Linda used my house for the example in a recent introductory webinar that I attended. It so happens that I heat substantially with wood, and the efficiency calculation for the 1000 Home Challenge involves the amount of wood burned in units of cords.
As is so often the case after I listen to Linda, my creaky old brain kicked it up a few RPMs and I began to wonder which is it better:
1) to burn more wood hotter to achieve more thorough combustion and reduced particulate emissions?
2) to damp the fire down to burn less wood but create more particulates and creosote/soot buildup in the chimney?
The answer is far from self-evident, and it probably doesn't really matter all that much, but it certainly reminded me yet again of the complexity we face when trying to create a metric to measure progress in building performance of any kind.
Leaving the math aside, it would seem to me that one would only want to generate the heat in quantities that are usable, since you really can't store heat energy without substantial remodeling for thermal mass storage, such as heating water for later space heat usage. So, damping down the fire to produce a reasonable amount of heat would seem the more reasonable practice, unless you are in fact heating water with the woodfire.
It gets more complicated. This is a very small stove (Lopi Answer is the model), it has a blower, and it essentially heats a 2000 ft² house. I don't believe any of the heat is wasted beyond whatever goes up the chimney. It's a new take on "right sizing"
Thanks for introducing yet another variable. Perhaps we can keep the list growing!
I understand the point you are making - Dampening down a stove can mean that combustion is incomplete and therefore emissions could be far worse than burning the fire hot.
Two of the case studies that we have on our website (www.thousandhomechallenge.org) use wood for heating. One officially meets the Thousand Home Challenge, the other will as soon as the application is complete. Both of these projects have been able to avoid the conundrum you pose: Gordy Scale, in Ontario, has a masonry heater. He burns it hot for a brief amount of time once every day, two, or three - whenever needed. The stove stores the heat and releases it over a long period of time. Operated in this way, masonry heaters are clean burning. Not an inexpensive heat source, but probably one of the most aesthetically pleasing heating systems a person could choose. Check out the case study for additional benefits. Same type of operation with the other project, John Livermore's super insulated home in Massachusetts. When the house cools down, John lights a very small fire, burning it hot. His house often gets all the heat he needs from the sun, and the house is pretty tight, and the windows are R 7, so it drifts down slowly.
I have recently heard about some miniature wood stoves that are appropriate for homes with a design heat loss of 10,000 Btus/hour. I haven't checked them out, but they sound like they fill a need. In many homes, even ones that are not well insulated, regulating the heat output from a wood stove is a challenge.
Another option is the solution Marc Rosenbaum proposes - nesting a wood stove with a ductless heat pump. The minisplit carries the load in the swing season and much of the winter, the wood stove is on reserve and available when you need more heat, thus reducing the amount of time when the wood stove is operated in a throttled mode. I realize there can be source energy considerations that would cause one to lean towards wood heat over electric heat, but wood, burned inefficiently is not a particularly good option.
I agree with you Ed. Hopefully the Thousand Home Challenge will challenge many to find options that do not pit energy reductions against environmental quality. While energy reductions may be easier to measure than many indices of environmental quality, and energy use/reduction is the measurable basis for meeting the THC, it is not sufficient in itself, and it is only part of the story of successful deep energy reductions.
Skiip Hayden will be presenting at the upcoming ACI 11 conference in San Francisco (select Hayden from the presenter list http://acinational.org/presenter-list). He is an advocate of heating with wood efficiently and cleanly. He has worked with manufacturers on creative solutions.