A More Efficient Woodstove

Efficiency---still the goose that lays the golden egg...

A new woodstove in the Bruhl house. Efficiency—still the goose that lays the golden egg…

Well, our trusty 1972 Vermont Castings Defiant woodstove, a hand-me-down from a close relative, finally had to be retired. It had developed enough air leaks that it was easy to overfire, and each time it got too hot it cracked or warped a little bit more, to the point where it was becoming difficult to keep it in check and was becoming a slight safety hazard. So, we went shopping for a new one, and settled on a large Dutchwest model (Dutchwest is now owned by Vermont Castings), the 2479, a non-catalytic stove that meets the strictest EPA emissions standards.

And, since I feel that efficiency is one large key to our sustainable future,  I’m writing this post because I’m amazed at the improved efficiency of this new stove. I didn’t quite believe that the old Defiant could be topped— I burn well-seasoned hardwood, and we burn hot, clean fires. But, whereas older stoves might have been 40-50% efficient, the new ones with carefully designed secondary burn chambers are 80% or more efficient, and the result is much more heat from the wood that is burned. The difference is noticeable—the new stove brings the house up to temperature far faster than the Defiant ever did, AND does it with less wood. Likewise, there are improvements in terms of air quality, older stoves typically release 30-40 grams of particulates an hour, but the new one, drum roll— is certified at 1.31 grams/hour.  Once it’s hot and in its secondary burn mode, when I step outside and look at the top of the flue, there is zero smoke. From outside you wouldn’t even be able to tell that a stove was burning, other than a few tiny heat shimmers coming out of the top. That’s efficiency.

The original Vermont Castings operation manual, which was also handed down with the stove when we got it.

The original Vermont Castings operation manual, which was also handed down with the stove when we got it.

So I did some reading about stove design, to figure out where this efficiency comes from. In this non-catalytic stove, the firebox is designed to tolerate a very hot fire, and is lined with refractory material to increase firebox temperatures. Fires are started just as with any stove, but once there is a bed of coals in the bottom you close the damper, and then gasses and smoke from the burning wood circle about and exit behind the coals, through an opening about six inches wide, low in the ceramic backplate. The throat of this opening has ten or so small holes that allow secondary air to be sucked in and mixed with the exhaust gasses and smoke, which are about 1100 degrees (Fahrenheit) after passing through the bed of coals, and the new mixture ignites or reignites and continues to burn in a separate chamber, behind and above the firebox, where temperatures can reach 1400 degrees. Apparently about 40 percent of the potential energy in wood is contained in these gasses, and without a secondary air system, most of them just go up the flue and are lost. And, the new stove seems to work exactly as advertised—when it’s in this mode you can hear it, a low flame roar emanating from the back, even as the visible fire in the primary firebox looks somewhat lazy.

High efficiency catalytic models are similar, except that they use a catalyst positioned across the exhaust stream which reignites the gasses at temperatures as low as 600 degrees. This enables such stoves to keep fires burning longer when they are turned down. The catalytic stoves require a bit more maintenance, though, and I think a non-catalytic model is the best choice for us; the normal pattern in our house is to make a larger fire to heat the house up when we get home after work, and then, since our house is fairly well-insulated, we don’t necessarily need a fire burning all night. In the morning it’s a few degrees cooler, but we’re usually on our way out the door, and we just wait until we get home again to restart it.

A video demo of how older stoves compare to new ones. I think this “old stove” wouldn’t be smoking quite this much if its door was closed, but the point remains—

Anyway, this efficiency matters. Efficiency, as I’ve said for years (one example here), is still the goose that lays the golden egg. Efficiency will be part of what saves us from ourselves. Efficiency is the opposite of wastefulness, and therefore a form of conservation. In terms of home heating, in our situation here we have something close to a free lunch. I cut wood sustainably from the property (with my  “solar powered” chainsaw, which I still think is the greatest thing since sliced bread). When biomass is harvested sustainably and then burned, there are no net CO2 emissions to the system, so heating our house is essentially carbon-neutral. And, it appears that this stove might require a third or more less fuel on average, which might save me many hours of labor each year. (I enjoy working in the woods, though, so I’m not sure this is a step in the right direction for me!)

So, more efficient, less fuel used, nearly zero particulate emissions, carbon-free heating. One more tiny improvement in our systems.

Top image credit: author.

2 thoughts on “A More Efficient Woodstove

  1. Peter Carothers

    TB, During your research did you find an efficiency rating for the stove we have been using all winter – the Hearthstone made by Heritage. It is cladded with soapstone on all four sides plus the top. We are very happy with it and, like you, note that the house is quite warm in the morning. PC

    1. Taborri Post author

      Hey Peter— I didn’t see anything about that stove in particular, but there is sure to be material online. But if you’re happy with it, then you should be set!
      -t

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