D11 Combustion Heating: Gas, Oil and Wood

Topic

A combustion boiler converts the chemical energy of a fuel into useful heat, but not all of that energy ends up as utilized heat; a fraction is lost—primarily through hot combustion gases escaping up the chimney—and boiler efficiency is defined precisely as the ratio of utilized energy to the energy lost via this route.

Condensing boilers achieve particularly high efficiency rates because they recover some of the heat that would otherwise be lost in the water vapor contained within the combustion gases—heat that a conventional boiler simply allows to escape unused up the chimney. Different fuels release vastly different amounts of energy per unit of mass or volume and emit varying quantities of carbon dioxide per unit of energy released; firewood sourced from sustainably managed forests is considered to have near-zero net emissions because the carbon released during combustion is essentially the same carbon the tree captured while growing, although burning it does generate particulates that require careful combustion to avoid compromising indoor air quality—an issue already addressed in the ventilation module.

Unless mechanically forced, the evacuation of these combustion gases occurs via natural draft: hot gases are less dense than cold outside air and tend to rise on their own. The strength of the draft increases with a greater temperature difference between the gases and the outside air and with a taller chimney—the same principle of rising hot air seen in the "chimney effect" described elsewhere in this catalog.

Thus, this module—which begins with the conversion of chemical energy into heat accompanied by inevitable losses—concludes by demonstrating that a combustion system's efficiency largely depends on recovering heat that would otherwise escape unused through the very chimney used to vent the gases.

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