Happy Thanksgiving everybody! We had a grand one (five pies, even!), so to to help ourselves recover from days of cooking, eating, visiting, and imbibing, Brandon and I began one of our most ambitious projects - a rocket mass heater. If you've traveled the interweb in search of alternative solutions to heating your home, you've probably seen references to this type of do-it-yourself wood stove. It's supposed to be super efficient, heating a small well insulated home, like ours, with a fraction of the wood needed for a traditional wood stove. It's also a snugly stove. Because it has a big thermal mass in the form of a bench made of mud and sand, it's made to be a warm seat, which I hope will be perfect for reading books, drinking tea, and cuddling the cats. Now, with winter knocking on our door, we're trying to get this stove installed in the house. The picture above is the mock up we built outside for our test run. It works!
Brandon marked the bricks and made some drawings of our mock up so we could re-create it inside. So far, the most challenging part of this project has been finding the materials. Since we are lucky enough to have friends who are potters and kiln builders, we were able to get some good fire brick to build the burn chamber and heat riser.
We dug clay from the yard, and also used some potters clay to make a brick mortar with sand we bought from Lowe's.
The instructions in the book suggest using your hands to mix the clay and sand. A drill with a mixer attachment works well too.
The yard near the barn resembles a brick yard right now. It also rained the entire weekend, so the recent excavations Brandon made for some electrical work have turned to muddy trenches, and the frequent trips out for materials during this project made a muddy mess of the yard and the floors in the house. We haven't even begun mixing the clay, sand, and straw to create the giant thermal mass bench, and this is already a messy project!
Why is that no matter how careful we plan and measure, nothing ever works exactly like we thought? Despite Brandon's careful drawings, the mortared structure isn't exactly like our mock up. Cross your fingers that when we finally light this thing in the house, it drafts like we hope.
In this photo you can see that we used non-flammable cement board on the floor, then a layer of insulating soft brick used in kilns. The inside of a pottery kiln can get thousands of degrees and the brick stays cool enough to touch on the outside. These brick are really light and porous. The third layer shown here is made from the hard fire brick, and this will be the bottom of the burn chamber, where the fire lives.
The sides of the burn chamber are fire brick mortared together. Nice and level, and square.
The mortar Brandon is using is the potters clay mixed with sand. Brandon had to fiddle with the amount of moisture to get it to stick to the bricks. We've never done much brick work before, so this was a new experience for us. Brick laying is heavy work!
On the left side of this photo is the hole where the fire wood goes in (straight down). On the right side is the hole where the hot gasses go up. In the middle is the brick work that makes the top of the burn chamber, since the fire burns horizontally in this type of stove. This is the part that didn't fit together the same as our mock up. We knew how big to make both of the holes, but somehow the brick layout between these didn't fit together like we thought it was going to. This makes me nervous that the stove might not draft as well as our mock up.
Have you ever used perlite before? It's the little white stones that are in potting soil. It's a light porous rock that not only helps with soil compaction, it also is used as insulation in masonry projects. To protect the wall and to prevent our hard earned heat from seeping into the wall, we put a piece of cement board along the wall with a gap behind it, and filled this gap with loose perlite.
The exhaust pipe for this stove is eight inch duct work, that will run along the floor in the dining room, against the insulated wall, and be covered with cob, a mixture of mud, sand, and straw.
Right behind the burn chamber, where there will be burning sticks, we also put a layer of soft brick against the wall.
This is the semi- final layout of the brick work for the burn chamber. Fire wood goes in on the right, near the wall, flames go through the center, and hot gas and bits of ash go up on the left into the heat riser.
The next step was the mortaring of the heat riser. This is a short tower of bricks that direct the hot gasses up.
We tried to make sure the inside of the heat riser was as smooth as possible. And even though we checked for level as we were building, somehow in the end, it's off level by at least a quarter inch. Sigh. What to do? We decided to ignore it and hope it doesn't matter. Good idea!
Despite the rain, we kept a fire going outside using wood scraps from the house renovation, and burned the paint off of a fifty gallon drum that was used for spaghetti sauce. This metal drum is a critical part of the rocket mass heater.
After the paint was mostly burned away, Brandon used a sanding pad on his grinder and gave the drum a cool looking sheen.
As you can tell, the cats were very excited by the prospect of a rocket mass heater. Snore.
We put more concrete board on the floor were we think the exhaust pipe and the cob bench will be. Stay tuned for the next installment of Rain and Brandon build a rocket mass heater! I know you are on the edge your seat. Me too!