry heater.   Denser wood like oak has the advantage of less wood needed.    A pound of dry wood, no matter the type, produces the same amount of heat.   

Chainsaw fells trees and cuts into logs ranging from 8 to 20 inches long.   Gas-guzzling, loud, hot, unreliable and notoriously dangerous, chainsaws are a necessary evil.  We do use handsaws for smaller logs and pruners to cuts kindling.  

Splitting Maul, swung by hand, splits the cut logs into cordwood.   If logs are mostly dry when split, then split cordwood can be immediately stacked in covered storage bin.  If logs are still green when split, then split cordwood is stacked in sun-exposed, one-layer thick windrows for one year before storing.    We do not use a gas-powered splitter for several reasons.  First, they do not save much overall time since the most time consuming step is moving the wood.  Second, they are expensive.  Third, we enjoy splitting wood by hand and so do many friends. 

Kindling is placed on top of the cord wood in firebox to ignite a top-down burn (fire burns from top to bottom so wood does not scorch and smoke).   Kindling---dry thin sticks---is everywhere:  falling off trees, the ends of sawed branches,  invasive shrubs like honeysuckle and buckthorn.    When collecting kindling take care to avoid sticking yourself in the eye. 

Wood Storage, the covered storage bin, needs to store enough cordwood and kindling for a winter.    We store kindling under the shed’s lean to.  We store cordwood in the containers which our steel roofing arrived, a total of 3 cords for a winter.    Prior to burning, a sixth of a cord is moved inside the attached garage to assure dryness and to make it easy to build a fire.

Trees, solar powered and eminently renewable, remain the unsung heroes of our heating system.  We harvest mostly dead and downed trees or those our neighbor cleared around his home.  Boxelder is our workhouse and the perfect tree for a masonry heater:  grows anywhere quickly,  sheds bark to dry well, easy to cut and split, burns great and since nobody likes it, we can always get plenty from our neighbors.   A single boxelder can be harvested many times via coppice and standard methods.    Since the masonry heater works best when burning hot and fast, we can use the entire tree, even the small branches and twigs.    Ash from the masonry heater is spread in a wide ring around trees and shrubs, inhibiting grass and feeding nutrients to the next generation of trees.

System Maintenance.  Wood heat is very labor intensive (see above).  I recommend it only to those who enjoy such or the health benefits from the steady activity.     By far the greatest amount of time is devoted to moving wood (retrieving, stacking to dry, storing, stacking in garage, carrying into house, stacking in firebox).   Creosote builds slowly inside the chimney since heater burns so efficiently. 

Report Card.   Warm Season = B.   Cold Season =  A.    (See blog post for details).

Biggest Challenge.  Leaving the home for many days in a row during very cold weather.   With our solar heating we do not fear freezing pipes.  But we will return to a cold home.  While we’re gone our “backup” is a friendly neighbor (who starts the fire).  This fits our value of community reliance rather than self reliance. 

Biggest success factor.  Adaptive process.   No matter the type of heater, burning wood is a learning experience.   Rather than over-thinking and investing in expensive splitters and wood sheds, our system has slowly evolved as we learn.   We not only save money, but it’s fun to creatively improve.

The Connection.    Imagine a cold winter night before a blazing wood fire, drink in hand.    Need I say more?   Yes actually because growing and harvesting trees pulls us out onto The Land and dreams us years and years into the future Land.  


  1. Wonderful post - always loved fireplaces and building fires. Have managed to go through a cord and a half - in a Pasadena winter! Fascinated that you can build your fire from the top down - never seen that before.
    Just a thought on your maul use - consider a large stump upon which to stand your wood for splitting. The angle is better on your back and there is less chance of a nasty accident.


    1. that's a really good idea. At my age you can't be too careful.