Pyrolysis... and setting your house aflame

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Ashful

Minister of Fire
Mar 7, 2012
20,058
Philadelphia
There was mention of a 60°C threshold in the name of avoiding pyrolysis, in another thread, which is about 140°F by my figuring. Having been working on my boiler a bit this week, I notice it's set to run up to about 230°F when there's a call for heat. That's internal boiler temperature, and actual water temperature to the radiators will come in below that, depending on flow rate etc.

I decided to fire one zone and check surface temperature on the pipes (copper tubing) leading from the boiler to that zone, and it's measuring 100°C ± 10°C. This tubing penetrates several floor joists up to 300 years old, and is captured between 250 year old flooring and an insulated ceiling in a few cases, pressed directly against the underside of the flooring.

I believe this is typical, business as usual for the tens of millions of oil- or gas-fired hydronic systems that were the norm in this country for decades, and continue to be used in existing homes. But if 60°C is a threshold for concern, then this is something to which we might give more attention.
 
Just checked surface temps of the "pre heating" splits around the woodstove. The hottest topped out at 160F, the others are in the 135F-150F range on the hottest spots. They are about 8-10" away from the cast sides of the jotul f400.

Pyrolysis... and setting your house aflame
 
I was at an outdoor event a few weeks ago with a large cast iron fire pot. I set up a Big Top down fire with many splits of varying moisture and species as far as i could tell. (Everyone brought some from home). I was amazed that after only 2-3 hours the 4-6" rounds that were standing on the ground with the tops leaning against the cast firepot had started to char significantly. Within another hour most of them were glowing red and starting to burn. These were unsplit and not very seasoned. I threw them in the fire. I wish i took a picture because i still have a hard time believing they started to burn so quickly. Before that i would have said no way would they even char much if at all.
 
Just checked surface temps of the "pre heating" splits around the woodstove. The hottest topped out at 160F, the others are in the 135F-150F range on the hottest spots. They are about 8-10" away from the cast sides of the jotul f400.

View attachment 322512
The pre heat is all fun and games till you look up and split in the warmer is steaming! _g

i like to do a pre heat on the wood when I’m running the stove low and slow. If I’m running it wide open the splits get preheated on the coals of their buddies.
 
The pre heat is all fun and games till you look up and split in the warmer is steaming! _g

i like to do a pre heat on the wood when I’m running the stove low and slow. If I’m running it wide open the splits get preheated on the coals of their buddies.
Ha, no drama here so far for the past few years of preheating. These boys get rotated into the front line pretty quickly.
The ones with fevers serve 1st.

Dang! Just took their temps again. Got a guy showing 170F.
 
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Do you guys that preheat store your wood outside until just before burning? Or to reduce Mc a little more? Just curious

Back to the pyrolysis… So it’s my understanding that the spontaneous combustion temp of the wood goes down over time as it’s exposed to high temps. Or maybe that’s just because the Mc is going down after being exposed to high temps?
 
There was mention of a 60°C threshold in the name of avoiding pyrolysis, in another thread, which is about 140°F by my figuring. Having been working on my boiler a bit this week, I notice it's set to run up to about 230°F when there's a call for heat. That's internal boiler temperature, and actual water temperature to the radiators will come in below that, depending on flow rate etc.

I decided to fire one zone and check surface temperature on the pipes (copper tubing) leading from the boiler to that zone, and it's measuring 100°C ± 10°C. This tubing penetrates several floor joists up to 300 years old, and is captured between 250 year old flooring and an insulated ceiling in a few cases, pressed directly against the underside of the flooring.

I believe this is typical, business as usual for the tens of millions of oil- or gas-fired hydronic systems that were the norm in this country for decades, and continue to be used in existing homes. But if 60°C is a threshold for concern, then this is something to which we might give more attention.

I believe I've seen somewhere mentioned in this forume that pyrolysis might start happening happening @175F, but nowhere in scientific documents, so take that "fact" with a handful of salt.
But you are making an excellent point regarding +100C/212F temp water pipes.

I think there is a slight difference in the environment itself.
While the joists are going to get extra crispy dry, it doesn't matter too much from igniting point of view, cause there is no open flame around?

When we talk about a piece of wood being close to open fire, in theory, the hotter that wood is, the easier it gets on fire? Considering ambers flying around, etc, maybe thus is concern? As well, the pyrolysis decreases the ignition point of wood.

So, while we won't see charring at +100C/200F, it makes sense to keep things around the fireplace as cool as possible, I guess.
Different point from pyrolysis, though.

Here is a picture from wikipedia, that is quite helpful to see what happens when:
Pyrolysis... and setting your house aflame


As for +60C I mentioned in the other thread, I'm just trying to be super overly safe, and the number might be higher (I'm no expert). But +60 is very hot to the touch anyway. Quick google search tells me that scalding happens in į seconds @ +60C, so even that seems a very high threshold to me.

P.S. thanks for starting a new thread
 
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Back to the pyrolysis… So it’s my understanding that the spontaneous combustion temp of the wood goes down over time as it’s exposed to high temps. Or maybe that’s just because the Mc is going down after being exposed to high temps?
That would be my understanding. It seems that only MC is changed in wood, when heated up to +100C, and less moisture equals to less energy is wasted when continuously heating wood, since there will be no water to evaporate thus wood will not loose energy when being heated up. But if the environment is able to heat only up to +100C, wood would (<- heh) never combust.

It seems pyrolysis starts only >100C, for instance "Most sugars start decomposing at 160–180 °C" (source: https://en.wikipedia.org/wiki/Pyrolysis)
 
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User error on my part i believe with the 170F laser reading on pre heating split.

It seemed way higher than made sense so i pulled the hottest one out of formation and stood it on the front of the hearth. Immediately lasered it and got 136F. That seems much more in line as it was not that hot at all to the touch.

I think the laser beam may have gotten distorted with it's path being to close to the stove on it's way to lighting the read spot on the split toward the back of the side wall of the fireplace. Anyone know if this is possible? Something is changing the reading?
 
i had pyrolosis then doctor gave me a cream ;lol ;lol ;lol aww cmon you know you liked it
Would have thought that You would have Loved that Burn!
 
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User error on my part i believe with the 170F laser reading on pre heating split.

It seemed way higher than made sense so i pulled the hottest one out of formation and stood it on the front of the hearth. Immediately lasered it and got 136F. That seems much more in line as it was not that hot at all to the touch.

I think the laser beam may have gotten distorted with it's path being to close to the stove on it's way to lighting the read spot on the split toward the back of the side wall of the fireplace. Anyone know if this is possible? Something is changing the reading?
The wood is reflecting some of the heat from the stove.

If you point an infrared camera at a glass door, you can see your heat reflection in it - which can be a little spooky sometimes!
 
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I hadn't thought of the ignition temperature of sugar before.

Ignition temp of carbon is around 700C or ~1300dF. reference: https://www.fireandsafetycentre.co.uk/blogs/safety-storage/auto-ignition-temperature

Down at the bottom of the list they list the ignition temperature of "wood" at about 600dF, which is the rough number I have been looking out for.

The setpoints on my boiler are pretty close to 165dF@20 psi low and 180dF @ 25 psi high.

I am perfectly comfortable putting damp, wet or oversaturated cotton towels under baking goods at any oven temperature up to 450dF- but I don't use them under the broiler.
 
There was mention of a 60°C threshold in the name of avoiding pyrolysis, in another thread, which is about 140°F by my figuring. Having been working on my boiler a bit this week, I notice it's set to run up to about 230°F when there's a call for heat. That's internal boiler temperature, and actual water temperature to the radiators will come in below that, depending on flow rate etc.

I decided to fire one zone and check surface temperature on the pipes (copper tubing) leading from the boiler to that zone, and it's measuring 100°C ± 10°C. This tubing penetrates several floor joists up to 300 years old, and is captured between 250 year old flooring and an insulated ceiling in a few cases, pressed directly against the underside of the flooring.

I believe this is typical, business as usual for the tens of millions of oil- or gas-fired hydronic systems that were the norm in this country for decades, and continue to be used in existing homes. But if 60°C is a threshold for concern, then this is something to which we might give more attention.
I literally have copper boiler pipes going directly through floor joists all over my house. Granted, my boiler heats the water to about 160f. But I can’t ever imagine that would be any risk of fire, ever. The house is 40 years old. There’s no other way I can think of to where u could get the pipes from the crawl space to the baseboards without going through wood floors. I would be shocked if this caused a legit concern.
 
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This was discussed on another site some years back...someone posted an article about a fire in an apartment building (IIRC) that happened due to hot water pipes...and again, IIRC it was supposedly documented that the fire was caused by pyrolysis from 180* pipes... I'll have to see if I can find it again, I think it was over on AS...
 
I'm glad to have privy to this info. I'm aware of pyrolysis bit never considered boiler pipes in contact with wood. My house isn't quite as old as Ashful's but it's seen it's fair share of history. I plan on converting to hydronics from forced air.... in stages. Fortunately, much will be in pex. Some in copper. I have noted previously, the brackets used that hold the hard pipes away from the wood a couple inches.
 
@Ashful
I'm surprised you heat your hydronic to above boiling... Mine are at 180 F.
I believe most are to avoid pressure issues?
 
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Yeah 230F seems way high, i thought maybe it was a steam system but it doesn't sound like it?
 
I'll ask my boiler tech's about it, next time I speak with them. But I'm running four floors of heating in this house, so system pressure and temperature are likely both higher than most. It's a big house on a surprisingly tiny boiler, so running higher temps may be a way to make more capacity from a smaller unit.

I know from some experience that I lose heat to the 4th floor any time the boiler pressure drops below 18 psig, so we're usually running closer to 20 psig. The boiling point of water is 257°F at 20 psig, so I'm in no danger of boiling at 225 - 230F, unless system pressure drops off.

But hearing so many of you say yours are set for 180°F has me wondering. My father, grandfather, and great-grandfather were all boiler tech's, in fact my grandfather and great-grandfather owned a plumbing and heating business, but they're all gone now. I sure do wish there were a way to email/text them, when questions like this come up. :p
 
One thing to also keep in mind, 180 (or whatever it’s set to) is the temp of the water INSIDE the pipe. The actual exterior of the pipe is likely cooler
 
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For 180F water, general guidelines state you can have 45k btu of radiators on a zone.

The amount of feet that is depends on the water temperature and rating of the emitter. You can run more than that, but the water will be much colder by the time it returns to the boiler, so rooms on the tail end of that line might not get enough heat.

I can easily see where the boiler could be set to run hotter if there are more linear feet of emitters and more heat is needed. I’ve tuned my boiler to run 150f water as I don’t need all that heat.
 
One thing to also keep in mind, 180 (or whatever it’s set to) is the temp of the water INSIDE the pipe. The actual exterior of the pipe is likely cooler
Even better than that, as that's water temp inside the boiler, and it seems to cool pretty quickly as it heads out from there. I measured surface temps once, and forget the exact numbers, but out of curiosity I can do it again this week.

For 180F water, general guidelines state you can have 45k btu of radiators on a zone.
Good number. But my system has no XOR logic on the zone valves, so in theory, all six could be running at the same time... PLUS demand for hot water thru the boilermate. The three largest zones are each 110 feet of aluminum on copper fin tube, with the remaining three zones being 15 to 60 feet each.

In reality, I doubt all six ever run at the same time, but two or three seems likely.
 
Most systems have a priority zone. Mine is hot water. After that, the other ones could probably run at the same time without too much of an issue. Less heat would go each zone as it’s going to be split up, but I bet the returning lines are going to still have plenty of heat to distribute. There’s no way a 15ft zone pulls off enough heat, and there’s little chance that a 60ft line does. Personally, I’d be afraid of short cycling with such small zones. That can break things! I attached my upstairs to the downstairs zone because of short cycling. I only had 20k btu on that zone. I attached it to my downstairs which has 45k on it. The water hits my upstairs first so my downstairs gets cooler water and keeps the whole place more comfortable, lol. 65k btu on a single line is way more than it’s rated for, but when it turns on, it’s not going to shut off in 3 minutes.
 
I hear what you're saying about low-BTU-load zones, but I don't think that's a problem here. Water coming back from that 15 foot (maybe even 20', I didn't measure) is plenty cool, because:

a) it's making a round trip of approximately 120 feet thru 3/4" copper tubing, most of which is uninsulated
b) it's only one 15-20 foot baseboard, but it's in a big attic room with big heat load on that baseboard
c) baseboard is mounted directly onto uninsulated and unplastered exterior stone gable wall

I'll run that zone tonight, and check pipe temperatures to/from that zone. I know the boiler kicks on and runs up to 220-230F when any zone calls for heat, not just one in particular. When there's no call for heat, it drops back to something like 180F, for hot water alone.