Rocket Mass Heaters

We intend to go entirely off-grid. I can’t say exactly when we’ll hit that goal, or what the final system will look like quite yet. Redundancy is important to us so that if a component fails or is down for maintenance it’s not an entirely dire situation. So is efficiency. Ideally, we want the greatest return for the least input- particularly where fuel is concerned. We’re confident that the rocket mass heater we’re building to heat our main living space meets this criteria.

A rocket mass heater burns (wood) fuel extremely efficiently. Seriously efficiently- which is important to us for several reasons not the least of which is that we live on the prairie. There are lots of systems that make this claim but I have never seen anything like the rocket stove. When we were in Oregon, one of our projects was a rocket water heater. When we fired up the system to test it we were encouraged to go around to the exterior of the building and put our faces close to the exhaust. (I know, I know- apparently I’ll do anything for a lentil burger.) I was utterly amazed when instead of a face full of smoke, I was greeted by a cool steam. The wood was burning so efficiently, and the system using all of the heat, that there was nothing but a light steam by the time it reached the outdoors. This is the promise of a rocket mass heater.

That’s the other very important feature of the rocket mass heater- efficient use of heat. When you think of a traditional forced air system, or even a regular wood burning stove, it’s clear to see that a lot of heat is wasted. The mass heater, on the other hand, stores the heat for slow release in easy to build, dirt cheap furniture. Excuse the bad pun. The heat travels down a course built into the furniture (in our case, a combination couch/day bed) transferring its heat to the cob that encases it and slowly releasing it. Essentially, the chimney is inside of the furniture exiting only when it has had a sufficient run to use up the heat. For example, a standard 6” system would have a run of about 30’ and be enclosed in about 3 tons of material.

When you hear talk of rocket mass heaters, you’ll hear frequent mention of the combustion unit and the thermal battery. The combustion unit is roughly J-shaped, and is made up of the feed tube, the burn tunnel, and the heat riser. Measured precisely, and with good mortar and insulation, the combustion unit burns fuel very efficiently and draws well. The thermal battery, as mentioned, is in the furniture itself (or the floor, though it’s not a recommended first time project) in which the heat is stored.

It’s important not just to get the measurement of the combustion unit right, but to carefully consider the run. You don’t want to place the pipes too low into the furniture (or maybe you do, for a much slower bleed and perhaps warming the floor in front of the bench) but you don’t want to go to high and risk a burning hot surface. About 6” from the top of your bench is recommended for a nicely toasted bum. Your initial run will be the warmest, with every length doubled back running slightly cooler- assuming that you’re doubling your run (we may be tripling ours).

You’ll also want to insulate underneath the combustion unit and behind your bench (couch, daybed…) if it sits against a wall. No sense bleeding good heat into the wall. Vermiculite (industrial, not agricultural), perlite, or pumice are all good materials. Use your imagination but avoid anything that may off gas, like foams.

For all of this you’ll need only the most basic materials. Bricks (which you can reuse or make), clay soil, sand, straw, urbanite or rocks (for heat storage) and a couple of barrels and stove pipe, or metal duct. The size of your barrels will vary between 15 gallons for the feed tube, and 30-55 gallon for the heat exchanger- depending on your specific design. Most, if not all, of your supplies can be found and reused.

If I’ve made it sound complicated- it isn’t, but there is some patience and tinkering required. You can’t (or rather shouldn’t) just throw your system together and encase it in cob. You build the system, and fire it. Check it for leaks, or a weak draw, or any other issues, rework your design and fire it again until you have a leak-free system that draws well and burns efficiently. You’ll know it draws well because it actually sounds like a rocket. I’m sure I’ll eventually tire of the sound when it first catches but I’m still just amazed every time that I hear one start. You’ll also want to decide how much radiant heat you want as that will decide how much of the barrel to leave exposed. Myself, I’d like to boil water for tea on top of the heater, so I’ll need to position the heat riser to within 2” of the barrel to create a hot spot.

Once your design is perfected, you’re ready to encase the entire thing in cob and wait for your new furniture to set. (It’s a good idea to test it once again, before your furniture is rock hard.) If you’re a patient person, you’ll have a mass heater that will last you for years. If you try and rush this, you will regret it- it’s not as easy as throwing out an old couch.

Now, having said that it’s efficient and simple to build- I’m not going to go through the entire process on this blog, nor could I without writing a book-length piece. Pick up the actual book- “Rocket Mass Heaters” by Ianto Evans and Leslie Jackson- it’s well worth it. You can buy it in book stores but for the (small) profit to go directly to the authors, you can order it at http://www.cobcottage.com/products . The book will walk you through the main components of the system, layout, calculations and exact measurements. And, apart from the fact that you won’t be able to build a mass heater based on my brief overview, the book also provides alternative models and adaptations, including a rocket hot tub, a Guatemalan cook stove, and an ingenious little coffee rocket.

As a prelude to building a mass heater, try your hand at a pocket rocket- a very simple rocket stove. It is really easy to build and throws an amazing amount of heat, but most importantly it will get you started, build your confidence and have you well on your way of being as hooked on these wonderful heating systems as we are! I’ll provide full details and pictures so that you can build your own pocket rocket in my next blog.

The combustion unit of the water heater

The combustion unit of a mass heater

Note the clean-out

Super cozy rocket mass heater, Cob Cottage, OR