Mrs. Homie’s mother’s house is heated by a wood-burning furnace. This is not a fireplace or a wood-burning stove. This is a furnace, in the basement, in which wood is burnt to provide warmth for the house (in the sense that the same furnace in another house may use natural gas, oil, an electric coil, or whatever).
How is it that the smoke generated from the burning logs goes up the chimney, but the warm air generated from the burning logs goes into the ducts, through the house, and into the various rooms to provide warmth and comfort?
Probably a heat exchanger, just like a furnace burning any other fuel. Basically two chambers that share walls so the hot exhaust gas in one chamber heats the clean air in the other chamber by conduction through the shared surfaces. The exhaust gas (and smoke) never mixes with the air that circulates through the home.
Most conventional combustion furnaces (oil, gas, wood) send a large amount of heat up the flue. They are inherently inefficient, losing as much as 40% of the availble heat up the chimney.
Actually, most modern furnaces send only about 20% of the available heat up the flue, and there are many [common] furnaces that would send as little as 4% up the flue.
Yes, but you are still losing a large amount of heat energy in the flue gases. A heat exchanger helps, but if flue gases are cooled too much, highly corrosive condensates can form in the flue and corrode it.
Which models are sending only 4% of their heat up the flue? Wouldn’t the latent heat losses alone in dried firewood be nearly 3-4% of flue gas loss by themselves?
OK…given that the high hydrogen content in natural gas, resulting from combustion of hydrogen, results in a latent heat loss of perhaps 4-5% or more, how can a furnace get to 4% flue gas loss? You’d have to have a stack temperature of ambient (or less), which I don’t think anyone, anywhere is able to do.
If the fuel is oil, coal, or biomass my question will be the same, only the magnitude of the latent heat loss numbers will vary.