Tuesday, February 26, 2013

Rocket Mass Heater Design/Build Workshop: Earth Sky Time Farm

Earth Sky Time is a community-based organic farm in the beautiful, wooded foothills of the Taconic mountains, just outside of Manchester, Vermont, USA. In addition to year-round sustainable agriculture, they also have a wood-fired bakery (featuring a massive Catalan wood-fired oven!) and make other prepared foods like humus and veggie burgers.

During the autumn and winter of 2012-2013, I was involved in the design and build of a wood-fired rocket stove sauna at Earth Sky Time. It was an exciting and ambitious project, and we strove to make it highly efficient, environmentally friendly, sustainably built, and locally sourced. You can learn more about the construction of the building itself in my last post: Building a Rocket Stove Sauna: Earth Sky Time Farm.

In October 2012, halfway through construction of the sauna, we held a workshop on designing and building a rocket mass heater, which I co-taught with Tristan Reaper. This allowed us to hold a fun event during the construction process, educate people about sustainable building methods, and get some extra hands on site to help us with the project.

The rocket mass heater is a super-efficient, clean-burning, wood-fired masonry heater. It can be designed to heat a home, make hot water, and/or for cooking. Its simple design means no professional stove-builders are needed - homeowners can design and build one on their own. The flexibility of its design means it can be built from a variety of materials, including locally sourced clay and stones, and salvaged materials like old bricks and barrels. This means it can be built at very low cost and with minimal environmental impact.

Here we're preparing an old barrel for use in the construction of the stove. The old paint and residues are being removed, and it's going to be given a couple coats of high-temperature stove paint. Building with locally harvested or salvaged materials can save a lot of money, but it takes extra time and craftiness to locate, acquire and prepare what you need.

Scroll down to see some photos from the workshop, and to see the finished Rocket Stove Sauna!

We began the weekend workshop in the classroom with an overview of masonry heater design. We talked about the forces involved in combustion and heat transference, the history of masonry heaters, and the development of the rocket mass heater. We looked at what sets this stove apart from the rest, from its unique design features to its founders' vision of a worldwide solution to environmental, social and health issues.

Then we got down to the nitty-gritty, explaining the mathematics involved in scaling the components of the stove in relation to one another, and also in relation to the space we are trying to heat. We showed how to measure the rate of heat loss at design temperatures for the space we are heating, and how to design a stove that can meet these requirements.

After the classroom element of the workshop, we went down to the build site and got to work building the rocket mass heater. Both the firebox and the masonry bench were built on a raised cement block platform. We insulated this platform with fireproof mineral insulations: beneath the firebox we used dry vermiculite, which is made from puffed micah. Beneath the masonry bench we used rock wool.

The first day of the workshop, we built the core structure of the stove using firebricks and refractory cement. While rocket mass heaters are commonly built with a firebox with a cross sectional area of 28" or 50", we scaled up our stove slightly to have a firebox of 70”.

The next morning, the internal chimney (or “heat riser”) of the stove was covered with a cylinder of hardware cloth and filled with an insulative mix. For this mix, we used clay-rich subsoil that we dug out of a nearby hill, and vermiculite. The trick with this mix is to make it structurally solid, while keeping its density low. The cylinder was then given a smooth final render of refractory cement.

The masonry bench was built with old bricks we received for free, leftovers from another job site. These were cemented together and filled with varying mixes: for the back wall of the bench, we used our clay-vermiculite insulative mix, while the wall of the bench that faced towards the center of the room was filled with a dense, sandy mix. These mixes slow down the transference of heat towards the walls, while maximizing the transference of heat into the room.

We built a manifold to connect the heat riser and the masonry bench. This is a tricky part of the stove design, because it's a common place to accidentally create a bottleneck that will hurt the stove's draw. The manifold is a chamber where the hot gases from the stove slow down slightly and enter the channels in the masonry bench. When the gases slow down, any ash they are carrying is dropped in this chamber. Water vapor that is released as the gases cool also collects here, so we built the manifold to be deep and basin-shaped. The manifold will also feature a clean-out door so ash can be removed occasionally.

After the workshop, we continued simultaneously building the sauna and the stove, with the help of volunteers from the Earth Sky Time community. If you'd like to read more about the construction of the building going on around the stove, you can read about it in my last post: Building a Rocket Stove Sauna: Earth Sky Time Farm.

The masonry bench was completed by installing 2 stove pipes of 8” diameter, and pouring a dense, sand-rich mix over them. Then the whole bench was given a final cement render. This final render slopes towards the center of the room, so any condensation in the sauna will flow towards the center of the room and not collect along the walls.

The diagonal feed of the stove is a feature I've been wanting to test out for some time. Normally the feed of a contemporary wood-burning stove is horizontal, and on a few models you can find vertical feeds. Vertical feeds are very popular on rocket mass heaters, because they allow long, straight pieces of firewood to be gravity-fed into the firebox. But I haven't been totally satisfied with vertical feeds. First of all, it increases the risk of the stove backfiring into the room, especially when the fire is just starting up and the heat riser is still cold. Secondly, while in theory it allows the wood to be gravity-fed into the firebox, in practice I've found it's not a safe bet that homeowners will take the time to ensure all their firewood is cut thin and straight enough for the feed to work. And thirdly, a vertical feed prevents the firewood from being pushed very far into the firebox. This means we have much less control over the temperature of the top of the barrel. The cooking variants of the rocket mass heater rely on the position of the firewood for this purpose. In our specific situation, we want to be able to control the temperature of that barrel-top for making steam in the sauna room.

So to make a long story short, for this rocket mass heater we built a diagonal feed. It still has the elegance and ease of use of a vertical feed, but without making any sacrifices concerning the draw and temperature control.

The rocket mass heater was then given the finishing touches: the exposed masonry was tiled using antique bathroom tiles we dug out of a friend's basement, the barrel was cemented in place, and the main stove door and 2 clean-out doors were installed. The barrel is salvage, of a good thick gauge; the stove doors were cut off of some old boilers we found, rusting out in a field. We gave all the metal components a little TLC and painted them with high-temperature stove paint.

The masonry bench of the rocket mass heater is usually just given a final render, some pillows are added and it's sat on as-is. But to adapt the masonry bench for use in a sauna, we built a 2-tier cedar bench over it. The planks on the bench have 1/4” gaps between them, to let the warm air around the masonry circulate through the room, and also to let moisture drip down through the bench.

And finally, here are some photos of the finished rocket mass heater, inside Earth Sky Time's brand new Rocket Stove Sauna!

After finishing the project, we tested the stove and the sauna for a few days and threw a few community sauna-nights to celebrate. The performance of this stove really surprised me – I had underestimated its capabilities, when combined with such a tightly built, well-insulated building like the sauna.

After leaving the stove and the building to drop to exterior temperatures, we lit a fire to see how it would perform with a cold heat riser. The temperature outside and inside the sauna was about 20 degrees F (-6.6 C). To my surprise, the smoke instantly followed the draw up the heat riser. We tried opening and closing the front door, which did cause the stove to backfire slightly during the first 5 minutes, but only with a wisp of smoke that left no trace in the room. And as soon as the stove was a little warm, there was nothing that could make it backfire.

As for heating the sauna, we knew from the get-go that we had oversized the stove for the building, to ensure we could reach sauna-worthy temperatures. But I was still amazed to find that by simply closing the doors and vent windows, within 2 hours of running the stove at half-capacity the sauna was heated to over 100 degrees F (38 C).

During the first sauna we all held in the building, the rocket mass heater easily maintained 145 degrees F (62 C). A pan placed in the center of the barrel top produced steam, and by pushing the firewood deeper into the firebox we could raise the temperature of the barrel-top to produce even more steam.

The exterior temperature that night was around 15 degrees F (-10 C). Afterwards, we left the vents closed to protect the masonry from cooling down too quickly, and to our surprise, when we returned to the sauna the next morning, it was still holding at 110 degrees F! (43 C). This is a testament to the tightness of construction, the great insulation of the building, and the high thermal mass of the masonry bench, which absorbs the heat from the exhaust gases and slowly radiates the heat back into the room, rather than letting it escape up the chimney. You can learn more about the design and construction of the sauna building in my last post, Building a Rocket Stove Sauna: Earth Sky Time Farm.

Building the sauna and its rocket mass heater at Earth Sky Time was a great learning experience for all of us. It was an opportunity to challenge ourselves, using the resources around us, to build a super-efficient building and stove that functioned together as one. All of the participants in the rocket mass heater workshop were great, and all of the volunteers that helped along the way as well. I hope we will all meet again someday – maybe while enjoying Earth Sky Time's new rocket stove sauna!

- Ben


  1. This is awesome, what's your next project?!

  2. Inspirational

  3. Wow that's a nice build !

    im planing to build a sauna.
    is it still working good?
    did you have to "repair" or change something ?


    1. I visited the community a couple of times since finishing the sauna, once a year afterwards and again a year after that. The sauna was still going strong! Both times I noticed it could be cleaned out more frequently though. If it's being used regularly during the winter, the stove should really be checked and ash removed at least every month to keep it running strong. But this particular stove is a beast - it was specifically built to withstand the wear and tear of a large community. It has larger-than-usual ash collection chambers and a minimum 8" diameter flue system so it was going strong despite not being cleaned very frequently.