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wood burning stove close up

Fall is in the chilly morning air. As I write this, I am waiting for someone to come check my radiant gas heaters before winter sets in. I am buying firewood, cleaning the chimney, and getting a check-up for the solar hot water system. I really like my utilities to work in the cold weather!

While all this goes on, I wonder how I can make my home even more efficient. I love the radiant heat of the cast iron heaters. With just the pilot light on, they throw out enough heat to warm a room. The thermal mass in the solar greenhouse absorbs heat from the low winter sun and the ambient heat of the room during the day. When the temperature drops, the heat radiates back into the space to keep a comfortable equilibrium.

Heat up mass instead of air

Radiant heat is even and predictable. It’s what thermal mass is all about, whether it’s a wall of adobe, concrete or water, a concrete floor, a pile of rocks, or even a city sidewalk! It’s part of what makes a rocket mass heater so efficient.

Rocket heaters are designed to burn small pieces of wood hot and fast. The heating chamber and the piping are heavily insulated to retain that heat. The heat rises and enters another chamber that is covered with a mixture of mud and straw (cob). The cob is the thermal mass that collects and stores the heat that radiates out slowly for comfortable, passive heating.

The exhaust then enters tubing or ductwork inside a cob bench or bed. This large mass absorbs more heat for even radiation later. The final exhaust and end product of the fire is water vapor that is expelled out of a chimney.

Instead of fire and smoke going up a traditional chimney, the heat from them is collected in thermal mass, and virtually nothing is exhausted to pollute the air. That’s the simple version.

Clean and efficient use of fuel

Rocket heaters use very little fuel. Small pieces of wood burn faster and hotter, so scraps and tree trimmings can create a lot of heat.

Think about a campfire – you always start with small pieces to get it going, and then you add the larger pieces to sustain the flame. Heating with a conventional woodstove is the same way. Rocket heaters just need the kindling to work effectively. Consequently, less wood is burned.

Because this stove burns so hot, polluting and toxic gasses are burned off before they reach the outside. If you think catalytic converters on a woodstove keep down pollutants, these burn much cleaner!

DIY or buy?

Many rocket heaters are homemade from simple, inexpensive materials. A 55-gallon drum, some masonry materials and piping can be found easily. If you have masonry skills, or know someone who does, that’s helpful. If not, there are tons of videos and websites for instruction. You can even take a workshop where you build one. There is nothing like a hands-on education!

Design is subjective, too. You have the option to integrate a cooking surface on top of the combustion chamber. A cob seating area/thermal mass can be as decorative or plain as you choose. You don’t need cob for it, either. Choose masonry, brick or tile. It can be as utilitarian or artsy as you want.

If you’re not handy, you can buy a rocket mass heater. I found several companies online.

Precautions

Before you dive into this project, read, read, read, and ask a lot of questions. You may need a building permit, but you may not be able to get one for a ‘rocket mass heater’. Check with your insurance company to make sure your insurance won’t be cancelled with this type of heat.

Build a prototype outdoors before putting one permanently into your home. Find out how to troubleshoot issues, like smoke coming back into the house.

Do your homework! Rocket mass heaters seem like a wonderful way to reduce air pollution and get the most heat out of your wood. I don’t see any reason to not install one!

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Nan Fischer

Nan Fischer has been living and building green for over 35 years. Nan’s emphasis on the BuildDirect blog is about how to make your dollar stretch further, while also moving toward a more sustainable lifestyle, as well as upcoming and existing technology to help us live in an ecologically-friendly way. Nan also authors posts on the website of her seed business, sweetly seeds.