January 18, 2012
Light Wood Fires From On Top
The Norwegian Institute of Public Health says for healthier air light your wood burning heaters from the top, not the bottom. An added benefit: more complete burning means higher efficiency. You get more heat from the wood.
- when the wood is lit from above, the heat radiates to warm the wood underneath. These pieces of wood will begin to emit gases which will rise, meet the flames and ignite
- if you light from below, the heat radiation will cause the wood over the flames to release gases which will rise. Without flames at the top of the wood, the gases will be released unburnt out of the combustion chamber to the chimney, where they will form particles
- the particle emissions from the chimney will be halved
- you gain maximum heating efficiency from your wood and thus lower your heating costs
Before anyone states "this is obvious": I've seen wood fires lit from the bottom many times. It is usually seen as easier to light from the bottom. Kindling can light bigger logs above it.
But it makes sense that a fire on top can burn the gases that get released from warming wood underneath them. So try to light the top.
Update: "pond" points to instructions on how to easily light from the top. What's still missing: How to easily load new logs into a burning fire bottom-up?
If you have wood that's dry enough to light from on top that might work. If the wood is not dry enough the kindling will not heat it enough to get it to ignition temperature before the kindling burns out.
If you had one match. . . would you light a fire from the top, or the bottom?
Seriously eggheaded advice. I heat my house with wood. I light a fire in the woodstove every morning. If I tried to light it from the top, I would have a hell of a time (in fact if it fails to light on the first try I will often take the wood out so that I can put paper and tinder underneath again for the second try... lighting it on top is ineffectual, it just doesn't transfer enough heat to the wood to catch it on fire). Given a big enough vertical stack, I suppose you could imagine a fire being lit on top (you'd still start it a few inches *below* the top) which then burned down through the layers. But that isn't the usual configuration. Further, 90+% of the wood gets added after the initial start. It goes on top because there is no easy way to stuff it in underneath the embers. That wood does indeed release smoke for a while until it's been turned to charcoal. Fixing that problem would require some sort of afterburner design to properly burn the pyrolysis gases.
Top Lit UpDraft (TLUD). It's how simple yet effective stoves are made. It does require dry material, but then all fires work better with low moisture feedstock. The alternative approach is an afterburner to get energy from the emitted gasses.
If I was down to one match I'd first reflect back on the foolish decisions I made that led me to that point.
Granted wet wood is a problem. Argues for the need for a starter heat source that is dry (some dry wood chips) or not wood.
Sure, start a few inches below the top. Requires a little planning and materials prep.
Adding wood at the bottom: Sounds like an engineering problem of fairly low difficulty. How can one add wood at the bottom of a roaring fire? Or how to use the fire's heat to warm up the wood in a way that causes the wood's out-gassing to get burned?
The Google search even returns videos and text web pages about Top Lit UpDraft (TLUD). Cool.
woodheat.org has a step by step tutorial on how to light a woodstove fire from the top, with many pictures:
I switched over to top-lighting last year. It works and it works as advertised.
If your bottom logs don't catch, then use more kindling.
Thanks for the tutorial. Looks like it involves more work splitting, but if top burning also results in significantly cleaner chimney and lungs, and more thermal energy per cord it could be well worth the trouble. I'll give it a try.
I'm delighted and a bit surprised to report that lighting from the top per pond's tutorial works very well, at least with recently split Black Locust firewood. I usually light only once per day, reloading several times over the next 24 hours. Does anyone have any ideas for a simple bottom reload system?
Could one use grating to create burning layers in a wood stove. In theory one could reload the bottom layer with larger chunks of wood that will start slowly while reloading more slender slices of wood up in the higher layers that can burn quickly. Basically, have a top layer that burns off the gases from the lower layers.
Another idea: Imagine putting wood on top of a stack of 2 or 3 grates. When the wood gets fairly burned have a way to lift up the top grate and put new wood on top of the middle grate. Then when it has burned lift it up and put wood on the bottom grate. By the time it finishes burning lift it and then slide out the top grate (which should be bare by then and slide the top grate underneath the previously lowest grate. So basically rotate the grates. One would need a way to lift grates with leverage.
Many stoves have a secondary chamber for burning gases released during the main combustion chamber. Thus there is no problem. Get a stove like a Yotul or such.
Reloading is not so difficult since I've fashioned a rake/shovel implement. This is used first to move the coals to the front of my 28 inch deep firebox. Then, wood is loaded as described in pond's tutorial, except for kindling. Lastly, the rake/shovel is used to move the coals to the top of the new pile. This technique works well, and costs less than a new woodstove.