# Wood-Burning Furnace: How Does Smoke Go Out but Warm Air Stay In? **URL:** \ **Category:** Factual Questions\ **Created:** [December 27, 2007, 2:28am UTC](https://boards.straightdope.com/t/wood-burning-furnace-how-does-smoke-go-out-but-warm-air-stay-in/431349 "2007-12-27T02:28:27Z")\ **Posts on this page:** 12\ **Page:** 1
**Author:** ![HeyHomie](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/heyhomie/32/207_2.png) [@HeyHomie](https://boards.straightdope.com/u/HeyHomie)\ **Post date:** [December 27, 2007, 2:28am UTC](https://boards.straightdope.com/t/wood-burning-furnace-how-does-smoke-go-out-but-warm-air-stay-in/431349/1 "2007-12-27T02:28:27Z")
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? ---
**Author:** ![JimOfAllTrades](https://avatars.discourse-cdn.com/v4/letter/j/ba8739/32.png) [@JimOfAllTrades](https://boards.straightdope.com/u/JimOfAllTrades)\ **Post date:** [December 27, 2007, 2:40am UTC](https://boards.straightdope.com/t/wood-burning-furnace-how-does-smoke-go-out-but-warm-air-stay-in/431349/2 "2007-12-27T02:40:23Z")
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. Check this link: > **[Heat exchanger](https://en.wikipedia.org/wiki/Heat_exchanger)** > > A heat exchanger is a system used to transfer heat between a source and a working fluid. Heat exchangers are used in both cooling and heating processes. The fluids may be separated by a solid wall to prevent mixing or they may be in direct contact. They are widely used in space heating, refrigeration, air conditioning, power stations, chemical plants, petrochemical plants, petroleum refineries, natural-gas processing, and sewage treatment. The classic example of a heat exchanger is found in There... for some more in-depth information. ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [December 27, 2007, 3:38am UTC](https://boards.straightdope.com/t/wood-burning-furnace-how-does-smoke-go-out-but-warm-air-stay-in/431349/3 "2007-12-27T03:38:48Z")
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.](http://en.wikipedia.org/wiki/Furnace) ---
**Author:** ![raindog](https://avatars.discourse-cdn.com/v4/letter/r/2bfe46/32.png) [@raindog](https://boards.straightdope.com/u/raindog)\ **Post date:** [December 27, 2007, 4:48am UTC](https://boards.straightdope.com/t/wood-burning-furnace-how-does-smoke-go-out-but-warm-air-stay-in/431349/4 "2007-12-27T04:48:19Z")
[QUOTE=Fear Itself] 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.](http://en.wikipedia.org/wiki/Furnace) [/QUOTE] 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. ---
**Author:** ![lissener](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lissener/32/6139_2.png) [@lissener](https://boards.straightdope.com/u/lissener)\ **Post date:** [December 27, 2007, 6:38am UTC](https://boards.straightdope.com/t/wood-burning-furnace-how-does-smoke-go-out-but-warm-air-stay-in/431349/5 "2007-12-27T06:38:28Z")
Also keep in mind that much of the heat is radiant heat, not heated air. ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [December 27, 2007, 2:17pm UTC](https://boards.straightdope.com/t/wood-burning-furnace-how-does-smoke-go-out-but-warm-air-stay-in/431349/6 "2007-12-27T14:17:11Z")
My last wood stove used outside are for combustion. Most good wood stoves do this. ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [December 27, 2007, 2:35pm UTC](https://boards.straightdope.com/t/wood-burning-furnace-how-does-smoke-go-out-but-warm-air-stay-in/431349/7 "2007-12-27T14:35:48Z")
[QUOTE=enipla] My last wood stove used outside are for combustion. Most good wood stoves do this. [/QUOTE] 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. ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [December 27, 2007, 3:03pm UTC](https://boards.straightdope.com/t/wood-burning-furnace-how-does-smoke-go-out-but-warm-air-stay-in/431349/8 "2007-12-27T15:03:28Z")
[QUOTE=the raindog] 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. [/QUOTE] 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? ---
**Author:** ![BJMoose](https://avatars.discourse-cdn.com/v4/letter/b/5fc32e/32.png) [@BJMoose](https://boards.straightdope.com/u/BJMoose)\ **Post date:** [December 27, 2007, 3:13pm UTC](https://boards.straightdope.com/t/wood-burning-furnace-how-does-smoke-go-out-but-warm-air-stay-in/431349/9 "2007-12-27T15:13:10Z")
[QUOTE=Una Persson] 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? [/QUOTE] I _suspect_ he’s talking about gas-fired furnaces, not wood-burning stoves. ---
**Author:** ![Una\_Persson](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/una_persson/32/346_2.png) [@Una\_Persson](https://boards.straightdope.com/u/Una_Persson)\ **Post date:** [December 27, 2007, 3:22pm UTC](https://boards.straightdope.com/t/wood-burning-furnace-how-does-smoke-go-out-but-warm-air-stay-in/431349/10 "2007-12-27T15:22:22Z")
[QUOTE=BJMoose] I _suspect_ he’s talking about gas-fired furnaces, not wood-burning stoves. [/QUOTE] 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. ---
**Author:** ![enipla](https://avatars.discourse-cdn.com/v4/letter/e/54ee81/32.png) [@enipla](https://boards.straightdope.com/u/enipla)\ **Post date:** [December 27, 2007, 3:27pm UTC](https://boards.straightdope.com/t/wood-burning-furnace-how-does-smoke-go-out-but-warm-air-stay-in/431349/11 "2007-12-27T15:27:43Z")
[QUOTE=Fear Itself] 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. [/QUOTE] Sure, but at least it isn’t sucking the warm air out of the room. ---
**Author:** ![Fear\_Itself](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/fear_itself/32/19637_2.png) [@Fear\_Itself](https://boards.straightdope.com/u/Fear_Itself)\ **Post date:** [December 27, 2007, 6:52pm UTC](https://boards.straightdope.com/t/wood-burning-furnace-how-does-smoke-go-out-but-warm-air-stay-in/431349/12 "2007-12-27T18:52:39Z")
[QUOTE=enipla] Sure, but at least it isn’t sucking the warm air out of the room. [/QUOTE] True, that is an important efficiency aspect of wood stoves, but that is not what the OP was asking about.