Rocket stoving to the moon!
We've left it a bit late but as the mercury plummets, we are racing to get our Rocket Stoves online. Both Howard in the tiny house that we built for him in the Dordogne, France, and Eva and I in our new place on Mallorca, Spain. With a bit of help from Ness who is cleverly waiting to see what works and what doesn't so that when we build her Tiny this spring she will have the benefits of our trails and errors. This is probably the third or fourth trial at building a rocket stove from a variety of materials (both recycled and new).
So what I thought I would do is quickly explain why we are so into Rocket Stoves, what they are and how you can get started playing around and making yours.
A Rocket Stove is a very efficient type of wood burner which is able to capture the heat normally lost out the chimney and deliver it directly to your butt over extended periods of time even after the fire has gone out. Between the more complete combustion and the extraction of heat from the smoke, Rocket Stoves deliver around 4 times the heat from the same amount of wood, whilst releasing only CO2 and steam into the atmosphere (instead of heavier wood gasses, which normally never get up to temperature to burn and break down to lower mass CO2).
To achieve this amazing feat they make use of basic thermodynamic principals. Mostly the facts that; hot air rises, cold air falls, and the amount of heat something can absorb is relative to its mass. E.g. heavy things can hold a lot of heat and light things very little (think of insulation).
Rocket Stoves then employ these to ensure first that there is plenty of oxygen, turbulence, temperature, and time for a complete burn of the wood, and second that this heat is extracted from the smoke (into a thermal mass) before it leaves the house. Normally in the form of a stone bench. In our case we used recycled concrete blocks and used two of the chambers and the channel for the smoke to travel before leaving through the chimney.
There are a few drawbacks that you should be aware of before you rip out your old wood burner out though:
1. The wood needs to be very small (preferably split) and very dry. Off-cuts from carpentry workshops and (unpainted/untreated)pallets etc. are great, big, hard to split, knotty logs not so much (especially green or wet).
2. The simpler versions of rocket stove mass heaters require someone to mind the fire almost continuously due to the small amount of fuel you can put in at a time as well as a possibility of backdrafts in some cases (where the direction of the burn reverses and the smoke and flames come out of the stove instead of going in). This makes them less ideal for Airbnb's for instance or extremely cold places that need a large influx of fuel to get the place warmed up when you first come in.
Having said all that with time these things can be remedied in many cases; wood can be dried above the stove for the next burn, wood can be split, guests can be educated and loading chambers with doors can be added to the front end to eliminate both the risk of backdrafts and the need to babysit the fire.
So how do you get started? I cannot recommend enough the simple, concise and highly useful guide written by the inventors themselves called Rocket Stove Mass Heaters which are available from their site: www.rocketstoves.com. It was my guide and inspiration as I worked my way through a long series of trials and errors using both metal (stainless and galvanized) pipes and later refractory bricks and old tiles as well as old water heaters and oil drums among other recycled goodies. It has a series of case studies and applications at the end including ovens and saunas etc. As well as little tricks and adaptations that people have come up with along the way. I still use it as a reference now just to double-check key dimensions and proportions. Also, check out our workshop program ;-)
As far as lessons learned so far? Personally, my thinking is towards thermal floors instead of benches, a feature we have applied successfully a couple of times now and which uses less and cheaper materials as well as less space within the (tiny) house. Also, the dimensions of the drum need careful consideration. In hindsight, the standard 55-gallon oil drum in our place now should have been a smaller 30 gallon one, mostly because of how big it feels in the space (12m2).
In Howards, we did use part of a water heater around the 30-gallon size and it fits the space nicely even though its only 7m2. One improvement has been to mount the boiler just through the wall from the stove to reduce the heat loss on the way but make the most of the (tiny) space and keep cosy aesthetic.
I hope this is valuable information to you and that, if you have not already, you too will one day explore the thrill of rocket science in your own home!
Your fellow rocket scientist,