It will not burst in flames. Yes, chips do get hot, specially ramial chips (from fresh branches), but a pile of leaves and kitchen scraps gets hotter for longer.

The gray stuff that looks like mold or ash is most likely the start of any number of decomposing fungi that like to feed on wood. That's a good thing. They will tend to become most prominent just underneath the surface, and will likely spread and grow to look a bit like a web running throughout the pile. Do you want to us it as mulch?

Any pile of organic matter under the right conditions can get hot enough to spontaneously combust, so yes your wood chip pile could overheat. I have seen many wood chip piles less than 4 feet in height generate enough heat to spontaneously combust, although many people have not and do no believe it can happen. This is enough of a concern that the National Fire Protection Association has established guidelines for storing wood chips, as well as other forms of organic matter that have been adopted by the Occupational Health and Safety Administration.

Here is a link that might be useful: wood chip pile fires

TXEB - I'm looking to use a good chunk of it as mulch (as things start growing for the Spring), but probably have too much for the garden, so am likely to use the remainder in my compost. Does it make a difference how I store it depending on the purpose?

kimmsr - thank you very much for that link - it is useful. My two piles are smaller than the 4m by 8m that it mentions at the bottom, but I shall make sure I keep it damped down at the weekend just to be sure!

jo -for the stuff you will use as mulch in spring, you want it to decompose a bit, but keep a coarsely mixed texture so it won't pack tightly when used. It would be good to turn it several times between now and ..wait .. when is your spring? The key here is to get through the initial decomposition which tends to release acids that are later consumed by other decomposing organisms. Around here for hardwood mulch most age it six or more months.

For any that you would like to turn into compost you can hasten the decomposition with a bit of added green stuff or adding urea or other natural nitrogen source (bloodmeal, manure, etc.), and keeping the moisture right. Mix in the green/N material, turn as often as you can manage (weekly to monthly), and keep moist (not wet) and it will decompose much faster. Wood chips make a great base for compost.

You can also sheet compost it in place - mix it up with some green stuff and spread it over the area where you would apply the compost, then keep it moist as it decomposes. You can speed that up with a breathable permeable cover (e.g., burlap) it that's practical (area not too large).

I have used freshly chipped wood as mulches numerous times and found that, contrary to some, the plants growing where that mulch was used grew greener and faster than where well aged wood chips were used. There is some research that indicates that fresh wood chips have a fairly high amount of Nitrogen in the mix, something that escapes to the atmosphere as the chips age.
Perhaps this by Professor Linda Chalker-Scott will answer some questions

Here is a link that might be useful: About Wood Chips for mulch

Thanks Kimmsr - that link is very interesting, and I'll definitely follow the hints and tips inside.

TKEB - we're moving into Spring now - I really got the trees sorted at about the latest date I could (due to a holiday and the tree-man not being available!). I like the idea of sheet composting (hmmm - does that mean I really ought to mow the lawns, too? ;-) )

I would think the answer would depend on several things.

1) The size and variation of the wood chips
2) The wood itself as far as C:N (Green vs. dead wood)
3) The size of the pile of wood chips

None of these questions are clear from the OP.

Robertz -
a) I'll have to check the size of chips tomorrow or Saturday (it's dark out now!), but from memory, there's not much which is bigger than my thumb in either length or width, and goes right down to very small pieces.
b) The majority is green wood, along with bark and leaves - it came from trees which were felled completely last week, and others which were being trimmed (only one which had any real dead wood in it, and that was only a few branches). From what I can see, all were hardwoods, if that makes a difference.
c) The piles are (approximately!) 4ft x 5ft x 2 ft (open to the elements) and 2 ft x 4ft x 4ft (covered with a tarpaulin).

Cotton rags saturated with linseed oil and stored in closed containers are known to spontaneously combust which is why woodworkers are told to lay them out in a single layer and dry before being disposed of.
A 6 inch thick pile of wood chips is not likely to spontaneously combust while a 2 foot thick pile may.

"A 6 inch thick pile of wood chips is not likely to spontaneously combust while a 2 foot thick pile may"

The research shows it takes a lot more than a 2-foot deep pile. It is very rare and there are much larger piles widely distributed around the U.S. It is a very rare occurrence.

... delete duplicate ...

This post was edited by TXEB on Thu, Oct 3, 13 at 7:37

I don't think the pile sizes mentioned are big enough to have any concern about fire. The material itself might be able to generate heat, but the size does not appear to be large enough to retain the heat.

And that, robertz6, is the reason temps increase in compost (and decomposing wood chip) piles. The heat is a result of the biological/chemical processes that occur during decomposition. The temperature increases because the heat is retained. If the pile is sufficiently large such that a near adiabatic condition is created, then spontaneous combustion might be a possibility. If the pile is not that large and heat is lost, then the temperature reached will probably remain well below any autoignition temperature. A simple probe thermometer is wonderful tool if there is reason for concern.

Some of my wood chip piles did generate some heat, but no where near enough to burst into flames. Some were piled as high as ten feet. Getting a temperature was impossible as it was difficult impossible to push the thermometer into the chips plus it would have been too short to get a good deep internal temperature.

Having said that, I would not recommend piling moist ramial wood chips higher than four feet or so just to be on the safe side.

Lloyd

Heat? In Manitoba ???

Did it last through a weekend?

;-)

Heck, even our snow gets hot!

Lloyd

Heck, Lloyd - that's cause for a picnic.

Granted I always dealt with tree company piles, therefore much dead wood. And some of them were quite large, 30 cubic yards t their biggest, and 8 feet high. I probably dealt with 4 or 500 cubic yards all told, half of them to fill a boggy part of my yard. IME, there will be the surface chips, then a layer of biological activity starting maybe 4 inches in, and the layer might be 4 to 10 inches. It is fuzzy and warm, but nowhere warm enough. Below that layer, nothing. Best guess is that the air and moisture combination below the layer is not conducive to decay. And you can undo the pile six months later, and the chips in the core of the pile have no signs of decay, whereas the fungus layer has become a solid block .

Lloyd, that's impressive.

It would take hundreds of degrees for it to set on fire.. but what I'd be most concerned about is

1) if any of the trees has resin on it... like pine and berch trees etc.. resin is easily flammable.

2) Sun... if not shaded by trees... sun will heat it up further more

3) The height of it.. the higher the pile the more heat generated.

I recently learned keeping regular charcoal for grilling out inside your car can flame up due to heat. I kept a bag of it in there all summer long incase of an emergency until I heard about this.

I've been trying to find answers regarding Vaseline wrapped in cotton balls for a quick fire starter ... if this is safe to keep inside a hot car since I learned about the charcoal... It all has me concerned now!!! I just looked and haven't found the website I saw weeks ago of temp heat to combust. They mentioned wood. I remember soft and hard wood also makes a difference.

I'd call a mulching company and ask them. If I think about it during hours I might make a call myself and ask a few places. Just small regular bags I've bought from local stores that sat in the sun were super hot.

When I taught fire science classes for the Michigan Fire Fighters Training Council I found a number of students did not grasp the concept of spontaneous combustion either and some of these were employed in the landscape industry. However, when I supplied them with copies of the National Fire Protection Associations guidelines for storing wood chips they did learn about this potential problem.
I have also attended fire investigation classes taught be instructors that did not believe in spontaneous combustion.

Spontaneous combustion is very real. Of all the things mentioned here so far, I would be most concerned about linseed oil soaked cotton rags (or similarly any other drying or readily oxidized oil), storage of charcoal, and the storage of Vaseline impregnated cotton balls.

I've had personal experience with a bag of charcoal, stored by an apartment neighbor below in a closet off his patio, that began to combust. It can and does happen.

In compost and wood mulch piles it can and does happen. It has been well studied, and some of the keys are reasonably understood. Trying to keep it practical for the homeowner, a few keys are to keep temperatures in the pile below 170 ðF (pile size helps mitigate this), and to keep it properly moist for composting. Doing that should preclude spontaneous combustion of both compost and mulch piles. For those who want to dig into the processes and mechanisms that can lead to spontaneous combustion in compost and wood mulch, see this review article.

Oh yes, it is real. I once left linseed-oil-soaked rags in a milk crate in my van, closed up on a hot summer day. I had just finished a job, parked the van at home and left to do something else. Fortunately my brother was sitting in the house and looked out to see black smoke coming out of the van. He opened it up and yanked out the milk crate, which also had some power tools in it....I used those partially-melted tools for years afterwards, which served as a good reminder.

regarding the hot mulch piles, I am seriously thinking about making a pile this winter with coils in it to circulate heat into my floor-heat in the house. It has been done. The size and ingredients are important. One recipe I read is two-thirds wood chips to one third cow manure. I can get fresh chips dumped but I don't have easy access to cheap manure (nor do I have a bucket-loader). I was thinking about buying some round bales and unrolling them, which is easy enough, and layering fairly fresh hay alternately with wood chips and coils of tubing in the center. Incorporating hay will make a much better end product for the garden as well. A pile of this maybe six feet high by 12 to 15 feet across will surely heat up pretty good and for some months. I really only want it to be quite hot for about 3 months. It should start to cool down in March just in time to put a cold-frame on top and grow early lettuces and radishes.

What is "quite hot"?

I read about a wood chip pile water heating system in Mother Earth News, probably way back in the 70s. IIRC it was actually on the cover of the mag.

Don't burn your house down! :-D

If it reaches a core temp of 150-170 that should be ideal for warming floors. A mixing valve on the loop allows tempering with return water to achieve the desired floor loop temp which is about 100F. If the internal temp in the pile is less than maybe 120-130 it would probably not be able to maintain the floor loop temp at 100.

regarding the possibility of combustion with such a set-up close to a house, there must be inexpensive moisture monitors that can be put inside the pile somewhere with a lead to a display, similar to the temp monitors.

brown -after you pass ~140 ðF, you're living on borrowed time. As temp go above that, necessary microbes are being killed. By the time you get to 160, they are being lost rather rapidly.

From the literature I've seen, you should expect 1,000 BTU/ton-hr, and that can be sustained for 6 months in a static pile with a temp of 140 ðF. For the pile size you suggest, I come up with about 19,000 BTU/hr. With reasonable heat transfer pipe insulation you should be able to get well more than 100 ðF water with a 140 ðF pile temp, given sufficient residence time (which translates to pipe length) in the pile.

The issue with moisture meters, or temps for that matter, is they will only measure a very small region. The danger begins when another small region gets hot, then dries enough but remains hot enough that the governing effect of moisture evaporation to limit temp increases is lost and then temp starts to climb rapidly.

It's a great idea, and it has and will work, but for a residence it needs to be done with great care.

The website I found on combustion with many things... I THINK they said it would take like anywhere from 500 to 700 degrees for Vaseline to combust. Paper was a few hundred perhaps more. **** I do not have a photograph memory..!!!!!!!!! But it was up there in temps. According this this website.

I saw a youtube video and I haven't tried it myself.. but this guy could not get Vaseline to light on it's own with a lighter. It would only set immediate fire with the cotton ball wrapped around it... therefore a tiny dab of Vaseline will stay lit for a couple minutes on average up to about 5 minutes if you add a little more. But too much Vaseline on a single cotton ball will not catch fire with a lighter.

I know this is O/T but I assume the safest way to store Vaseline cotton balls is separately... chapstick can work.. just don't have it pre-made. I made a substantial amount of them too... lol. geez what IF I get stranded somewhere with no help and need to make 5 fires a day. Most survivalist only carry enough for 3 fires. Also do not carry it in plastic.

I agree with the moisture.. if I had a huge pile I would spray it down with a hose and getting it in the middle like everyday.
Use it for a nice backyard fire and toast some marshmallows ... consider giving some away for free on craigslist???

flo - the cotton is probably more combustible than the Vaseline. The Vaseline just helps it burn longer, and more steadily.

Thanks for the thoughts, TX.

I guess the trick is getting the pile in the correct size range (depending on the ingredients). Too big and it will run away on temperature, perhaps, and too small and it will not get hot enough to be useful.

pnb - all of the work I've seen on extracting heat from compost process has been done with static piles, with one exception (some crafty Brits). The problem with a static compost pile as a thermal energy source is that it is an out-of-control process, meaning there is no control. That presents a number of challenges to make the capital investment worthwhile.

Composting for a thermal energy source will need to be much more precise than what we tend to practice around our gardens. The composition of the mix, including moisture, should be controlled with a measure of accuracy generally unseen in gardening. Capturing and using the heat generated from the decomposition of organic materials for controlled use is no longer just getting decomposed materials - those are down on the list as waste materials. Then there is pile size, geometry, etc.. It's not a simple list.

I believe the engineering approach would be to oversize the pile to assure that, within a reasonable range of expected heat generating capacity, it would more than meet the demand, then add a means of exhausting the excess heat as waste. There are a number of ways to exhaust the excess heat. The trick is being able to control it.

There has been a lot of work done with heating greenhouses, which is less demanding than residential space heating. A pretty good assessment is available in an undergraduate thesis from the University of Waterloo in Canada (good school!). If it were me, I'd start small and try to work out the bugs and control issues with something like hot water.

Presumably the pipes in the pile are polyethylene, for good thermal transmission. Even without combustion, they could distort or get damaged by heat. But the point remains that for combustion to happen in a wood chip pile you have to have at least enough nitrogen, and be in certain windows of moisture and air circulation. Which is why wood chips piles really don't combust most of the time. If you have anything other than ramial, fresh, green chips, it does not combust.I doubt that a purely dead wood pile would combust without additions, under any environmental circumstances. Stated otherwise, a fungal dominated pile.

Supposedly hepex can withstand very high temps indefinitely, I believe to 180 at least. So that should be ok even in the event of a power outage or pump failure that would allow the water in the pex to be still for long periods.

I agree that building a pile for heat is much more complex than merely for compost. Even a tiny chance of a combustion is troublesome, so my inclination is to start on the small side rather than the large side and accept the likelihood that it may not produce enough heat to be worthwhile, the first time around, just to see how it goes. I have installed a new floor-loop under my kitchen, which will be attached to my existing boiler, but I plan to set up some tees and shut-offs and a separate pump such that it can be shunted to some other heat source, like this outdoor pile or an outdoor wood-burning rocket stove. So if I miscalculate the pile size too small I can simply bypass it or even run through it from the boiler as an adjunct.

hePEX is probably the right material for ease of use. The rating is a heat/pressure pair - e.g., 200 ðF/80 psi / 180 ðF/100 psi, etc. But it's probably not the best for heat exchange.

Just be sure to have adequate setback and fire breaks in between the source (pile) and the building. That includes the tubing, etc.

I'm thinking that if you're actually withdrawing that level of BTU's from the pile, of course it's going to keep the temps down from where they would be otherwise. So the risk overheating and/or combustion is generally lower, assuming you're withdrawing heat constantly.

I haven't read up on spontaneous combustion very much, but my impression is that it starts in a very small, even microscopic spot. Comparing bulk temp to combustion temperature of the material will usually tell you it won't burn, and yet it does anyway. Now and then.

I was thinking that same thing today - running the circulator pump obviously will keep the pile from overheating if there is sufficient footage of tubing in the pile. It will be important to use the smallest possible pump because the juice adds up. My electric bill jumps about $40 a month in jan/feb to run one circulator pump as it is.

TX, hePex is the standard stuff used around here to heat slabs, so the transfer is pretty good I guess. Not as good as flex copper tubing, I suppose.

Like I said, there are a lot of ways to remove excess heat, and the key is control of whatever approach is taken. One approach would be to use an immersed coil, maybe a secondary coil to be used solely for pile temp control. How much heat can be removed will depend on more than tubing footage. It will also depend on the temp difference of the "coolant" and the pile, and the volume flow rate. For that to work you need a heat sink on the other end. The on/off could be thermostated.

There are a number of options. Another good reason to pilot the entire process on a small scale.

HePEX can handle the heat so long as the pressure is also controlled. The question you need address is the load it will experience at temp. That's not normally an issue with radiant floor heating where the surrounding matrix is set rigid at low temp, before heating.

So back to the original question about overheating ...

It's haying time here in SE TX. Lots of fields being mowed, raked and baled. Late yesterday afternoon I was headed across the county, driving through cattle pasture and hay field country. I turned at a major intersection, romped on the gas, and as I looked down the road I saw a lot of white smoke drifting/blowing across the road. About a half-mile later as I am passing long rows of nicely lined up freshly baled hay in the field on my right (these were wrapped 5' or 6' round bales), there was one bale in a mile-plus long row about 150 - 200 yards off the road burning quite nicely. I would have taken a pic, but there was no shoulder and I had a large truck coming up fast in the rear view mirror (plus I was running late). The bale looked to be about 1/4 gone, completely black outside, and a few flames reached about 2-4 feet up mostly from the near top sides. It was glowing red, like hot coals, where the center was. As there was nothing around (hay mowers were ~3 miles further down the road), it was a clear and dry day after a very rainy weekend, the hay had obviously been freshly baled (probably last week between two rainy weekends), I rather strongly suspect it was one that got too hot and spontaneously ignited. First time I've ever seen this.

When I came back through ~ 3 hours later it was getting quite dark, but the fire had been extinguished with very little left of the bale.

I've been using tree chippings delivered in a large truck load for the last 3-4 years which I spread in my back yard 2-3' deep across the entire yard. I have no grass there. While I'm hand delivering it to the yard the 4-5' pile gets hot but I do not think anywhere near hot enough to ignite.

My main concern isn't self ignition but some moron from the condo in my backyard throwing a lit cigarette butt onto it or a stray rocket on the fourth or New year eve igniting it.

On very dry days and these holidays I keep the mulch wet by spraying it down with water from my well.

I also have a chiminea in my yard, and wet the mulch down before using the chiminea to eliminate the possibility that a cinder could ignite it.