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Mary MacKay
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Interest heats up in super-efficient, wood-fired rocket stoves

Erica Wisner guides a group of interested Prince Edward Islanders through the step-by-step process of making a rocket mass heater from scratch at Island Forest Foods in Brookvale, P.E.I.

CHARLOTTETTOWN, P.E.I. - A group of Prince Edward Islanders was recently all fired up to learn more about rocket stoves. The building of these super-efficient, wood-fired heaters from scratch was the focus of the Pyromania 101 workshop at Island Forest Foods' in Brookvale.

"It is rocket science, just a different sort of rocket science. We believe in simplifying things down to their (basic level)," says Ernie Wisner.

He and his wife, Erica Wisner, are the leading innovators in rocket stove technology and have built more than 700 rocket stoves worldwide.

"They're called rocket stoves because of the sound they make, because when they're burning they roar," Ernie says.

"It's this low-level rumble. It's sort of like having a cat, a purring that goes on. You get really used to it, and it gets really comforting."

The technology for this particular rocket mass heater was developed about 30 years ago by Ianto Evans of Cob Cottage Co. in Oregon, whose design came from his work in creating super-efficient cook stoves to be used in developing countries.

Evans tailored that technology to work as an innovative and efficient space-heating system.

Ernie apprenticed with Evans for two years, learning various building techniques, such as straw bale and cob.

"(Erica and I) started out with the grand plan that we were going to teach a wide spectrum of appropriate technology, and what it actually has turned into is everybody wants us to teach these - the rocket mass heaters and the (custom cob oven rocket stoves), not solar hot water heaters," he says.

"They burn a fraction of the wood. They really are super efficient. ... We built one in our house in Portland that we were renting, with the landlord's permission. We lived with that for four years and ran it on all of the sticks that fell down on 1.7 acres. We never bought wood."

The Wisners, who are now based in Washington state, are on a cross-continent tour sharing the science of these rocket mass heaters.

"These were designed to be built with all natural and recycled materials," Erica explains.

"If I spent six months scrounging I could probably make one of these for probably about $20 and that would be for the perlite. The ingredient list is bricks, insulation, one or two 55-gallon drums, clay, sand, straw, ducting. So that's pretty much it. It's not really that complicated," Ernie adds.

A super-hot chimney right above the fire draws not only smoke but flame up quickly, blending hot fuel and air into a quick, hot, clean-burning fire that takes little wood and leaves little residue.

The magic of a rocket stove starts with the combustion unit. Wood feeds in downward, a small batch at a time. The fire burns sideways and then up inside a super-insulated burn chamber.

"The burning sideways issue is pretty simple to explain. It's because it's a thermal siphon," Erica says.

"It's marvelous how it works, but there's good reason for it. It gives a much cleaner burn, making the fire go around corners and keeping it at temperature while it does that. You get time, temperature and turbulence that gives you a very clean fire."

The rocket mass heater channels the smokeless exhaust through a massive earthen masonry heat exchanger before it is vented to the outdoors.

The thermal masonry mass acts as a heat battery, slowly releasing warmth over time.

"The heat moves into that mass at one inch an hour ... and that penetration keeps on going. So it basically spreads out at one inch an hour to the surface of the bench. If you do that five or 10 nights in a row, pretty soon the whole mass is warm," Ernie says.

"Now it's kicking off that heat 24 hours a day, so you're running your stove less. It also means you're not going into that (heat-and-cold) spike cycle that you get with a woodstove."

Building codes vary from county to county in every state and province in Canada, so that can present challenges.

The code the Wisners helped write for Portland, Oregon, has been approved.

They are now working toward building a stove at a certified Environmental Protection Agency testing facility in the hopes of getting an Underwriters Laboratory (UL) listing, which would help with insurance issues.

In a new build, a rocket stove can be incorporated into the structure, but for existing homes the masonry mass typically becomes part of the furniture in the form or a bed or bench.

There are benefits to the horizontal nature of the thermal mass, which is specific to this type of rocket stove, one being that it is a lesser weight per square foot than a vertical mass would be. For example, a bench ends up being about 18 inches high and approximately 150 pounds per square foot or less.

"The other thing you get, which is a huge part of why they are more comfortable and efficient, is you've got a full body heating pad. You can actually sit or lie on the bench and get luxuriously warm," Erica says.

"It's not going to fit where your woodstove was. You're going to have to go through this design process to figure out how you can make it an asset and a welcome part of the house, rather than something you wish you'd thought through before you stuck it in the middle of the floor.

"Quite often it's the thing that makes the room."

— The Canadian Press

Organizations: Forest Foods, Cob Cottage Co., Environmental Protection Agency

Geographic location: P.E.I., Prince Edward Islanders, Portland, Oregon Brookvale Portland Washington Canada

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