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Woodburners!
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woodburner



Joined: 06 Apr 2009
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PostPosted: Sun Dec 01, 2013 5:53 pm    Post subject: Reply with quote

In which case, extrapolating your experence, is not helpful in advising someone trying to run a relatively small steel box burner.
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Little John



Joined: 08 Mar 2008
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PostPosted: Sun Dec 01, 2013 6:02 pm    Post subject: Reply with quote

vtsnowedin wrote:
woodburner wrote:
Lighting is best done from the top, not the bottom. Look up something like upside down fires. This dramatically reduces smoke production on starting, and gives instant flames. It also means the first hour of burning will need no attention other than to shut the vent when the fire is established. If you have a glass door that gets sotted up, you aren't running it hot enough, and it indicates tar will be going up the flue. Loading it up last thing at night and closing the vents is a no-no IMO.

What nonsense. Heat smoke and flame rise. If you want to build a fire apply your match to the kindling placed on the bottom. And if you want heat through the night it takes a full fill up and the vents set to the idle (not closed off air tight) position.
I say this after fifty years of heating my house with wood without any other back up. Right now it is +2 deg. F outside and I have just raked over the coals from the night fire in the furnace and thrown on a couple of maple log splits. It is quite comfortable inside here at the computer table.
Yep. Light the kindling from the bottom.
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Little John



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PostPosted: Sun Dec 01, 2013 6:11 pm    Post subject: Reply with quote

woodburner wrote:
In which case, extrapolating your experence, is not helpful in advising someone trying to run a relatively small steel box burner.
Well, I've use a variety of closed box burners as well as open fires, again of all sorts both inside and out. Every one of them started better by lighting from the bottom. Additionally, the structure of the fuel stack, prior to lighting made a big difference. The driest, largest surface area by volume and most flammable material was loosely placed at the bottom (paper and kindling) with as much air round the material as possible,. This was then stacked on top with progressively more massive lumps of fuel until the last thing to be placed on would be the log or the coal. If it was done carefully, then it could all be put on in one go at the start prior to lighting. Then loads of air allowed in at the bottom of the fire, whilst starving it of oxygen at the top. this would create a flue effect where the air would rush into the bottom of the fire creating a furnace effect.

There's a reason the grate has slots in it and is not a solid sheet of metal. It's cos that's where the oxygen most efficiently gets into the fire and feeds the flames to burn the fuel from bottom to top.

Finally, the other obvious reason for promoting the fire to burn from bottom to top is because of the fact that heat rises and, in doing so, it pushes all of the moisture out of the fuel yet to be burned.
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woodburner



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PostPosted: Sun Dec 01, 2013 6:46 pm    Post subject: Reply with quote

Read the links I posted, and learn something. Then look up some of your choosing about upside down fires and learn some more. I used to light from the bottom, until understanding combustion taught me otherwise.

You don't need much air from underneath for wood fires, and only then for starting in most cases. I am not interested in coal, it's dirty stuff, and makes dirty fires.
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Little John



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PostPosted: Sun Dec 01, 2013 7:23 pm    Post subject: Reply with quote

woodburner wrote:
Read the links I posted, and learn something. Then look up some of your choosing about upside down fires and learn some more. I used to light from the bottom, until understanding combustion taught me otherwise.

You don't need much air from underneath for wood fires, and only then for starting in most cases. I am not interested in coal, it's dirty stuff, and makes dirty fires.
I was taught by a variety of woodsmen out in the woods as well and my mam so you're okay WB. I think I'll be fine thanks. If, on the other hand, you are making a different point about catching the wood gas that escapes from wood due to not getting burned up on initial combustion, then of course oxygen needs to be mixed with that gas to promote combustion of it. However, we are into different territory there because this ideally requires a second capture chamber to fully utilise and consume such escaped wood gas an d, in any event, you will only get large amounts of wood gas escaping if the fire is not burning well.
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vtsnowedin



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PostPosted: Mon Dec 02, 2013 10:01 am    Post subject: Reply with quote

I am not against designs that inject secondary air at mid stream to more completely consume the wood gas before it enters the chimney. Stack temperatures above 250F are energy lost. But the design of that should be simple and not involve a lot of turns in the flow that can collect ash at low points. Complicated designs that only work properly etiher wide open or shut down tight can't provide the range of outputs you need when heating a house in a full season with it's temperature variations. The also require much more maintenance and frequent cleaning to keep everything working as designed. Something simple and of adequate size works better. You donít have to fill a large fire box full but you can't double fill a small one.
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RenewableCandy



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PostPosted: Mon Dec 02, 2013 2:06 pm    Post subject: Reply with quote

I can vouch for Clearview: we have one here which came along with us when we moved (buyers of the previous Chateau Renewable didn't want it, the twazzocks). A decade on and still roarin'!

I did put a post up about upside-down fires on here a while back, but I've never tried one in our stove. I think you have to pack all the stuff together really tight or the heat doesn't travel downwards and I'm afraid I just can't be arsed.

Seconded the flat top and rear stack exit so you can boil a kettle. Wish we'd had the presence of mind, etc.
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Little John



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PostPosted: Mon Dec 02, 2013 2:25 pm    Post subject: Reply with quote

In order to get a fire to burn downwards, one has to to fight against the natural tendency for it to burn upwards. The reason this is a natural tendency is because hot air rises. This means that the unburned material immediately above the burning material is going to be both hotter and dryer relative to any other unburned material in the fuel pile due to the constant supply of hot air rushing past it form below. It really is as simple as that. Now, of course, if a fire is left to its own devices after lighting from the top, then it will burn downwards in the absence of any material above it to burn, but it will burn less well as a consequence. Or, at least, it will burn less well for a good portion of the burn. That is to say, if the purpose of the fire is to get it to release its heat at a reasonably rapid rate.

Putting aside more esoteric concerns about the secondary capture of wood gas, as mentioned earlier,. the only remotely plausible reason I can think of for this fad in some quarters for top down burning is to reduce the chance of the heavier logs above the kindling, in a normal bottom up burn, from collapsing and snuffing out the burning kindling below. Or, at least, this seems to be the favoured rationale on the net that I have read. My answer to that rationale is that if this happens its because the person who set the fuel pile up prior to lighting it did a crap job in the first place. The central skill in making a good fire is the manner in which the woodpile is constructed and built upon as the fire takes hold.
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PS_RalphW



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PostPosted: Mon Dec 02, 2013 3:32 pm    Post subject: Reply with quote

The biggest fire I ever lit was a brush pile in the Czech Republic. It was 3 metres high and 10 metres long. It wasn't until after we lit it that we realised it had been built in a 5 metre deep depression.

The smokiest was a 3 metre high stack of fresh cut conifers on a very foggy and wet December day out in the wilds of Norfolk. The fire brigade turned up after complaints from car drivers 200M away, but left when they saw we had all under control.

The most disastrous was a small bonfire burning brash whilst clearing regrowth on Devils Dyke, a North-South linear ditch several miles long, and ancient monument and a nature reserve of un-improved grassland. It was March and there was a strong south wind. The fire jumped a couple of feet into a light growth of nettles and it immediately spread to the open grassland of the main monument, burning about 100 metres before the fire brigade arrived. I walked home that day covered head to toe in soot, scaring the local estate lads silly Twisted Evil
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kenneal - lagger
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PostPosted: Tue Dec 03, 2013 3:48 am    Post subject: Reply with quote

There are two separate issues here, top down lighting and top down burning fires.

Regarding top down lighting, the problem with lighting a fire in the conventional way is that until the firebox heats up sufficiently a lot of wood gas is driven off and does not light. This is because the hot gasses from the burning kindling rise up through the cold wood and are cooled and, although they drive out some wood gas, they can't ignite it.

With a top down fire far less wood gas is driven off initially as the heat from the kindling warms the wood underneath and at the side and then goes on to warm the chimney and promote a good draught. The wood immediately around the burning kindling starts off gassing and this lights because it rises through the flames of the kindling and there is plenty of air in the fire box. This small but intense fire heats up more quickly as there is more air in the firebox than smoke. Once the fore is burning well and the fire box has heated up you can then start adding logs to the top.

Having tried both methods I find that top down lighting is better. Don't criticise it until you have tried it.

Regarding top down burning stoves, they are started with a normal straight up draught but once the draw in the chimney has been established the damper is closed and the draught and inertia of the air moving in the chimney immediately draws the air and gasses down through the hottest part of the fire, heating it and causing it to burn like a blow lamp in the secondary burn chamber below the fire.

Examples of stoves available
http://ecape1820.tripod.com/id18.html
http://www.hearth.com/talk/wiki/downdraft-stove-operation/
http://www.hydro-to-heat-convertor.com/gasification.html

There was a discussion about this on PS a few years which included the URL for a video of one working but I can't find the thread. the video was awesome!
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woodburner



Joined: 06 Apr 2009
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PostPosted: Tue Dec 03, 2013 10:01 am    Post subject: Reply with quote

RenewableCandy wrote:
I can vouch for Clearview: we have one here which came along with us when we moved (buyers of the previous Chateau Renewable didn't want it, the twazzocks). A decade on and still roarin'!

I did put a post up about upside-down fires on here a while back, but I've never tried one in our stove. I think you have to pack all the stuff together really tight or the heat doesn't travel downwards and I'm afraid I just can't be arsed.

Seconded the flat top and rear stack exit so you can boil a kettle. Wish we'd had the presence of mind, etc.


RC, It's no more difficult (probably less so) than lighting at the bottom. I split logs to be about 2" thick to promote quicker drying, and this means setting the fire is easy. They are the length such that I can set them sideways across the base of the fire box. Next is a few bits of torn up corrugated cardboard in the middle (from all the boxes of stuff we get delivered Embarassed ) then 4 sticks 1/2" to 1" thick placed front to back, and lastly a piece of larger (but not much larger) wood placed across the back of the sticks.

Open the vents, light the cardboard, close the door as far as the latch and watch it go. The first half minute might be a bit of smoke, but once it goes there will be flames for ever after. You can watch the smoke burning at the top of the box, yet still be visible as smoke lower down. Bigger logs can go on once the fire has burnt most of the first lot.

It takes longer to read this than to do it.

There are some who refuse to understand the physics of radiation in top lighting, and understanding that however hot, smoke (the fuel) will not burn in the absence of oxygen. With top lighting, the fires start off in a place where the available oxygen can match the smoke production, or exceed it. This means all the smoke can be burnt. The age old "wisdom" of bottom lighting may sound correct, but it makes for smokey (inefficient) fires, while top lighting gives clean fires that annoy neighbours less. An example of radiation transferring the heat and nothing to do with up draughts, is the electric toaster. Leave the thing running long enough and you can set light to the bread. (Yeah, yeah, I know, they have timers fitted now, but they didn't before pop-up types. We still have one of the old ones in the attic.)

As KL says, anyone who believes top lighting is no good should try it before criticising. Then try making a top lit up-draught gasifier TLUD, from a few tins, and be amazed that you can sniff the gases after a minute or two without coughing, or your eyes smarting.

Another top lighting example

If you should get sucked into biochar, from trawling the stove videos, read this before you think it will cure the world's ills.
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Little John



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PostPosted: Tue Dec 03, 2013 10:10 am    Post subject: Reply with quote

kenneal - lagger wrote:
There are two separate issues here, top down lighting and top down burning fires.

Regarding top down lighting, the problem with lighting a fire in the conventional way is that until the firebox heats up sufficiently a lot of wood gas is driven off and does not light. This is because the hot gasses from the burning kindling rise up through the cold wood and are cooled and, although they drive out some wood gas, they can't ignite it.

With a top down fire far less wood gas is driven off initially as the heat from the kindling warms the wood underneath and at the side and then goes on to warm the chimney and promote a good draught. The wood immediately around the burning kindling starts off gassing and this lights because it rises through the flames of the kindling and there is plenty of air in the fire box. This small but intense fire heats up more quickly as there is more air in the firebox than smoke. Once the fore is burning well and the fire box has heated up you can then start adding logs to the top.

Having tried both methods I find that top down lighting is better. Don't criticise it until you have tried it.

Regarding top down burning stoves, they are started with a normal straight up draught but once the draw in the chimney has been established the damper is closed and the draught and inertia of the air moving in the chimney immediately draws the air and gasses down through the hottest part of the fire, heating it and causing it to burn like a blow lamp in the secondary burn chamber below the fire.

Examples of stoves available
http://ecape1820.tripod.com/id18.html
http://www.hearth.com/talk/wiki/downdraft-stove-operation/
http://www.hydro-to-heat-convertor.com/gasification.html

There was a discussion about this on PS a few years which included the URL for a video of one working but I can't find the thread. the video was awesome!
The only reason you are going to get a lot of wood gas escaping from poorly burning top wood in a fire lit from below is due to incorrect choice of top wood on the fire, given it's current combustible state. If a fire is lit properly and managed properly as it builds in intensity, then the wood that is placed on top of it at any given time is going to be sized appropriately to that fire at that given time.
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kenneal - lagger
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PostPosted: Tue Dec 03, 2013 5:14 pm    Post subject: Reply with quote

If start with just kindling and sit there and gradually feed in larger wood you can start a fire smokelessly. Personally, I don't have the time to do this so a top lit fire is quicker, easier to manage and works just as well.
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Little John



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PostPosted: Tue Dec 03, 2013 5:37 pm    Post subject: Reply with quote

kenneal - lagger wrote:
If start with just kindling and sit there and gradually feed in larger wood you can start a fire smokelessly. Personally, I don't have the time to do this so a top lit fire is quicker, easier to manage and works just as well.
I would still call that into question K. That is to say, assuming the wood is appropriately dry (if it isn't then it won't matter which way you try to light it), then it is simply a matter of using the correctly sized and positioned pieces of wood stacked such that there is a structural integrity to the pile and that there is plenty of room for the air to circulate. The simplest kind of structure is a lattice one where one layer of kindling is placed front to back and the next layer is laid right to left etc. This means that when the bottom layer disintegrates, the layers above still retain a structure that allows for the fire to develop and increase in volume. Similarly, there should always be a supply of appropriately sized logs from thin to thick

If you are basically saying that in the absence of a capacity, willingness or time to use dry, appropriately sized wood constructed into a fuel pile of optimal structure to promote air circulation, then it may not be any worse, or even slightly less worse, to light it from the top down, you may be right. However, it strikes me this is a solution to a problem that has been invented especially for it.


Last edited by Little John on Tue Dec 03, 2013 6:26 pm; edited 1 time in total
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woodburner



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PostPosted: Tue Dec 03, 2013 6:21 pm    Post subject: Reply with quote

Rolling Eyes
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