Better Wood Heat: Rocket Mass Heater FAQs Answered

Reader Contribution by Destiny Hagest and Permies.Com
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What Is a Rocket Mass Heater?

First, let me just start off by saying that as a full-time resident of a very rural community in hardiness Zone 3b, heat is a priority for me — a big one. However, as a working mother of an adorable tornado of a toddler, time is limited, and I just don’t have all summer and fall to help my husband gather and split wood anymore.

Simply put: I love rocket mass heaters. I love the idea of splitting less wood. Of doing less work. There, I said it.

Rocket mass heaters are essentially a wood-burning stove, but with a few interesting adjustments that make it crazy efficient. These heaters can burn up to 90% less wood (seriously, these things runs off of twigs). And thanks to the addition of thermal mass, they’re wonderfully effective at not only heating a space, but keeping it warm long after the fire goes out.

How Do Rocket Mass Heaters Work?

Rocket mass heaters burn a little differently that a typical wood stove, and by that I mean they burn sideways. Yep. Sideways fire. It’s pretty cool.

The rocket stove itself is essential a burn chamber that extends horizontally into a vertical stack inside of a large insulated barrel, which we call the core of the rocket stove. This core is super insulated, which creates a strong draft to pull the flame back into the core, and keep smoke from backing up into your house.

As that smoke is pulled into the barrel, it’s rapidly cooled, heating the barrel itself, which can be used as a cooking surface at that point. Now, here’s where the magic happens.

From the barrel, rather than just going up and out a chimney, the smoke weaves and winds its way through a circuitous route of exhaust piping, typically wound through a thermal mass.

The idea is that, most often in a conventional wood burning stove, most of the heat of the fire is actually lost in the exhaust, and just goes right up and out of your house — what a waste!

With this design, every iota of heat is collected and transferred to the living space, and as a result, what eventually comes out of your chimney is incredibly clean smoke — almost steam, so there are very few pollutants that wind up being emitted as a result of trying to stay warm.

What’s the Difference Between a Rocket Mass Heater and a Rocket Stove?

If you’re new to this idea, I want to make sure I cover this topic top to bottom. There are a lot of exhaustive books on the design and function of rocket mass heaters, but let’s get back to basics here.

Firstly, what is the difference between rocket mass heaters and rocket stoves?

Simply put, they’re essentially the same thing, one just has an extra component. The rocket stove itself is the wood stove with the sideways-burning chamber and insulated barrel.

What makes a rocket stove a rocket mass heater is simply just the addition of thermal mass — typically a hand-sculpted cob bench — built around the exhaust piping.

What is Thermal Mass?

A big part of what makes a rocket mass heater so incredible is its integration with a thermal mass. The result isn’t just wildly functional and efficient – in many cases, it’s downright beautiful.

A thermal mass is essentially any dense material that has the potential to hold onto heat energy because of its mass. Think about when you touch a stone in a fire pit after the fire’s gone out — it’s still warm. That’s because even after the fire goes out, that stone is still holding onto and slowly releasing the heat energy of the fire.

The idea behind using the right thermal mass with a rocket mass heater is that your heat doesn’t just blast at you all at once — it’s stored up gradually in that mass, and is then gradually released back into the surrounding cooler air, adjust temperature gradually, and holding indoor climate at a stable level for longer.

Like I said before, many people choose to build a beautiful cob bench or bed around their rocket stove exhaust, as in the picture below, but there are so many possibilities with a design like this, and thanks to its relative newness, there are so many opportunities to innovate.

Maybe you need to heat a fish pond for an aquaponics system? Maybe you’re trying to grow lemons in Montana and need an auxiliary heat source? Rocket mass heaters can do it, and for a fraction of the wood expenditures that you would normally see with a conventional wood stove.

Can I Build a Rocket Mass Heater Myself?

Yes! This is the really cool part — even though it’s rocket stoves we’re talking about here, it’s really not rocket science to put one together.

Now, that being said, I’m a mom, so let me mom you for a bit with this warning: when putting things that you light on fire in your house, do your research. There are plenty of resources out there available for this type of project, with experts to give you all the information you need to create a safe and effective rocket mass heater in your own home. Don’t be sloppy or haphazard — become a student of the rocket stove if this is something you want to build.

There, lecture over. Now the cool part.

The thing is, rocket mass heaters are still relatively unheard of where mainstream heating is concerned, so there’s a lot of room for fresh professionals to rise to the call and continue to innovate alongside industry experts like Ernie and Erica Wisner and Paul Wheaton.

Not only can you build one yourself, but if you’re really ambitious (and really good at it), you might even be able to make a career out of it. Dare to dream, right?

Where Can I Learn How to Build a Rocket Mass Heater?

Okay, now for a list of all of those handy-dandy learning resources:

Better Wood Heat: DIY Rocket Mass Heaters (4 DVD Set)

If you’re a visual learner and need to see one of these in action, this DVD set from Paul Wheaton over is a good way to get a feel for how these things work are put together. The set covers two different styles — cob and pebble — and also features a DVD all about the core itself.

Rocket Mass Heater Builder’s Guide by Ernie and Erica Wisner

This is a brand new book put out by touted experts and MOTHER EARTH NEWS FAIR speakers Ernie and Erica Wisner, and is full of design information, as well as pro tips from the duo to help ensure a greater chance of success for you.



Destiny Hagestis personal assistant to Paul Wheaton, founder of and, as well as a content curator and freelance writer. You can catch Destiny hanging out in the forums at quite regularly, and visit herLinkedIn profile, and follow her on Twitter.

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