Friday, December 4, 2015

Rocket Mass Heater - Making Some Progress

This morning the world outside the house was frosted with ice.  Inside, we weren't frosted with ice, but it was chilly enough downstairs that fuzzy slippers and a thick sweater are much appreciated.  Getting undressed in the chilly front room where I keep my clothes practically requires a pep talk and a strategy to prevent prolonged skin exposure.  Burrrrr...  We need to finish our rocket mass heater soon!  We have made some progress toward that goal.  In the photo above, you can see the wire cylinder I made to fit around the heat riser.  I used bits of wire that I had left over from building the chicken coop.  

I tried to make it round, and small enough that when the barrel is placed over it, there will be a two inch gap between the cylinder and the barrel on all sides.    

Brandon finished sanding the barrel we burned the paint from.  Since this will get really hot when the stove is lit, we don't want any bits of paint left on it that will make a toxic fume in the house.  

By the way, since I've been asked this by most people who I tell about our rocket mass heater, we do have a smoke and carbon monoxide detector.  It's a fancy one too, that will alert Brandon's phone if it detects anything.  Not that we ever plan to use the rocket mass heater while we aren't there.  And we have a fire extinguisher!  I even got to practice with the fire extinguisher one before, when Brandon's tractor caught fire, so I know where it is and how to operate it.  

We filled the cylinder with a insulation made from perlite mixed with potters clay.  This is the step that I don't really understand the reason for.  The book says it's important, and I think at one time I could glean the reason for it, but by the time we were ready to do it, and Brandon asked me why we were doing it, I couldn't remember any more.  We did it anyway, even though it wasn't a small effort. 

The insulation recipe we used was five parts perlite to one part clay. Brandon used his hands to mix it together in small batches.  It was loose and fluffy, but if we squeezed a handful it would form a clump that would crumble when pressed.  At one point we ran out of perlite, and took a math break while we figured out the cost per quart when factoring in the gas it takes to go all the way to the city for the jumbo bag sold at Home Depot versus driving to the nearest small town Tractor Supply store for the small bags available there.  In the end, it made more sense economically to make the longer trip to the city for the big bag, but I think realizing we could get take out for dinner while there was the deciding factor!  

We didn't compact the insulation mix as we filled the space between the brick heat riser and the wire cylinder.  I would pour the insulation mix in the top, and a goodly portion of it would pour out of the wire onto the floor.  I did a lot of sweeping and reapplying while Brandon stayed busy mixing.  Where the wire met the brickwork around the burn chamber, there were large enough gaps that I used some sand and clay mortar to fill in so my perlite would stop pouring out.  

Finally, we got the whole thing full!  By this point, the many feet we have in the house (twenty, including us, Puck, and three cats) had tracked clay and perlite everywhere, despite my sweeping.  I'm still finding crumbles of perlite in strange places.  

We added a some clay and sand mortar left over from the brick work to the top of the perlite.  I'm not sure, but I think as the stove is used, the heat will bake the clay mixed with the perlite, and create a solid tube of insulation.  I think it may eventually burn the wire away too, but by that time the clay will be baked solid and it won't matter.  

The next step was to place the barrel over the top of the heat riser, and prop it up with some bricks to that the flat top of the barrel is a couple of inches above the top of the heat riser.  Easier said than done!  More math required.  Despite our careful measurements and barrel cutting, we ended up using broken pieces of left over bathroom tile on top of the fire brick to shim the barrel into the right place. 

I was excited to finally get to see the barrel in it's home.  I think some of the folks I've told about the rocket mass heater seemed doubtful that we would like having a metal drum in our dining room.  I know this was Brandon's concern too.  I like it though.  It sort of industrial and unexpected.  A bit weird maybe, but so am I.   It's first thing that can be seen when you walk in our main door too, so I'm glad to find that I don't think it's too ugly.  

I'm pleased with our work so far, but still nervous that it's not going to draft well.  There are some things that we didn't plan for.  For one thing, we always planned to have the barrel near the wall and the fire box out in the room.  When it came time to build it though, we turned it around so we could stand nearer the hot barrel and get it further from the sloped ceiling and wall, which puts an extra bend in our exhaust pipe and may change the air flow to the fire box.  Also, the two inch gap I planned between the barrel and the insulation cylinder, in actuality, varies from one and half to three inches.  Also, the heat riser is off level by at least a quarter inch, but we made the barrel level.  I think this will make the top of the barrel have a hot spot, but I don't know.  I just don't know if these things matter, and I won't know until we light it.  The book says to make adjustments, but that's not going to be an easy thing.  Hopefully we will find out soon, and take the chill out of this house.  Next is attaching the barrel to the exhaust using sand and clay.  I'll let you know how it goes.  

1 comment:

Anonymous said...


This is so awesome! I am so curious to see how it works. You guys have put so much thought into it, I'm really impressed. Love, love, love seeing the progress.


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