Maths behind a woodburner

[thinfourth]

I am planning to build a woodburner for my workshop

To save sodding about too much is there any basic rules regarding flu size, air inlets, baffles, secondary air etc


The basic idea i have is big rectangular box sitting on its end. Fire in the bottom half and the top half full of 1" box section. the outside of the box section exposed to flu gas and the internals open at one end with the other end in a box which would be fed by a fan blowing hot air round about butt height

 

[stormin]

Sounds a bit like you're over complicating it. The heat will radiate from the outside of the box anyway. Unless you have some need to duct the heat some distance away I can't see the point of the box section heat exchanger.

 

[thinfourth]

Some heat will radiate from the box

But loads of heat from the flu gases will go up the flu and vanish

 

[stormin]

Some heat will radiate from the box

But loads of heat from the flu gases will go up the flu and vanish

You need some heat going up the flue to get it to work. To little and your flue temperature will drop leading to deposits forming inside the flue and lack of draft onto the fire.
A simple baffle inside to stop heat going directly up the chimney and force the air around the firebox is usually enough for most commercial stoves. Controlling the air into the combustion chamber controls the rate of burn and to some extent the flue gas quantity/velocity.

 

[Rig Pig]

Don't over size the wood burner, they provide a vast amount of heat from a small fairly simple box. The best way of making a good one is copying a known good design. How much heat are you looking for?

 

[arthur]

Our house is heated by word burners, as well as we sell a good few second hand ones in the yard, as said good advise is study those that work, flu size is often much bigger than that of intake air vents, and defo look at size, small burner kick out a massive amount of heat!

Gasification is the way forward.

 

[outofthefire]

I'd line it with fire bricks. Personally I prefer to have a damper in the flue so you can shut down the air flow last thing at night, so the logs just smoulder. Open everything up in the morning put some wood in and generally there's enough hot embers to get it going again.

 

[sparkysy]

@ outofthefire
Do you wrap your last log on the fire with newspaper before you shut it right down? it will definitely still be good in the morning, good trick me dad taught me!

 

[outofthefire]

@ outofthefire
Do you wrap your last log on the fire with newspaper before you shut it right down? it will definitely still be good in the morning, good trick me dad taught me!

No, i've not heard that before, i'll give it a go though.

 

[sparkysy]

Use a whole paper or 2 wrap it reasonably tight around a log about the diameter of your forearm, it works a treat!!

 

[GrumpyOne]

Build a Rocket heater, flue is a lot cooler than a normal woodburner.

 

[Kent]

take care, copy a good design, defiantly line with fire brick and fit co2 detectors

 

[thinfourth]

Build a Rocket heater, flue is a lot cooler than a normal woodburner.

A rocket heater appears to be a insulated combustion chamber with a large area for the heat to transfer from the flu gas into the room

Most plans that i can find easy looks like a huge concrete thing as they are all mass heaters. For this a mass heater would be useless as i want fast acting for a workshop.




My orginal plan we a combustion chamber with a large area for the heat to transfer from the flu gas to the room

Just i was going to use forced air for heat transfer

I am more looking at is flu is X area then air intake should be Y

 

[J.B. Welding]

Hi Thinfourth, hope I can help a bit-your idea with the fan and tubes is a good one it works well in a big workshop, my mate has one similar in his woodshop and the fan does a great job at warming the whole shop quickly. If your putting a baffle in it should be about 2/3 the area covered, fire bricks make a huge difference all around the firebox and the ones from electric storage heaters are proven to work. A 5 or 6" chimney on an 'average' sized stove with a 3 or 4" primary air inlet putting air into the bottom of the fire will let you get the fire roaring to start it quickly, shutting down a bit to get good combustion as the firebed is established.
Having preheated air coming in over the fire-ideally along the edge of the baffle or top of door opening will give an efficient burn. Try to keep your chimney about 150-200 degrees c to stop creosote problems and if possible insulate the bit outside to keep it warm to stop above problems. Hope that helps-you could put a few too many heat exchange tubes in to start then blank them off if the chimney gets too cold, also a thermostat controlled fan will keep you in the right zone.

 

[Parm]

Hi Thinfourth, hope I can help a bit-your idea with the fan and tubes is a good one it works well in a big workshop, my mate has one similar in his woodshop and the fan does a great job at warming the whole shop quickly. If your putting a baffle in it should be about 2/3 the area covered, fire bricks make a huge difference all around the firebox and the ones from electric storage heaters are proven to work. A 5 or 6" chimney on an 'average' sized stove with a 3 or 4" primary air inlet putting air into the bottom of the fire will let you get the fire roaring to start it quickly, shutting down a bit to get good combustion as the firebed is established.
Having preheated air coming in over the fire-ideally along the edge of the baffle or top of door opening will give an efficient burn. Try to keep your chimney about 150-200 degrees c to stop creosote problems and if possible insulate the bit outside to keep it warm to stop above problems. Hope that helps-you could put a few too many heat exchange tubes in to start then blank them off if the chimney gets too cold, also a thermostat controlled fan will keep you in the right zone.

Although not my question I think you've hit the spot here and some of us others may also find this usefull

 

[julianf]

Night storage heater bricks are better than nothing, but theyre not ideal. Theyre structurally stable under temperature (unlike other blocks) but they do not insulate that well. They are good for smoothing out hot spots etc. but for insulation, you really want the light weight blocks.

We have night storage heater bricks in our log burner, so im not just recounting what ive read... : )


If i was designing a stove, with no consideration to construction issues, i would have a low(er) temperature fire box feeding into a highly insulated secondary chamber that was kept very hot. The function of the first chamber would just be to get the solids into a vapor, and the second chamber would be where the full oxidation would occur.

Of course, on a normal stove, both happen in the same place, but there is advantage to separating them - largely that the temperature of the 2nd chamber is independent of you chucking lumps of cold fuel on.

I would not try to strip any heat at all from the 2nd chamber, but do that to the gasses once they had left it - in a 3rd chamber, if you like. I might even run the air for the primary chamber from this (so it was pre-heated) to get the whole job hotter.


Basically, if you burn stuff hotter, you burn more of it. You dont get tar etc on the stack when the fire is burnt very hot, as all the tars etc burn (and then you get the heat from them - which is what you are after anyhow!). So, having the burn in areas where youre not trying to get heat from means a more efficient burn, and more heat out at the end.


I think i said before - we have no other source of heating or DHW at the moment, other than solid fuel - so you end up putting in some though when youre feeding a fire all the time.

 

[outofthefire]

Basically, if you burn stuff hotter, you burn more of it. You dont get tar etc on the stack when the fire is burnt very hot, as all the tars etc burn (and then you get the heat from them - which is what you are after anyhow!). So, having the burn in areas where youre not trying to get heat from means a more efficient burn, and more heat out at the end.

If your burning everything hotter aren't you risking a chimney fire or is the theory that there is nothing combustible going up the chimney?

 

[J.B. Welding]

If your burning everything hotter aren't you risking a chimney fire or is the theory that there is nothing combustible going up the chimney?

Technically, the ideal firebox temp. is about 750-900 degrees. At that temperature the wood will happily give off the gas that burns and tars will also combust, given that enough..but not too much.. secondary air is provided, also at the above temperature. A hotter than 950c temperature in either the fire box or secondary combustion chamber can encourage nitrogen to bond with excess oxygen forming nitric oxides with are not really desirable exhaust gasses..
Although it is possible to achieve temperatures above 1000c burning wood it is not recommended for the environmental reasons mentioned.
The idea is to burn off all the gas and tar from the wood and extract the heat before the chimney area-so chimney fires are avoidable by
1) burn untreated wood under 20% moisture
2) burn off tar and keep the chimney up to temp. so that nothing combustible sticks to the chimney,
3) limit excess oxygen in the chimney which could fuel any fire therein.
4) clean chimney every so often, just incase!!

If you ever look at your chimney stack and see dark smoke coming out something is wrong-a whiff of white or no visible reek at all is what to aim for really. What that's got to do with the Pope though I have no idea so don't ask.

 

[julianf]

I did not say that the flue gas up the stack was hotter - that would be a waste.

You would, ideally, burn hotter, and then suck out the heat.

And, yes, a chimney fire can *only* happen after a lot of poor burning. Burn hot and you don't have to worry about tar in the stack, as it will have been oxidised in the fire itself before it ever gets near the stack.

Our Rayburn gets hot enough to make the plate glow cherry occasionally. At those temps, you're not going to get creasote forming when the gasses cool, as there is no chance of it passing through un-oxidised (unless the smoke is being blown through too quick - hence my earlier talk of a hot secondary chamber for high temp dwell)

 

[KemppiFrog]

[QUOTE="J. What that's got to do with the Pope though I have no idea so don't ask.[/QUOTE]

That is about putting straw in the mix to inform the faithfull when the cardinals have elected a fresh pope;
Having knocked out quite a few log burners. It is interesting to see how the big guys build fairly simple but effective units. While we wonder about extra chambers and features for a few more %. Cost effectiveness?
KF