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Woodstove flue temperature

I just finished replacing a woodstove chimney as part of a fire repair job. The old pipe somehow caused a fire in the roof framing, which was caught and extinguished quickly. It burned away enough framing and sheathing that I can't tell exactly what happened, but it may have been originally installed with inadequate clearance, or roofers may have inadvertently pushed it too close when re-roofing. The new chimney consists of about 6 feet of double-wall on top of the stove, transitioning to insulated pipe as it goes thru a floor, finished attic, and roof above (another 12 feet). The stove is a Vermont Castings "Vigilant", a big honker with an 8" outlet. All the pipe is 8" also.

Yesterday I test-burned the stove. It burned OK but it was hard to get a roaring fire using any combination of damper setting, door closed vs. ajar, etc. The owner's firewood is local Doug fir. I split a couple of pieces and metered them in the middle... 22%.

This is quite different than the woodstove in my shop, which is a Morso Squirrel. Tiny by comparison, but I can start a full-on nuclear conflagration in there using a bit of kindling from the scrap bin and fir firewood that's more like 17%. It just burns like hell.

I'd like to give the owner some operating parameters. I suspect the fires are slow-burning and not that hot, and that too much creosote is building up too quickly. There are a variety of stove-top and pipe-mounted thermometers on the market, and it appears that you can use one or both to try to get the optimal burn. There must be other strategies as well.

I told the owner to get some dry firewood for the remainder of the season. The house is leaky enough to provide plenty of air. I know the stove is way behind the times, but there it sits. What say you?

Asked by David Meiland
Posted Fri, 02/04/2011 - 08:29

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9 Answers

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1.
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David, I had a similar problem years ago -- when the Vigilant was at the top of the market in fact -- and wasn't able to get enough fire to keep the flue and stack clean (in MA). A local sweep recommended adding a couple of feet of stack height above the roof. It seemed to do the trick.

He also re-sealed the 6" (round) pipe where it entered the 8" (square) flue. I mention the measurement because I would wonder if that increase didn't also help the draw. Your situation seems to give a constant diameter ... you could argue the physics a couple of different ways, and I'm far too distant in time to make a good case, but perhaps his solution will give you some ideas.

The pipe temp guage was great for my (consumer) use, too, as it was an easy way to pay attention to the stove's performance when I walked by.

Best wishes,
Joe

Answered by 5C8rvfuWev
Posted Fri, 02/04/2011 - 10:43

2.
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David,
To get a hotter fire, you need:
1. To leave all the dampers wide open.
2. To split your firewood into smaller pieces.
3. To use dry wood.
4. If necessary, to increase the height of your chimney.

For safety's sake, keep an eye on the stove. Too hot is just as bad as too cool.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 02/04/2011 - 10:54

3.
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OK... two comments about added chimney height. Is there a way to apply math to this, or do I throw another section on there and see what happens?

Are the thermometers any use?

Answered by David Meiland
Posted Fri, 02/04/2011 - 11:41

4.
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David,
The usual rule of thumb -- this is probably in most building codes -- is that the top of a chimney must be at least 3 feet above the highest point of the roof structure (the ridge), and must be at least 2 feet higher than any part of the structure within 10 feet horizontally. [Later edit: see Dick Russell's later comment, since I may be wrong on this.]

Wood-stove thermometers are a good idea for newbies who don't know how to run a wood stove. Most experienced stove users know when their stove is too cold or too hot. If the owner has any doubts, wood stove thermometers are cheap.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 02/04/2011 - 11:52
Edited Fri, 02/04/2011 - 12:48.

5.
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David,

to follow on Martin's #4 -- in my case the high point was about 20' distant and the final height of the chimney (which was at an intersecting ridge) was 5' above the ridge it was on. It was originally just at the code height.

Of course Martin's right that the thermometer is mostly for 'newbies,' but even after years of experience I found it a good reminder on occasion.

Answered by 5C8rvfuWev
Posted Fri, 02/04/2011 - 12:09

6.
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I'm already at code height + on this one, but it sounds like you are both saying that added height might improve draft. What's a little counterintuitive to me is the idea of adding more pipe out in the cold, where the flue gas temp would presumably quickly drop and cause problems...?

It's an easy roof. I can walk up there and add another section and reset the rain cap. I'll have the owner split all the wood in half again and see where we are then.

Answered by David Meiland
Posted Fri, 02/04/2011 - 12:12

7.
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David,
As masons have long known, the higher the chimney, the better the draft. It's another reason to build a two-story house.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 02/04/2011 - 12:20

8.
Helpful? 0

Martin, I think your post #4 on height of chimney is off on one minor point. I believe the three foot spec is height above point of penetration and not the high point of the roof (the ridge). One reference for this is:
http://www.fireplacesnow.com/smchinfo.asp

Answered by Dick Russell
Posted Fri, 02/04/2011 - 12:45

9.
Helpful? 0

Dick,
Thanks for your correction.

Answered by Martin Holladay, GBA Advisor
Posted Fri, 02/04/2011 - 12:48

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