Batch Rocket Mass Heater for Greenhouse

For decades our 20 x 48 foot greenhouse sometimes freezes tender plants when outdoor temperatures go below zero. Inside we do have a few thousand gallons of water in tubes and tanks that heat up when the sun shines but often winter weeks go by without sunlight. In order to grow more than chard, onions and salad greens, we needed a way to add heat. 

Snow also blocks sunlight from entering.

Until we remove snow and upper regions slide down.

A section of glazing showing five layers and 

diagonal bracing to support snow loads.

The best reference for building rocket stoves and heaters.

The firebricks used in this project are held together with cob, a 

mixture of sand and clay, instead of a high temperature cement 

that cures hard. That way any rework only requires the dry glue 

to be moistened and it comes apart. The ratios of sand to clay 

above show that at above 5:1, the mixture, on drying, does not crack.

The completed heater with barrel riser surrounded by seven 

water tubes, an eleven foot heated bench and exhaust stack.

Inside the two black barrels, one on top of the other, is the ceramic

 fiber riser that powers complete combustion gasses of burning wood. 

The disc removed from one end of the lower barrel was suspended 

inside the upper barrel to prevent hottest exhaust from reaching the 

top. During firing, the color change indicates the region that typically 

reaches 700F charring the vegetable oil coating. The blue ring on top

 hides a foot of insulation that limits heat transfer to regions above.

Paint was burned off the barrels outdoors to prevent fouling indoor air.

This view of the upside down lower barrel shows the riser hole.  The more 

complex outlet hole for the hot exhaust gases to exit the riser stack 

and enter the horizontal tube inside the bench was made later. 

This illustrates the transition from the  barrel stack to the horizontal flue 

run inside the bench with this cob (mixture of clay, sand, silt and stones) 

also in good contact with the adjacent water tubes.

This view shows the bench with water tubes behind before two additional 

water tubes were added on each side of the barrel stack. Cement tile 

backer board contributes and hides thermal mass.

The bench frame consists of two instrumentation racks lying on 

their back, end to end. An 8 inch steel flue runs the length inside 

and was covered by a foot of clay/sand/stone slurry that has now 

dried in good contact with the flue. The volume above now stores 

more than a ton of  surplus bricks to maximize storing heat during 

firings. These are still readily available for future projects.

Another view of the bench with its sheet aluminum top 

left over from building large solar collectors.

This shows burning off the paint on one of two boxes with doors that 

make it easy to inspect the transition between the riser/barrels and 

horizontal flue and between the latter and the vertical stack.

This shows the detail of the bottom of the burn chamber with 

two stainless steel pipes that introduce fresh air to the base of the 

riser to enable complete combustion of exhaust. Under these is 

space for a stainless steel tray that catches ash and makes 

them easy to remove.

This view shows the ash section of the wood stove that is the sole 

source of heat in our home. Many years ago I replaced its cast iron 

grate that burned through with stainless steel pipes and they've 

performed wonderfully. The ash bin below also works very well, 

requiring us to empty it only once per week. 

Illustrates the first row of wall fire bricks and include those 

that define the port width. Forcing the exhaust through a 

narrow channel creates two vortices just above the far end of the 

secondary air pipes and creates the "roar" as it travels up the 

riser, making the noise that gives "rocket" heaters their name.

Firebox and riser base now complete and also showing the open 

box door of the barrel/horizontal transition through which any ash 

buildup can be removed.

Firebox now with a calcium silicate fiber roof, one inch of 

ceramic fiber insulation and sheet aluminum to limit 

infiltration. The ash tray is also in place.

Four each four inch high calcium silicate rings were stacked on 

top of the firebrick riser base before insulating the assembly with 

ceramic wool batting. The barrel clamp ring fastens the lower barrel 

to its lid that has a hole for the riser and flue exit.

This shows the one meter tall ceramic fiberboard riser portion.

The open area of the hexagonal riser and other items that carry 

exhaust gasses are larger or the same as an eight inch circle

 to prevent restricting flow.

The hexagonal riser now covered by two inches of ceramic fiber 

blanket, a sheet of aluminum and also showing the barrel lid 

with the flue gas exit transition hole. Two vertical steel 

angles were added to insure the riser inside stays put.

The door assembly includes a ceramic glass window and an air 

inlet door that admits the proper amount of primary and secondary

 oxygen for optimal combustion. The latter is opened only during the 

hour or two per day that there is need for additional heat. If the 

sun comes out for even a few hours, typically no additional heat 

is needed even when nighttime temperatures approach zero. 

Xena, our greenhouse cat, supervised everything!

Moisture in burning wood exits as steam when it's freezing.

When it's above freezing, there is no visible exhaust. The rocket mass 

heater did keep the greenhouse above freezing during a -20 F very windy

 period but it did require the fire burn for quite a few hours. Even with 

nights in the teens, this week in February we haven't needed fires because 

of sunny days! When the fire is out, I now block the bottom of the 18 foot 

tall stack to prevent the very buoyant warm gasses inside from sucking 

heat out of the thermal mass (the vacuum is strong enough to hold up 

a thin aluminum sheet). This draw does make starting fires very easy 

since it takes less than a minute to ignite a wad of newsprint

that is placed under a few twigs to make the burner roar!

The little flap on the heater door now has its white paint burned off 

where it covers the round air intake hole. I only open it a half inch 

during burns to limit the rate. The broken tiles on the bench behind 

are some of 74/88 that arrived broken. They are sending replacements 

so I can finish tiling the heater apron. The low profile ash bin is a bit 

small and has to be emptied every three or four burning days.