Focus on energy Q&A

Some brands of wood boilers that I have looked at talk about high temperature “gassification” heating that supposedly is more efficient and produces less creosote. I’m a little unclear if this is just some advertising hype or an actual benefit. Of course, these same systems are also quite a bit more expensive than other wood-fired boilers and at least one system encourages users to purchase a heat storage tank that apparently heats the water up, stores it and then pulls the water when there is a demand, which seems good, but once again I’m not sure if this is absolutely necessary, a nice perk or advertising hype. Any insight into these indoor wood boilers would be much appreciated. – Jason Johnson, Unity

Wood does not burn. As wood is heated, gases are formed. The gases burn, not the wood. These gases require mixing with air and sustained temperatures of about 1,000 degrees to complete the combustion process. In a conventional stove, neither the mixing process nor the attainment of high temperature is achieved; hence much unburned material enters the chimney. Some of these gases condense on the chimney wall, forming creosote. Some enter the atmosphere as air-polluting smoke. Modern Environmental Protection Agency-approved stoves have combustion chamber geometries, refractory lined flame paths, and sources of secondary combustion air that tend to solve the unburned material problem. The sources of secondary combustion air are not under control of the stove operator. The output of the stove is therefore controlled by the time interval between fuel loadings. I have said many times that every wood-stove household needs a father-in-law who thinks he is Humphrey Bogart operating the boiler on the African Queen. Any wood-burning device with a large fire box and a control mechanism that limits airflow will be inefficient and polluting. That is the role of the water storage system with some units. Water storage eliminates the need to limit the air supply. Advice to new-home builders: Many homes now in use are more than 100 years old. We assume that the home you intend to build will be useable for at least the next 100 years. Very soon – perhaps in a decade or two – petroleum based fuel will be pre-empted by the transportation sector and not available for residential space heat. Plan accordingly.

Answer provided by Dick Hill, retired emeritus professor of engineering at the University of Maine.