Just read "Wood Heat: Is it right for you?" in Mother Earth News, issue 236. No, it doesn't talk about rocket mass heaters, but briefly weighs different wood heat options. Cost-wise: it offers the website www.hearth.com to calculate fuel cost. It also suggests that you calculate in paying yourself minimum wage for time spent chopping the wood if you do it yourself. It also offers the EPA website: www.epa.gov/woodstoves/basic.html, and says stoves they certify have an average of 70% efficiency. (I think RMHs have 90%?) The article talks about different options, including wood boilers or central furnaces, although these are harder to find high efficiency. Pellet stoves could save money and hassle, but you can't make them yourself, and they require electricity to run their fuel auger, fans, and controls. (reliant on the grid) Lastly, the article says that one mistake not to make is running your chimney out the wall and up in order to leave the house. Running the chimney straight up through the roof of the house will minimize smoke coming back down and rolling into the house whenever you open the door.
It's quite common for efficiency ratings to be complete lies. To be true they would need to measure total energy in a given amount of wood as compared to total energy delivered to the home. But the rules concerning these tests allow them to make adjustments which skew the results. For instance there were many slowburn units sold in the 70s and 80s which were called 12 hour burners or 24-hour burners. They choked off the air and consumed fuel slowly. When wood is burned too slowly it doesn't completely combust. Some tests allowed manufacturers to simply express their efficiency as it related to the amount of heat produced. The test did not account for all of the smoke, creosote and carbon monoxide delivered to the neighborhood. These and other unburned constituents of poorly burned fuel represent lost energy. Had the rules required accounting for 100% of the potential energy the efficiency ratings would have been lower. Other tests allow manufacturers to make adjustments because the fuel is not burned in a 100% pure oxygen environment. Most importantly, the efficiency of regular woodstoves are hugely dependent on operator competence.
I haven't followed recent developments on this front but I have worked out a very accurate means of calculating rocket stove efficiency based on measuring the temperature rise of a known weight of cob within a rocket stove and burning a known weight of perfectly dry wood of known energy content.
Location: Victoria British Columbia-Canada
posted 8 years ago
Update. I discovered that some efficiency tests allow you to vent a certain amount of hot air up the chimney and still count that as a positive. The gist of it is that since the wood-burning unit requires draft it's somehow acceptable to ignore energy lost up the chimney while creating this draft. Of course this is total nonsense. Lost heat is lost heat and it doesn't matter how you spin it. Manufacturers should have absolutely no say in how efficiency tests are conducted because it leads to this sort of malarkey.
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