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2nd Year of running the woodburner. Didn't think it was drawing very well
the last few days, wasn't sure if it was because of the cold, still,
weather or something else. So removed the baffle to look at the base of
the 7" rear exit flue. Maybe more than 50% blocked by light
crispy/brittle tar bits, removed all that (a bucket full) and it's now
drawing and burning far better. B-)

The flue was swept before we started this season in the last week of
October, similar amount of burnt tar then after the entire winter (5
months?) as I've just removed after a week under three months.

There has been a fire in it most days since the 23rd Oct from about 1500
with the last logs going in about 2100. The stove is quite large 10kW
output but with a boiler rated aroud 7kW so I guess the flue gases are
generally quite cool. We are burning seasoned hard wood (mostly beech but
some birch and a little possibly oak), with the bark falling off and
shakes opening up in the end grain. Generally it'll get through 4 x 9"
long 4" sized logs every hour, so something over 15 logs/day.

Is that level of light crispy tar build up sort of expected? I don't mind
digging it out every 6 weeks or so, more concerned about buildup in the
flue and risk of a fire up there...

--
Cheers
Dave.



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On 13/01/13 17:35, Dave Liquorice wrote:
2nd Year of running the woodburner...


That sounds a rather fast soot build-up to me. When I was heating this
place by woodburner alone (Jotul, similar size, similar fuel, similar
rate of consumption, no backboiler) I swept it myself twice a year and
got out out half a bucket of soot from the flue, and only light
sweepings from within the stove itself.

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On Jan 13, 5:35 pm, "Dave Liquorice"
wrote:

We are burning seasoned


define "seasoned" - have you done it or are you buying it in?

hard wood (mostly beech but
some birch and a little possibly oak), with the bark falling off and
shakes opening up in the end grain. Generally it'll get through 4 x 9"
long 4" sized logs every hour, so something over 15 logs/day.


at what stove air vent level?
IOW is the fire "smouldering", "bit of flame & smoky when door
opened", "plenty of flame & no smoke", "roaring blaze"

& which vents are you using (if there is a choice)?

Jim K
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On Sun, 13 Jan 2013 11:15:44 -0800 (PST), Jim K wrote:

We are burning seasoned


define "seasoned" - have you done it or are you buying it in?


Bought in but as I said bark falling of and shakes in the end grain. The
supplier stores them in 400kg dumpy bags in a well ventilated barn. We
have 'em stacked at the side of the carport. Bought in to sit beside the
stove a couple of days before they end up inside it.

at what stove air vent level?


3/4 on the top "air wash" vent, under grate vents closed. Never riddle
the grate but the fire bars do seem a little wide apart so maintaining a
decent ash bed isn't easy. It could be that it just needs a harder burn
occasionally.

IOW is the fire "smouldering", "bit of flame & smoky when door
opened", "plenty of flame & no smoke", "roaring blaze"


No smoke, lazyish bluey yellow tinted flames that are frequently not
attached to the logs. ie they are burning the gases emmited by the logs.
Once a log has been in there for 30 to 45 minutes if you bash it with
tongs it will fall apart and be glowing anything from dull red to red all
the way through depending on how long it's been in.

Of course a fresh log burns with bright yellow attached flames initially.

--
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Dave.



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On Sun, 13 Jan 2013 18:43:33 +0000, Dom Ostrowski wrote:

That sounds a rather fast soot build-up to me.


It's not soot, at least not soot as in fine powdery possibly slightly
oily stuff that I remember as soot from the open coal fire when I was a
lad. This is hard brittle black lumps and some very light and crispy
browner bits.

When I was heating this place by woodburner alone (Jotul, similar size,
similar fuel, similar rate of consumption, no backboiler) I swept it
myself twice a year and got out out half a bucket of soot from the
flue, and only light sweepings from within the stove itself.


I think the boiler makes a heck of difference to flue gas temperature,
thus flue temperature and how much tar will condense out. When it was
swept back in Oct I didn't think that much came down, it certainly didn't
sound like much came rattling down. But as I didn't do it I don't really
know.

--
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Dave.





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On Jan 13, 8:18 pm, "Dave Liquorice"
wrote:
On Sun, 13 Jan 2013 11:15:44 -0800 (PST), Jim K wrote:
We are burning seasoned


define "seasoned" - have you done it or are you buying it in?


Bought in but as I said bark falling of and shakes in the end grain. The
supplier stores them in 400kg dumpy bags in a well ventilated barn. We
have 'em stacked at the side of the carport. Bought in to sit beside the
stove a couple of days before they end up inside it.

at what stove air vent level?


3/4 on the top "air wash" vent, under grate vents closed. Never riddle
the grate but the fire bars do seem a little wide apart so maintaining a
decent ash bed isn't easy. It could be that it just needs a harder burn
occasionally.

IOW is the fire "smouldering", "bit of flame & smoky when door
opened", "plenty of flame & no smoke", "roaring blaze"


No smoke, lazyish bluey yellow tinted flames that are frequently not
attached to the logs. ie they are burning the gases emmited by the logs.
Once a log has been in there for 30 to 45 minutes if you bash it with
tongs it will fall apart and be glowing anything from dull red to red all
the way through depending on how long it's been in.

Of course a fresh log burns with bright yellow attached flames initially.

--
Cheers
Dave.


mmm can you describe the baffle? presume your descriptions of flame
are with this in place?

Jim K
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On 13/01/2013 17:35, Dave Liquorice wrote:

2nd Year of running the woodburner. Didn't think it was drawing very well
the last few days, wasn't sure if it was because of the cold, still,
weather or something else. So removed the baffle to look at the base of
the 7" rear exit flue. Maybe more than 50% blocked by light
crispy/brittle tar bits, removed all that (a bucket full) and it's now
drawing and burning far better. B-)


My wood burner is broadly comparable with yours in power and a back
boiler. My observations are that I have had the same thing happen as you
but it was because I was burning up the last bits of coal on it.

Mine is capable of running dual fuel. The most likely cause of your
build up recently is that the top of the chimney where the flue gasses
meet the cold external air is starting to neck a bit with condensing
tar. My sweep recommends burning a small amount of smokeless fuel in the
mix to make the burning wood soot less inclined to plate out as tar and
easier to dislodge. I don't yet know if his advice is sound.

I don't know about yours but mine said the baffle plate should be
checked weekly for any obstructions - probably CYA H&S regulations. I do
it about monthly or if I have trouble lighting it.

The flue was swept before we started this season in the last week of
October, similar amount of burnt tar then after the entire winter (5
months?) as I've just removed after a week under three months.


The air draw is decreased so more tar, soot and smoke formed. You can
probably keep it running by checking the baffle plate a bit more often
and then getting the sweep in to check over the chimney. My money would
be on a constriction right up at the top from what you have described.

Just seen your description of the flame patterns which sounds more like
the situation I get when I damp mine right down for the night. Relights
nicely from the embers. Do you have a carbon monoxide alarm in the room?

There has been a fire in it most days since the 23rd Oct from about 1500
with the last logs going in about 2100. The stove is quite large 10kW
output but with a boiler rated aroud 7kW so I guess the flue gases are
generally quite cool. We are burning seasoned hard wood (mostly beech but
some birch and a little possibly oak), with the bark falling off and
shakes opening up in the end grain. Generally it'll get through 4 x 9"
long 4" sized logs every hour, so something over 15 logs/day.

Is that level of light crispy tar build up sort of expected? I don't mind
digging it out every 6 weeks or so, more concerned about buildup in the
flue and risk of a fire up there...


The worry is that you have a build up right at the top and plating out
of tar in the pipe and at some stage the whole lot decides to make a
chimney fire which tends to devalue the steel lining somewhat.

--
Regards,
Martin Brown
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"Dave Liquorice" wrote in message
ll.co.uk...

2nd Year of running the woodburner. Didn't think it was drawing very well
the last few days, wasn't sure if it was because of the cold, still,
weather or something else. So removed the baffle to look at the base of
the 7" rear exit flue. Maybe more than 50% blocked by light
crispy/brittle tar bits, removed all that (a bucket full) and it's now
drawing and burning far better. B-)

The flue was swept before we started this season in the last week of
October, similar amount of burnt tar then after the entire winter (5
months?) as I've just removed after a week under three months.

There has been a fire in it most days since the 23rd Oct from about 1500
with the last logs going in about 2100. The stove is quite large 10kW
output but with a boiler rated aroud 7kW so I guess the flue gases are
generally quite cool. We are burning seasoned hard wood (mostly beech but
some birch and a little possibly oak), with the bark falling off and
shakes opening up in the end grain. Generally it'll get through 4 x 9"
long 4" sized logs every hour, so something over 15 logs/day.

Is that level of light crispy tar build up sort of expected? I don't mind
digging it out every 6 weeks or so, more concerned about buildup in the
flue and risk of a fire up there...


In my experience you need to have a really good roaring blaze every now and
again to keep the flue clear of such debris.

AWEM

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In message o.uk, Dave
Liquorice writes
On Sun, 13 Jan 2013 18:43:33 +0000, Dom Ostrowski wrote:

That sounds a rather fast soot build-up to me.


It's not soot, at least not soot as in fine powdery possibly slightly
oily stuff that I remember as soot from the open coal fire when I was a
lad. This is hard brittle black lumps and some very light and crispy
browner bits.

When I was heating this place by woodburner alone (Jotul, similar size,
similar fuel, similar rate of consumption, no backboiler) I swept it
myself twice a year and got out out half a bucket of soot from the
flue, and only light sweepings from within the stove itself.


I think the boiler makes a heck of difference to flue gas temperature,
thus flue temperature and how much tar will condense out. When it was
swept back in Oct I didn't think that much came down, it certainly didn't
sound like much came rattling down. But as I didn't do it I don't really
know.


It appears to be a little *underaired* as you sound to be gasifying the
wood. I'm no expert though. AJH might have some better knowledge. Our
Clearview gets through a heaped garden barrow of dry logs in a full day.
I haven't yet fitted a boiler or swept the flue:-(

There is lots of stuff on the manufacturers sites about combustion. ISTR
the Clearview one warns about boilers dramatically reducing the
combustion temperature and leading to sooty windows.

Do you run the fire closed down overnight? This quickly blackens the
glass on ours.


--
Tim Lamb
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On Sun, 13 Jan 2013 12:51:06 -0800 (PST), Jim K wrote:

No smoke, lazyish bluey yellow tinted flames that are frequently not
attached to the logs. ie they are burning the gases emmited by the
logs. Once a log has been in there for 30 to 45 minutes if you bash it
with tongs it will fall apart and be glowing anything from dull red to
red all the way through depending on how long it's been in.

Of course a fresh log burns with bright yellow attached flames
initially.


mmm can you describe the baffle? presume your descriptions of flame
are with this in place?


Yes, Ascii art is easier than words:

+--+-------+ Whe
= | a +---- d = door
d v a flue v = air wash vent fed from =
d b +---- g = grate
d b | b = boiler
d b| a = baffle
d b|
dggggggggg-+
d |
+----------+

The baffle is really just an extension to the top of the boiler to force
the gases up and round the boiler it's a stubby T in shape. The central
"leg" allows access to the rear of the boiler and flue exit.

--
Cheers
Dave.





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On Jan 13, 9:30 pm, "Dave Liquorice"
wrote:
On Sun, 13 Jan 2013 12:51:06 -0800 (PST), Jim K wrote:
No smoke, lazyish bluey yellow tinted flames that are frequently not
attached to the logs. ie they are burning the gases emmited by the
logs. Once a log has been in there for 30 to 45 minutes if you bash it
with tongs it will fall apart and be glowing anything from dull red to
red all the way through depending on how long it's been in.


Of course a fresh log burns with bright yellow attached flames
initially.


mmm can you describe the baffle? presume your descriptions of flame
are with this in place?


Yes, Ascii art is easier than words:

+--+-------+ Whe
= | a +---- d = door
d v a flue v = air wash vent fed from =
d b +---- g = grate
d b | b = boiler
d b| a = baffle
d b|
dggggggggg-+
d |
+----------+

The baffle is really just an extension to the top of the boiler to force
the gases up and round the boiler it's a stubby T in shape. The central
"leg" allows access to the rear of the boiler and flue exit.


presumably the boiler part is in use? does it produce a "hot" output
or just help another heat source?

between 3 and 9 for how long would the boiler be producing "hot"?

generally sounds to me as though the boiler may be cooling the gases
etc so much that the tarry nasties are condensing on the first/next
coolest surface - your flue exit .....is there a flue liner of some
sort above all this? or just an open masonry flue ;( ???

Jim K
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In message , Andrew Mawson
writes
"Dave Liquorice" wrote in message
ill.co.uk...

2nd Year of running the woodburner. Didn't think it was drawing very well
the last few days, wasn't sure if it was because of the cold, still,
weather or something else. So removed the baffle to look at the base of
the 7" rear exit flue. Maybe more than 50% blocked by light
crispy/brittle tar bits, removed all that (a bucket full) and it's now
drawing and burning far better. B-)

The flue was swept before we started this season in the last week of
October, similar amount of burnt tar then after the entire winter (5
months?) as I've just removed after a week under three months.

There has been a fire in it most days since the 23rd Oct from about 1500
with the last logs going in about 2100. The stove is quite large 10kW
output but with a boiler rated aroud 7kW so I guess the flue gases are
generally quite cool. We are burning seasoned hard wood (mostly beech but
some birch and a little possibly oak), with the bark falling off and
shakes opening up in the end grain. Generally it'll get through 4 x 9"
long 4" sized logs every hour, so something over 15 logs/day.

Is that level of light crispy tar build up sort of expected? I don't mind
digging it out every 6 weeks or so, more concerned about buildup in the
flue and risk of a fire up there...


In my experience you need to have a really good roaring blaze every now
and again to keep the flue clear of such debris.


Also, is the flue insulated?

--
Tim Lamb
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On Sun, 13 Jan 2013 21:29:40 +0000, Tim Lamb wrote:

It appears to be a little *underaired* as you sound to be gasifying the
wood. I'm no expert though.


Neither am I, that's why I'm asking. B-) I was under the impression
that to get the most out of the wood you where supposed to gasify it.

Do you run the fire closed down overnight? This quickly blackens the
glass on ours.


No, the last logs go in about 2100 so that by the time the CH thermostat
drops back to 15C at 2200 it's not roaring away heating the thermal store
for no real benefit. It'll still have a full bed approx 12 x 18" of
burning charcoal that doesn't half chuck out the heat...

--
Cheers
Dave.



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On Sun, 13 Jan 2013 13:54:42 -0800 (PST), Jim K wrote:

presumably the boiler part is in use? does it produce a "hot" output
or just help another heat source?


The boiler feeds a 300l thermal store via a load controller that should
ensure that, apart from initial start up, the circulating water is always
above 60C. Clip on thermometers at the boiler show the flow from the
boiler in the range of 65 to 75C and return around 60 to 65C.

between 3 and 9 for how long would the boiler be producing "hot"?


I guess all the time it can, the thermal store is depleted by the CH and
HW. The CH will come on and off as required and the woodburner generally
keeps up with that demand, plodding along heating the store when the CH
is not demanding heat.

generally sounds to me as though the boiler may be cooling the gases
etc so much that the tarry nasties are condensing on the first/next
coolest surface - your flue exit


Could well be right, the flue pipe immediatly out the back of the stove
up to the cIosure plate is single skin metal tube, so quite "cold" and
looses heat well. I guess the best solution is to give it a good burn
every now and again. What do you reckon once a week, fortnight or month?

.....is there a flue liner of some sort above all this? or just an open
masonry flue ;( ???


Above the closure plate flexible twin wall flue liner not quite sure how
much insulation is around that in the old stone chimney. I didn't see
enough empty vermiculite bags to have filled the entire void. I wasn't
overly happy about the installers...

--
Cheers
Dave.



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"Dave Liquorice" wrote in message
ll.co.uk...
On Sun, 13 Jan 2013 13:54:42 -0800 (PST), Jim K wrote:

presumably the boiler part is in use? does it produce a "hot" output
or just help another heat source?


The boiler feeds a 300l thermal store via a load controller that should
ensure that, apart from initial start up, the circulating water is always
above 60C. Clip on thermometers at the boiler show the flow from the
boiler in the range of 65 to 75C and return around 60 to 65C.

between 3 and 9 for how long would the boiler be producing "hot"?


I guess all the time it can, the thermal store is depleted by the CH and
HW. The CH will come on and off as required and the woodburner generally
keeps up with that demand, plodding along heating the store when the CH
is not demanding heat.

generally sounds to me as though the boiler may be cooling the gases
etc so much that the tarry nasties are condensing on the first/next
coolest surface - your flue exit


Could well be right, the flue pipe immediatly out the back of the stove
up to the cIosure plate is single skin metal tube, so quite "cold" and
looses heat well. I guess the best solution is to give it a good burn
every now and again. What do you reckon once a week, fortnight or month?

.....is there a flue liner of some sort above all this? or just an open
masonry flue ;( ???


Above the closure plate flexible twin wall flue liner not quite sure how
much insulation is around that in the old stone chimney. I didn't see
enough empty vermiculite bags to have filled the entire void. I wasn't
overly happy about the installers...



You can get thermometers which clamp to the single wall flue above the
stove.
This lets you know if you have the flue gasses warm enough.

--
No plan survives contact with the enemy.
[Not even bunny]

Helmuth von Moltke the Elder

(\__/)
(='.'=)
(")_(")



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On Jan 13, 5:35*pm, "Dave Liquorice"
wrote:
2nd Year of running the woodburner. Didn't think it was drawing very well
the last few days, wasn't sure if it was because of the cold, still,
weather or something else. So removed the baffle to look at the base of
the 7" rear exit flue. Maybe more than 50% blocked by light
crispy/brittle tar bits, removed all that (a bucket full) and it's now
drawing and burning far better. *B-)

The flue was swept before we started this season in the last week of
October, similar amount of burnt tar then after the entire winter (5
months?) as I've just removed after a week under three months.

There has been a fire in it most days since the 23rd Oct from about 1500
with the last logs going in about 2100. The stove is quite large 10kW
output but with a boiler rated aroud 7kW so I guess the flue gases are
generally quite cool. We are burning seasoned hard wood (mostly beech but
some birch and a little possibly oak), with the bark falling off and
shakes opening up in the end grain. Generally it'll get through 4 x 9"
long 4" sized logs every hour, so something over 15 logs/day.

Is that level of light crispy tar build up sort of expected? I don't mind
digging it out every 6 weeks or so, more concerned about buildup in the
flue and risk of a fire up there...

--
Cheers
Dave.


That is a really dangerous circumstance.
You could be gassed or start a chimney fire.
I hope you have a CO detector.

It can be got around by having a good hot blaze once a week to burn it
off in most stoves before too much accumulates.
But don't do it when there is a big accumulation, a chimney fire can
destroy the chimney/burn the house down.

It is an inherent thing in non-pyrolysing stoves when reducing zones
occur in the fire. Unburnt hydrocarbons condense out in the chimney.
(Tar and creosote).
It's made worse if the wood is not fully dry.
If you have white smoke, this indicates you have a cold chimney/wet
wood. and tars will be forming.
Most of the time there should be no visible smoke or at worst a slight
bluish haze.

Most tarring happens in the top foot or so of the chimney (Where
combustion gases meet the cold air).
If you get it elsewhere, it is an indication that the chimney is
running cold. This can cause damage due to acid attack, especially on
masonry chimneys

Sweeping often won't dislodge this stuff BTW. It usually has to be
scraped off.
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On Mon, 14 Jan 2013 09:29:12 -0000, David WE Roberts wrote:

You can get thermometers which clamp to the single wall flue above the
stove.


Or an infrared thermometer? That would have more uses. CPC have what
looks like quite a decent and cheap one on offer ATM. Dual laser,
adjustable emissivity (0.1 to 1.0), 12:1 distance:spot. £32.95 + VAT
IN0545703 normally £38.47 + VAT.

This lets you know if you have the flue gasses warm enough.


How warm is warm enough?

--
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Dave.



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On Jan 14, 3:59 pm, "David WE Roberts" wrote:

http://www.energywatersaver.co.uk/st...ometer-review/


This review of a Stovax product says that the thermometer is graded in three
zones:
"The display is very simple, just three zones of operation; Creosote, Best
operation and Too hot. The temperatures are displayed in both Celsius and
Fahrenheit."


that looks neat! £12 odd on amazon

Jim K
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On Mon, 14 Jan 2013 09:34:54 -0800 (PST), Jim K wrote:

"The display is very simple, just three zones of operation; Creosote,
Best operation and Too hot. The temperatures are displayed in both
Celsius and Fahrenheit."


that looks neat! £12 odd on amazon


Still think an IR thermometer has more uses (and I want one) but the this
hook has been severly weakened with that price. B-(

Looking at images of stovepipe thermometers it seems that the "ideal"
flue temperature is between 120 to 250C.

--
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Dave.



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Dave Liquorice wrote:
2nd Year of running the woodburner. Didn't think it was drawing very
well the last few days, wasn't sure if it was because of the cold,
still, weather or something else. So removed the baffle to look at
the base of the 7" rear exit flue. Maybe more than 50% blocked by
light crispy/brittle tar bits, removed all that (a bucket full) and
it's now drawing and burning far better. B-)

The flue was swept before we started this season in the last week of
October, similar amount of burnt tar then after the entire winter (5
months?) as I've just removed after a week under three months.

There has been a fire in it most days since the 23rd Oct from about
1500 with the last logs going in about 2100. The stove is quite large
10kW output but with a boiler rated aroud 7kW so I guess the flue
gases are generally quite cool. We are burning seasoned hard wood
(mostly beech but some birch and a little possibly oak), with the
bark falling off and shakes opening up in the end grain. Generally
it'll get through 4 x 9" long 4" sized logs every hour, so something
over 15 logs/day.

Is that level of light crispy tar build up sort of expected? I don't
mind digging it out every 6 weeks or so, more concerned about buildup
in the flue and risk of a fire up there...


I've got a woodburner, multifuel actually, but I wouldn't describe myself as
a guru.
It's been installed since Oct 2011, so just over a year. I burn mainly scrap
timber - door frames, windows, joists etc, the occasional tree (cherry,
eucalyptus, oak, sycamore) and a little coal.
I don't have a damper, the hole at the back of the burner has a 5ft piece of
corrugated flue liner affixed to it and bent up into the (brick) chimney,
draws well with vents open, smoulders for hours unvented and I've never had
any build up of tar/ creosote.
It's used daily from Oct to April, from about 5pm to midnight, using about
same volume of timber as you, but this time of year, a kilo of housecoal
gives off a hell of a lot more heat than wood alone, I pay £7.50 for a 25kg
sack, so about 4 or 5 will last me over winter, **** the smokeless ****e
off, and the stuff the garages try to sell at £9 for 10kg




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On Jan 14, 8:00*pm, "Dave Liquorice"
wrote:
On Mon, 14 Jan 2013 09:34:54 -0800 (PST), Jim K wrote:
"The display is very simple, just three zones of operation; Creosote,
Best operation and Too hot. *The temperatures are displayed in both
Celsius and Fahrenheit."


that looks neat! £12 odd on amazon


Still think an IR thermometer has more uses (and I want one) but the this
hook has been severly weakened with that price. *B-(

Looking at images of stovepipe thermometers it seems that the "ideal"
flue temperature is between 120 to 250C.

--
Cheers
Dave.


It depends on what sort of stove your have.
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On Mon, 14 Jan 2013 23:07:25 -0800 (PST), harry wrote:

Looking at images of stovepipe thermometers it seems that the "ideal"
flue temperature is between 120 to 250C.


It depends on what sort of stove your have.


Carry on.

--
Cheers
Dave.



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In message o.uk, Dave
Liquorice writes
On Mon, 14 Jan 2013 23:07:25 -0800 (PST), harry wrote:

Looking at images of stovepipe thermometers it seems that the "ideal"
flue temperature is between 120 to 250C.


It depends on what sort of stove your have.


Carry on.


The thermometer on the front of my stove is usually indicating somewhere
between 175deg.C and 215deg.C. I guess the first section of flue would
be similar.

Whilst not multifuel the clearview claims to meet the requirements of
smoke control areas. Once warmed up, there is no visible plume. There is
a baffle plate between the firebed and the flue with extra air forming a
curtain next to the door glass.

In a fit of nervousness, I shone a torch on the start of the flue last
time the fire was cold. Thin film of dark grey powder only.


--
Tim Lamb
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On Jan 15, 1:16*pm, "Dave Liquorice"
wrote:
On Mon, 14 Jan 2013 23:07:25 -0800 (PST), harry wrote:
Looking at images of stovepipe thermometers it seems that the "ideal"
flue temperature is between 120 to 250C.


It depends on what sort of stove your have.


Carry on.

The combustion gases need a certain buoyancy to overcome the
resistance of the chimney and stove.
The bouyancy depends on the temperature difference between inside and
outside temperatures.

The bouyancy required depends on how convoluted the gas passages/
chimney is, the size of the passages, the length of the passages and
the hieght of the chimney..
It varies too on how warm the chimney is and how much energy it takes
to warm up the chimney and maintain that temperature.

Conversly, to be efficient the flue gas temperature need to be as low
as possible.
But not so low as the cause condensation.

So your flue gas temperature needs to be as low as possible so long as
the chimney is still functioning/drawing and there is no white smoke
which indicates condensation is taking place in the chimney. It's
hard to achieve this under all conditions.
In practice, natural draught chimneys are bound to be inefficient
because they only work well over a narrow range of conditions and they
are upset by the outside air/wind conditions.
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