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chimney fire

chimney fire

chimney fire

Hi guys,
I am involved in a fire investigation. It concerns a fire occurred in private home, in particular the fire was started in a cavity where there are a chimney and other plants.

I have already isolated the cause of the fire and I have studied the fire patterns, in particular I have attached a photo that shows the classic V pattern and that highlights the ignition point in an elbow of the chimney.
Furthermore I have to point out that the chimney was really bad insulated, bad supported and there were too much 45° elbow in the path(six elbow). The chimney go through the roof, and wood fiber insulation is in front of the chimney (separated by 3 cm gap of air)

The chimney was connected to a pellet stove. The pellet stove had a nominal heat output of 17 kW and the IMQ test made on the stove shows a mean flue gas temperature of 167°C.

The fire has occured the 28 december, and the chimney was installed just 2 month before this date. In my opinion a soot fire has occured, in fact the stratification of soot could be reinforced by the loss of pressure due to the elbows and to the heat loss (bad insulation).

Talking with the firefighting departments, they told me that just the smoke spills can cause the fire. I have tried to demonstrate if the radiant heat emitted by stratification of the hot smoke (using the temperature of a mean flue gas temperature of 167°C with Boltzmann formula) was enough to ignite the wood fiber insulation, but I cannot demonstrate it.

Can you help me to analytically demonstrate the possibility of the ignition due to the hot smoke stratification? I accept every advice thanks.

RE: chimney fire

Providing length of pipe and diameter will help. Is it double or triple insulated pipe? As to cause are you considering the excessive combustion air in from an improper open draft into the fire area? Excessive oxygen up there and resulting in unexpected fire inside the pipe and resulting red-hot pipe? It's a common problem with wood burning. Radiant heat from that red-hot pipe will do it.

RE: chimney fire

Hi oldestguy,
the total length was 13 meter. I am attaching the chimney path that I have pieced together. The fire has occured in the last elbow. It was a simple DN 130 inox chimney with 3 cm of mineral wool connected to the chimney with adhesive tape.

RE: chimney fire

This is what is termed "an accident waiting to happen". Likely installation location could have loosened the "insulation". Sheet metal pipe joints also not so tight.

RE: chimney fire

I perfectly agree with you...
But the respondent expert says that, since the test smoke mean temperature is 167°, it is impossible that a fire can occur. Furthermore he says that since the chimney was new, it is just two mont old, no soot fire can occur. So I was trying to analytically demonstrate it, in order to avoid misunderstanding.

RE: chimney fire

In my 87 years of life I have lived most of them in houses or areas where wood was burned for heat. I've fed a lot of wood burning stoves and furnaces, cleaned a lot of chimneys. It was not uncommon in winter at night to see the outward exposed ends of metal chimneys to be glowing with dull red and even see flames coming out the end. In handling sheet metal pipe elbows that were made from sections fitting together, sometimes riveted, to have them come apart or leak. Inside these houses I have seen charred walls even with some wide spacing from these pipes. Installing a pipe covered as you describe might easily be damaged, especially at a joint. Three CM isn't much. With a fire raging upward inside and a faulty joint, the hot smoke thing may be present, but my experience is the red-hot metal nearby and a faulty covering is more likely the cause. The upper end of the chimney likely is cooler than near the stove where creosote forms more easily, meaning a source of fuel is way up there feeding the flame. Being new is meaningless, not knowing the exact history of creosote forming and the full info of that covering as to being intact or protection level from that heat. If there are any heating codes, I would doubt they approve of such an installation. There would then be reasons for rejecting such a job as suitable.

RE: chimney fire

Where was the "167 C" measured?

This temperature should not be sufficient to start a fire.

Are you sure the point that you identified was the point where the fire started ... or was it only the point where the fire inside the chimney made its presence known outside the chimney?

If the wood stove was operated regularly in poor combustion conditions (and it happens a lot) it's conceivable that there could be enough build-up in two months. Shutting down the air supply so that the stove doesn't make too much heat because the house is getting too hot, is a recipe for making lots of unburned material condense in the chimney as it cools. Then one day when one wishes to have more heat because it's cold outside, the stove operates with excess air, the fire spreads up the chimney (starting at the stove). All those loose joints in the sloppily built chimney let in extra air which feed the fire once it starts.

I've been in a house that had a chimney fire, and slept through it. Fortunately, our chimney was properly isolated from the structure of the house, and the fire threw all manner of black debris out the top (and it landed on the snow outside) but it didn't do any damage inside the house!

RE: chimney fire

It was measured during a standard test near the pellet stove.What are you meaning as "poor combustion condition"?

RE: chimney fire

Starved for air. If the stove is wanting to make too much heat, you restrict its air supply to slow down the fire - that's how it's done. But without enough oxygen, the wood still decomposes because of the heat, and there is a lot of unburned crap going up into the chimney. And, with the stove making less heat, the temperature in the chimney is lower, and some of that stuff (creosote) condenses on the chimney walls.

RE: chimney fire

Seems like a pretty skinny pipe; even without the elbows. My strictly gas fireplace has nearly 50% larger diameter, for about the same length run.

This http://nvlpubs.nist.gov/nistpubs/jres/61/jresv61n5... suggests that auto-ignition temperatures well below even your assumed value are possible, depending on the exact nature of the wood fiber in question. This http://fire.nist.gov/bfrlpubs/fire81/PDF/f81010.pd... however, does not support that concept.

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RE: chimney fire

We have many homes around my area that are heated using pellet stoves. As a general rule, creosote is not an issue with them as they are typically efficient burning units. What can throw that out the window is the pellets that were being used. I would look at the stove in question and read the recommended installation instructions as well as the operating manual. Most will state that they are to use a hardwood pellet only. They will also give specific venting instructions.

While we do not see a lot of creosote build up under normal operations, we had a retailer sell cheaper softwood (mostly pine and poplar) pellets for a while. During that time we had one house where the vent was completely plugged with creosote after only a month and a half. We have also seen where the stoves will put out sparks and embers through the vent.

The picture is not very clear, but what was the build up of creosote like? Was the thick scale that had cracked with fine particles? That is the kind of build up that can be set on fire at relatively low temperatures. If the build up was a more even, thin layer, I would look more at the angle of embers possibly exiting the chimney through a bad joint.

For me the key elements are
What were the pellets that were being burned?
Do they meet manufacturers recommendations?
Does the chimney meet installation guidelines?
What is the creosote build up look like?
What else could have possibly started the fire?

RE: chimney fire

From the pic, I can't tell if that flue is a double-wall design... I'm going to assume it was else it's not to code. Double-wall flues are quite cool on the surface... if I'm really cold working outside, I often go inside for a few and warm my hands by holding the flue. Any significant heat loss would not be due to lack of insulation surrounding the pipe (and there really shouldn't be any surrounding it anyway).

In terms of creosote, I simply cannot imagine how a 2-month old system would have enough to cause concern. I have burned two years of the lowest-grade pellet in my stove and had little more than an inch or so of ash in the pipe cleanout T (7-8 cubic inches). Brushing the pipe caused little more to come out. the systems are just too efficient to cause a significant buildup in that short amount of time.

13 meters is a pretty good run. You should find out what stove was being used and the EVL for it. The six 45-degree bends alone mean this pipe would need to be 4", at a minimum (looks like it form the pic). The long run, however, makes me question if the stove had enough power to push through that much pipe... did it have an external blower? I would expect a very poor burn if the stove wasn't able to properly push through that length, which could lead to the owner turning it to the highest setting for more heat (along with a lot more unburned particulate matter), which can snowball...

Is it possible the flue was not properly sealed at that last 45-degree? Perhaps it was disconnected during final assembly and no one caught it?

Dan - Owner

RE: chimney fire

Double walled stove pipes stay cool due to ventilation. Without ventilation the second wall is just a spacer. Was the wall cavity ventilated? If not it could get very hot. It does not appear that this installation would meet any building code. It used to be common practice for stove pipes to run across a room before going outside in order to get more heat extraction. This is not legal any longer.

RE: chimney fire

It is a single wall chimney. The chimney is a DN 130.

RE: chimney fire

MacGyverS2000 which criteria did you used to estimate that the chimney should be 4".
It does not have external blower. Do you know a criteria to estimate if the stove had enough power to push through the pipe?

RE: chimney fire

Single-wall pipe, all bets are off. I'm unaware of any pellet stove installers who use single-wall for flue installs, and using it requires the approval of both the stove manufacturer and local code. A stove worth its salt in efficiency will not have nearly the same heat output in the flue as, say, a wood stove... but a double-wall tube should be used anywhere there will be close proximity to combustibles (don't know what else the flue comes near in this installation). Make sure local code allows single-wall, or else everything else is irrelevant come lawsuit time.

Most stove manufacturers will provide an EVL (Effective Vertical Length) limit on their blowers... essentially, how much pipe 3" can the stove can blow through before it can't sufficiently evacuate the stove for a clean burn. 45-degree bends are roughly equivalent to 3' of straight pipe, 90-degree bends roughly 5'. Anything more than 15-20' is suggested to upgrade to 4". Technically, vertical runs are considered as half the actual distance (1' vertical = 0.5' EVL), but I'm leery of that one. Your setup is pushing the equivalent of 60' of 3" pipe. I can't say what the EVL is for 4" pipe, but that's still a long path... I would have expected an upgrade to 6" pipe, at a minimum.

Dan - Owner

RE: chimney fire

Hi guys,
Here in attachmente the technical characteristics of the Stove, the model is HIDRA 18.
I have met the stove manufacturer, they told me that chimney pulling should be almost 11 pascal.

Considering that the chimney is 13 meter, the pressure difference is computed as follows:

ΔP = g*H*(rho o – rho i)

ΔP = pressure difference, in Pa
h = height of the chimney, in m
rho o = air density at 20°C =1,2 kg/m^3
rho i = air density at 167°C=0,82 kg/m^3

So, the ΔP = 48,92 bar

Considering that the flue gas mass flow is 5,3 g/s, so the velocity in a pipe of DN 130 is 0,49 m/s. Consequently since the velocity is low, the pressure drops are very low.

So I have some question now
1) Since the pressure difference is higher than 11, could be this a problem for the chimney?
2) Since the smoke velocity is 0,49 m/s, could be this a problem for the chimney? Is there a minimum velocity in the chimney?
Thanks in advance

RE: chimney fire

Very low flow velocity will (A) allow particles to settle out and (B) allow heat transfer to significantly cool the gases before they get very far down the pipe, which will (C) promote condensation of any creosote that may form.

RE: chimney fire

BrianPetersen Is there a minimum velocity that could create problem in the chimney?

RE: chimney fire

A "minimum" velocity (a very loose term here, mind you) would depend upon the insulation of the piping... a perfectly-insulated pipe could move as slow as it wanted since heat would not be lost to the outside environment. But that's an ideal world, one we don't live in.

I still greatly question, even with very high heat loss, how much creosote could build up on the pipe in only two months time. Soot assumes a poorly burning flame, something that should have been recognized looking at the stove itself... you wouldn't have soot buildup if the flame was burning well, regardless of how much the gases cooled on their way up the flue.

I have to wonder if that portion of the flue had a leak at a joint, leading to a well-sealed portion of the chimney run getting hotter and hotter.

Dan - Owner

RE: chimney fire

I refer you to section 4.2.2 of the attachment.

You are operating well below 160K below room temp, so what were the manufacturers recommendations?

What is Fig 2 of EN 303-5??

The quoted gas temps for this unit is a mean of 89C. Where did 167 come from? Are you sure no one mixed up degrees F and C??

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

RE: chimney fire

I keep coming back to the possibility of buildup... and I have a few questions.

In the two months since the stove was installed until the fire, was it run every day?
If the outside temps were mild, did they continue to run it but at a significantly reduced setting?
How insulated was the flue run?

I do not have hard numbers, but IF they ran it every day for two months straight at its lowest settings (or cycled it on/off a lot at a low setting) and the run was not insulated very well, the buildup of soot MAY have been heavy.

Can you look at any other portion of the flue to determine how much buildup, if any, there was?

Dan - Owner

RE: chimney fire

@MacGyver 2000
With the term "minimum velocity" I mean if it exist a minimum velocity in the standards, in which this velocity create too much creosote. This is my main question by now, because in my opinion 0,49 m/s is too slow.

The insulation was 3 cm of rockwool, but sometimes there were uninsulated spaces.

I made a survey in the house now and after 2 month without cleaning the chimney the result was 23 cm of creosote(as shown in the photo).

This was the official report of the laboratories and nobody mixed up the value.

RE: chimney fire

Holy crap... then yes, I'd say this system was poorly designed all around. That is a LOT of soot for a mere two months. As I said earlier, my buildup was 7-8 cubic inches after two seasons of burning (around 5-6 tons of pellets)... that looks like 110-120 cubic inches worth after what, maybe 0.5 ton of pellets?

Of course, you still have to prove there was a build-up of soot near the joint that failed, as well as proving the soot got hot enough to combust and start the fire.

On a side note, I love that cleanout... I have to unscrew a cap on the bottom of my flue, which is a royal pain.

Dan - Owner

RE: chimney fire

More than to prove there was a build-up of soot near the joint that failed,I have to prove the cause of the build-up of soot. So that's why I am trying to prove that the system was poorly designed.

Since the chimney was 13 meter, it is difficult to prove that draught wasn't enaugh. But the velocity was just 0,49 m/s that is low but I need a comparison scientifically proved that indicates that 0,49 is low.

Do you agree?

RE: chimney fire

Ask the stove manufacturer... they may be able to run a computer simulation that shows temp along the flue run, exhaust speeds, and possibly particulate buildup based upon quality of pellet used. You'll need to know what pellets were used (and be prepared to send a sample), as well as the amount of runtime on the stove and at what heat setting.

Dan - Owner

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