Geodesic Dome Greenhouse - Part 10 - ROCKET MASS HEATER

This details how I build a Rocket Mass Heater for my geodesic dome greenhouse.  It's quite amazing how efficiently it burns and the heat that it generates!

Below is the transcript from the video:

Welcome Back!

This segment details the rocket mass heater construction for the Geodesic Dome Greenhouse.  I’m not going to detail the measurements of the heater, but the information can be found in a book entitled “Rocket Mass Heaters:  Superefficient Woodstoves You Can Build” by Evans & Jackson.

The entire burn tunnel and thermal battery is designed to be under the floor.  When it’s complete, the only exposed part will be the 55 gallon drums and the wood feed hole.  I started with a sand base and laid some old patio pavers under the burn tunnel area and then laid out some antique clay bricks that a friend of mine gave me.  I’m not sure how long they will hold up to the heat, but the price was right!  The second course of bricks is laid on their ends to provide the proper height in the burn tunnel.  I decided to run the chimney down all the way and cut it to fit against the burn tunnel area.  The mortar is set very thin and is used more to fill gaps between the bricks.

After the chimney is set, a couple of bricks are set on the top of the burn tunnel and mortar is applied around all the open joints.

You will see a series of concrete bricks in a circular pattern around the chimney.  This the outline for the  55 gallon drum that will placed over the chimney stack which will be supported by the bricks.  This area will help to collect any ash before it goes into the thermal battery.

The vent through the thermal battery area is galvanized flue pipe.  Please note that the galvanized coating will burn off the steel and is toxic.  The first several burns of the system should be done while the area is well ventilated.  All the joints are connected with Mylar tape, but it is mostly to hold things together while I assemble the pieces.  The adhesive will quickly burn off.

For the thermal mass, I chose to bury the system with sand instead of the traditional cob.  The sand is cheap and is spread through the rest of the floor of the greenhouse. It will also be easy to dig up in the event the flue pipe needs any maintenance in the future.

The base for the 55 gallon drums consists of some extra cement bricks mortared together.  It’s nothing fancy, just enough to prevent the sand fill from getting through any of the cracks.

Once the mortar has set, the entire area is backfilled with sand and compacted.  Eventually, brick pavers will be set on this sand base throughout the entire greenhouse.

This overview shows the wood feed hole, burn tunnel, and part of the chimney.  It also shows the barrel base and then how the exhaust leaves the area and goes into the thermal mass battery.  I ran out of sand so I couldn’t complete covering the area.

The outer section of the chimney is made from two pieces of flue pipe screwed together.  You may have noticed that the galvanized coating is mostly burned off since I have been doing some test burns prior to finalizing the installation.

Again, I broke away from the traditional design and decided to insulate the chimney with rock wool instead of vermiculite.  It will be interesting to see how well it holds up to the heat inside the chimney.

Back at the thermal mass, the flue extends out of the area using a tee.  The bottom is capped and a bunch of holes are drilled into it to let the condensation drain.  The remaining area is backfilled with more sand.  The flue pipe exits through the side wall which is protected with a thimble made from the lid of a 55 gallon drum.  The vent is capped with a tee to help prevent back drafting.

To force a stronger draft, I decided to build the chimney taller than most heaters.  I needed about one and a half barrels to cover the height of the chimney.  I got two identical barrels that had removable lids and cut the bottom out of each barrel. 

The bottom barrel, which was cut shorter, is set upside down on the cement block base and the top barrel is set right side up so that the barrel bottoms butt together.  The original lid clamp is set on the bottom lips and tightened to seal the joint.

It’s a perfect day to test fire the heater!  The dome is covered with a blanket of snow.  It doesn’t stick to the steeper sections of polycarbonate so some natural light can get in.  A few hits with the rake handle and I can knock of some of the other snow and it lightens up most of the area.

To start the heater, I cheat a little and put a small duct fan in the exhaust vent.  This creates a nice suction in the burn tunnel when the system is still cold and guarantees that there is no smoky back drafting.  From lighting the match to it being fully running takes about 2 minutes and I can remove the duct fan.

I start it with some newspaper along with a bunch of pine kindling.  As the fire builds I add some larger pieces of kindling and then add small pieces of hardwood.  By this time I can take out the duct fan and it’s drafting on its own.  The system has no problem running with larger pieces of wood. 


When it’s up to temperature it mostly exhausts CO2 and water vapor.  There is hardly any smell of smoke.  Burning for about 16 hours, it used less than a wheelbarrow of wood and left about 2 cups of ash.

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The fire tunnel is below the floor level and the inner chimney rises out of it.  The outer chimney is fitted and then fire-proof insulation is packed between the chimney pieces.  The exhaust duct is installed for the thermal mass and buried under the floor.  It then runs vertically and then out through the side wall.  A 55 gallon drum is installed over the chimney.

When the fire is going, the heat drafts up the chimney forcing the fire to burn sideways, preventing it from burning up the feed tube.   Some of the heat radiates out of the barrel, cooling it down and creating more draft.  The warm exhaust is forced through the thermal mass battery where more heat is absorbed into the mass.  It then rises vertically where more heat radiates out of the exposed duct work.  It then vents out of the building at about 85 degrees.

Eventually, I plan on covering the barrel and blowing the heat radiated from it down under the floor of the greenhouse, using the entire floor as a huge thermal mass.