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I've heard that wrapping my wood stove's chimney - the parts inside the house that go from the stove the the actual chimney - with copper pipes will increase the amount of heat generated.

Has anyone ever tried this? What diameter copper would work best for a typical 7" stove pipe? And how long should the copper be, given a 3' length of stove pipe?

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You just need a more efficient stove. Played around with it once. You just create a really bad pine tar still. The creosote and soot that gather on the inside of the pipe due to the cooling cause even worse heat transfer and a fire hazard. –  Fiasco Labs Dec 24 '12 at 18:09
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4 Answers

Not much. Copper is a good conductor of heat, but it wouldn't help much unless you made the actual stove pipe out of copper. Increasing the surface area of the stove pipe would, though, so if you want to solder copper fins onto it, that would help, and there's no need to worry about galvanic corrosion since the stove pipe is stainless steel.

That might end up being sorta ugly. If you really want to get a lot of heat out of this thing, look into getting (or making) a heat exchanger.

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The theory I heard was that the copper adds a lot of surface area that can radiate heat. –  chris Dec 19 '10 at 3:28
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Copper, while a good conductor of heat, is not as good as air. If you are talking about a mostly flat sheet of copper, you are talking about an extra barrier. Air quality aside, what would heat the house better? A copper stove pipe or no stove pipe at all? No pipe at all. Simply put, without fins, making the stove pipe out of copper, or a heat exchanger, you won't be transferring extra heat. A heat exchanger isn't hard to make, and it would probably double the amount of heat you got out of your stove, so if you're looking for extra heat, I would definitely recommend a heat exchanger. –  Michael Dec 19 '10 at 3:54
    
Also highly recommended if you hook a very small fan up to the intake of a heat exchanger (never the outlet, as it will be very hot) is putting a timer on it. You don't want to heat your house up to 85-95 degrees. Make sure that the fan doesn't get hot either -- use a 12"-long clay pipe or something of that nature as the thermal barrier between the fan and the stove, and make the inlet below the stove. DO NOT USE PVC. (Max temp for PVC is ~140 degrees F) –  Michael Dec 19 '10 at 4:16
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I bet it would work spectacularly if you force air through the copper pipes. On the other hand, to get a decent volume of air through narrow pipes you would need something like a compressor, which is loud and expensive. –  Fake Name Dec 20 '10 at 9:14
    
The stove already has a fan, which blows air between the back of the stove and a sheet of steel across the back of the stove. –  chris Dec 20 '10 at 14:06
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In a woodstove, where the fire doesn't get very hot, you get a lot of unburnt gases. These are compounds that could burn but only at a higher temperature. Instead, they evaporate. As they travel up the chimney, the cool and condense. Over time it builds up. If you then burn a really hot fire, they can finally ignite, and your chimney burns. This can happen when you use a lot of pitchy softwood for a long time (Douglas Fir, for example) and then burn some really dry hardwood (e.g., maple).

If you do something clever to extract more heat from the chimney, you will get more condensed goop in your chimney, so be sure to clean it regularly.

Another way to deal with this problem is to always make a really hot fire. Then almost everything will burn, leaving very little goop in the chimney, and very little ash. That's going to be too much heat for your house, so you have to surround your fire with an enormous thermal mass. That will moderate the temperature. This is the principle behind a "Russian stove", among other names.

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The point about extracting heat leading to more goop is a good one, but as Michael points out in the comments below, there are products to do this. I'm not sure how much difference a few extra BTUs will make, given that the outside temp. can be -20C a lot of the time, and is pretty much always below 0C when the stove is on. It's a newer stove, and we only burn well seasoned hardwood, so the fires are usually pretty hot. –  chris Dec 22 '10 at 0:54
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The "goop" you are referring to is creosote, a black glossy hardened tar that will develop in any wood stove and chimney. This is worse with unseasoned or soft woods, but will develop from all woods. If ignited, a chimney fire will result. A chimney fire can get so hot and uncontrollable that it can actually crack the flue liner or severely damage the mortar in an unlined flue. Chimney fires are a leading cause of house fires here in the north country. All wood burning chimneys should be inspected and properly cleaned every year. –  shirlock homes Jan 18 '11 at 11:39
    
I disagree about the soft woods contributing more creosote than hard woods. If anything, hardwoods tend to contribute more creosote than soft, as people tend to burn under-seasoned hardwoods (given the same amount of seasoning time, soft woods tend to dry out much faster than hard). If anything, the soft woods burn hotter than hard woods (and faster), usually providing the temperature required to ignite the creosote from the hard woods. –  Jon Fournier Jan 24 '11 at 15:06
    
An interesting effect during creosote fires is the evolution of large quantities of volatiles as the creosote reevaporates. It can momentarily snuff the chimney fire and then reignite with a soul cleansing thump which doesn't mean much to someone who's never heard it before. It's the equivalent of a really bad muffler backfire on a car and can open pipe seams, crack chimney tiles and purge burning materials into places they're not intended to be. The second pop is usually followed by a call to your local fire department and a lot of praying. –  Fiasco Labs Dec 24 '12 at 18:04
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The gas leaving the stovepipe needs to have a certain temperature to be able to heat the chimney to actually create proper airflow by thermal venting.

The pipes you are linking to are actually meant to insulate the pipe, making it cooler on the outside, the gasses hotter, allowing the chimney to get hotter and generate more updraft, pulling more air into the stove, burning more wood, making more warmth.

Of course you can overdo this and have too much heat escaping as hot air. This is why the kind of stove, length of stovepipe and kind of chimney have an influence on the efficiency of the whole system.

If you have good draft, you might be able to extract some additional heat from the gas. If not, you may be able to profit from insulating the pipe to increase draft.

All this of course means you have to let air into the house from outside. If your house is new and well-built, it may be too tight to heat with wood. Try if your oven works better if you open a window slightly.

Also, dont mess with your stove without knowing what you do, or asking a professional. After all, you are dealing with a column of flame passing through your house.

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Air is very bad at transferring heat. This is why all modern insulation is based on air bubbles. You can only use the heat that is created by the fire, no more -no less. You cannot use copper or anything else to manufacture more heat than that provided by the fire. As mentioned elsewhere you need a hot chimney to help the hot exhaust gasses escape.

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By wrapping the pipes in a conductive material like copper, you're increasing the surface area available to carry heat to the surrounding air. More surface area means more heat conducted into the air; this is how radiators work. –  Niall C. Dec 24 '12 at 16:33
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