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Author Topic: What causes a chimney to smoke?  (Read 8418 times)
sbchapman
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« Reply #15 on: January 14, 2013, 04:24:45 PM »

I have a Woodwarm and a Clearview and they behave very differently - completely different stoves with different characteristics.

However, both burn 'clean' i.e. no visible smoke except a heat haze from the flue top when up to temperature. Initially adding wood to a previously smoke-less stove causes both to smoke until they have settled down and reached equilibrium - I dont know if this the temperature change in the firebox due to the door being opened, the new wood being partially pyrolized, or what. The only way to reduce the smoke with new wood is to run the stove full bore with all vents open and to be honest probably the same amount of smoke if produced but it is diluted by the vast amounts of air going up the chimney.

The main thing I've found is to run the stove with the vent opened slightly until the new wood is burning happily, then the controls can be shut down to produce the desired balance of heat & flames. Oh, and on the woodwarm, if the stove BODY (not flue) isn't up to at least 150C it will smoke unless it's just embers/charcoal with no partially burnt wood present.

Steve
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charlieb
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« Reply #16 on: January 14, 2013, 04:46:18 PM »

Related question:  I've just replaced a DEFRA approved squirrel with a slightly bigger a and slightly older morso. The squirrel had a 'tertiary' (I think) air supply that brought pre-heated jets of air into the back of the stove which once up to temperature created funky jets of flame.  Replacement older stove doesn't have these - just ordinary baffle - and I get the impression that slightly more smoke ends up going up the chimney.    My question is: why don't all (good) manufacturers make all their stoves with tertiary air supply?  Presumably any smoke (or woodgas) going up the chimney is lost fuel and it's not as if the non-cleanburn stoves are cheap!   Is there any disadvantage of the 'cleanburn' technology?
 
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dhaslam
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« Reply #17 on: January 14, 2013, 06:25:28 PM »

Just had a quick look at Woodwarm specs below. Surprised to see max burn temp 400C could it be your trying to burn at to high a temperature for the stove and therefore not getting complete combustion.

http://www.woodwarmstoves.co.uk/_assets/manuals/new%20instructions%20fireview%20march%202010.pdf
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I think that the Woodwarm instructions are a bit mixed up. In their instructions  the flue temperatures are up to 411C for the bigger stoves so the combustion temperature would be a lot higher.

Carbon monoxide burns  at just over 600C  and higher temperatures are needed to reduce particulates, mainly  composed of unburned carbon.    Complete combustion can be aided by  better stove design that allows  reduced particulate emissions at lower temperatures but temperatures of  900C plus  are the norm in efficient stoves.

The Walltherm stove claims combustion temperatures of up to 1000C  with flue temperature of about 110C
http://www.wallnoefer.it/cgi/sdcgi.exe?USERID=952013302333504142F94263&SID=E&mid=bks

The Navitron stoves claim combustion temperatures of 900C also with quite low flue temperature
http://www.navitron.org.uk/product_detail.php?proID=500&catID=161 
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mike7
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« Reply #18 on: January 15, 2013, 01:10:54 AM »

When people speak of flue temperature, do they mean temperature of the actual flue gases or of the (outside) of the flue casing? I just wondered...
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Bodidly
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« Reply #19 on: January 15, 2013, 10:12:51 AM »

A couple of pictures of the two chimneys at a similar stage in the burn.

This is the Woodwarm on single lined flue.


The Saey on a double skinned flue.

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sbchapman
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« Reply #20 on: January 15, 2013, 11:46:45 AM »

Maybe the clue is in your last post? Single skin vs doubleskin flue? Is the single skin insulated (pummice, beads, vermiculite etc?)

re flue temperature, I always thought it was the outside temp, i.e. what a stove thermometer fixed to the flue reads.

edit: Actually just seen you mentioned the flue difference in your first post Smiley
« Last Edit: January 15, 2013, 11:48:16 AM by sbchapman » Logged

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Ivan
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« Reply #21 on: January 15, 2013, 12:44:03 PM »

Also, there is a big difference when the chimney is on the end wall of a house - it will cool the smoke much more than a central-mounted chimney
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Bodidly
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« Reply #22 on: January 15, 2013, 12:50:14 PM »

I have no doubt that the exiting smoke is much cooler I just thought if I worked a fire right the particulates would be burned off just leaving clear emissions that are mainly steam. So do you think the contents of the exiting gases in the pictures are pretty much the same? And is it just the speed they exit caused by the insulated flue making dirt in the smoke apparent?
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biff
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« Reply #23 on: January 15, 2013, 03:09:46 PM »

That could very well be the answer SBchapman.
                             The double skinned flue can keep the temp high,right to the exit point.Plus it looks like the single skinned flue is on an end wall and Ivan,s point that End walls shed heat quicker than interior walls and as a consequence the flue will also run cooler.
                                          Biff
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