Rocket Mass Heaters
- E. Wisner
I am sitting on a heated bench on a cool spring morning, enjoying the breeze from the open door.
Last night we burned a double handful of kindling and most of one small log. The fire lasted less than an hour, yet the warmth lingers now.
Wood heat is the soul of an independent home. Yet, romantic as the cheery evening fire may be, the honeymoon is over when your feet hit the cold floor in the morning. Somebody has to tend the fire, continuously, to keep the house warm. Experienced woodstove operators may boast of keeping one log burning all night … a trick that sends most of that log’s fuel value up the chimney as smoke. Where that practice is widespread, the air quality, creosote problems, and fuel requirements eat up a lot of time and attention.
These problems are not new. Fuel shortages and expense, smoke problems, and inconvenient 'conveniences' are as old as civilization. Around the world, different engineers have proposed similar solutions: more efficient stove shapes, materials, or systems that recapture waste heat in warm masonry mass. African jug stoves, American fox stoves, Roman hypocausts and steam baths; Russian, Finnish, and German masonry heaters; Chinese and Central American hillside homes with their furnace under the floors.
Our heater is kind of like that, but instead of custom masonry tiles and sub-foundation installation, it is made with scrap metal and earthen masonry, on an existing slab-on-grade floor. It was literally dirt cheap, aside from the stovepipe chimney.
We (over)built ours as near to code as we could figure, including a manufactured chimney. Then we invited our city inspectors to take a look. They didn’t quite know what to say. We learned that in Oregon (and most of the USA), if your heater is clean and safe, over 1860 lbs, and is your only way to heat or cook, it is exempt from DEQ and many local regulations as well. So are antique woodstoves. (One inspector made the parting remark, "Well, dirt's old...")
Now we’re helping to draft some building code so they can officially approve it. If this all goes according to plan, we'll be happy to share the progress with other owner-builders.
Here’s how we built our living room heater:
2) Plan the Layout:
I like paper; Ernie likes to lie down in the space and draw on the walls and floor.
- For our building code efforts, the barrel has a heat shield and clearances like a normal woodstove; and the fuel feed is away from combustible walls.
- The main mass is a bench, insulated from the walls; foundation is an existing 4-6" concrete slab floor.
- We planned cleanouts for each major bend inside the system.
- The exit to outdoors could have been horizontal, but our final design includes a vertical stovepipe chimney courtesy of the property owner. ($560 or so)
3) Collect the materials:
- bricks, old concrete rubble, and rocks
- ducting, (15 recycled sections from ReBuilding Center for about $25, 3 new parts @ Home Despot for another $30. Nothing a real scrapper couldn’t get for free.)
- a steel barrel, (about $10 delivered)
- perlite for insulation ($30, Home Despot)
- scrap metal wrapper to hold insulation around the heat riser (on hand)
- We dug up some local clay (free), and ordered a truckload of sand ($110)
- Decorative tile and pigment… another $25…
Tools we mostly had on hand:
- tinsnips, pliers, crimpers for ducting,
- shovel and tarp for mixing cob,
- buckets and wheelbarrow
- level, masonry gear.
- Boots, gloves, and strong coffee ($12).
(If you’re counting, the other $190 included a bartered piece of triple-wall metalbestos for the thru-ceiling, a very pretty thrift-store ash bucket, a $60 permit, and pizza for our crew. Total was about $1000.)
Over half our cost was the new stovepipe chimney and thru-roof details. The heater worked just fine with its blood-warm exhaust ducting stuck straight out the window, during test firing. So we could have saved $560 with a little discretion and a dryer-vent-style exhaust.
5) Brick Masonry: We dry-stacked the first layer of footing; insulated under the combustion area, and start laying the brick with lime mortar. Insulated and double-sealed the brick burn tunnel.
6) Mixed a lot of cob for the thermal core.
(Cob is mostly sand, with enough clay to stick it together. Like fireclay mortar, but less fussy. Most cob has straw / fiber reinforcement, but our “thermal cob” is just stiff, clay-sand mortar.) We packed it smooth and careful around the ducting, then prodded in a dense mix of rock rubble and cob mortar for the rest of the core.
7) Seal the system.
- Used cob (sand-clay mortar) to seal the heat riser and barrel in position.
- Connected the barrel and ducting, with an easy-to-reach ash cleanout.
8) Test-fire once everything is chinked in place. This is the hardest time to light a stove, when it’s all wet mud. Newspaper, kindling, wood, except it all goes in vertical and upside-down. It passed!
9) Drying: Now we’re sure the proportions are correct, and nothing has been accidentally blocked during construction. We let the core dry out – leather-hard within a few hours, and almost as hard as cement within a week or so. By this time we had dustcovers over it, and we left the finish work for later.
Mix plaster, re-wet cob, float a smooth finish, set tile or burnish and paint. Some people go sculptural at this stage, but we chose simple curves and lines with tile details. We tinted the earthen plaster with an iron-oxide concrete pigment.
We may add a lime plaster eventually, but we like showing off the earthen plaster showing for now.
So all in all, it might have taken a few supply runs, and then three weekends of actual work, for a team of about 4 people. We had different helpers at each stage, friends and family and curiosity-seekers. Several of them want a stove like ours when they get a chance.
We run our heater for a few hours a night in our coldest weather, and wake up to a warm house like an early summer’s morning.