Top Down Fires (upside down fires) - how to light them and why they work

[Zak33]

We had a good chat about lighting a stove here on the forums so I thought we'd go forwards and discuss Top Down Fires

When I was a kid I was taught to light a little fire, and then gently add larger and larger pieces of wood above it to get it going properly. And while this works, there are a few major flaws in it. And in particular with a modern wood stove, the'y're fundamental flaws.

I am going to assume using a STOVE and good quality, well seasoned fire wood with a moisture contrent of way under 20%. The lower the better because otherwise burning wood takes up loads of energy just drying the water out from within before the firewood does a proper job or warming you up!

Right here goes - a Top Down Fire, or an Upside Down fire involves laying out your largest pieces of wood first at the base, then medium sized and finally building a little fire of kinding on the top. You light the small fire on top and as it burns, it begins to heat up the wood below it and falls in on itself to catch the lower wood alight.

But there's a more complex thing occuring - the air around the wood and the wood itself, up to about 275'C is only just letting go of the moisture and some creosote. The outter edges of the wood that are burning, are hotter, clearly, but you're not getting the full use from the wood yet.
This is primary combustion

As the wood heats up further, it begins to let out the combustible gases from within itself, and for them to burn they need a flame becuase the stove isn't hot enough yet to help the combustion. These are secondary gases. It's where LOADS of your heat comes from IF you can use these secondary gases.

If you light a fire under, like I used too....and stack wood on top of it, the combustible gases whoosh up the chimney and don't burn and you lose the benefit. Becaus there is no flame above.
If you have the fire and the associated flames ABOVE the wood, as the gases excape, they are burned better by the fire on the top and not only do you get more heat, the stove heats faster AND the chimney gets less crap in it

But we're not done

Moving on...

As the fire temp increases further (I'm talking about the core of the fire temperature, not the temp of your whole stove!!) different gases are released and they COULD burn better, but they need a mixture of the right high temp (nearly 600'C) and the right amount of oxygen.
Too much air and the temp drops too far, not enough and the combustion slows. Those secondary gases are methane and methanol, and you all know those are fuel gases... to get them to be released properly (and burn efficiently) you need a hot burn. Flushing loads of cool air in won't work - That's why we have air controls on stoves. Most stoves allow the air wash to control this and on my own stoves I have found different settings for both. Good fires, burning well but, using less wood and creating huge heat.

Finally you are left with charcoal, and that too needs to burn but its not as calorific. We still want it though and it will burn as the next layer of wood is added to the stove.

BUT remember... you are no longer in Top Down mode ... the fire is HOT and adding the logs drops the firebox temp until the door is closed and the temp comes back up.. the wood heats.. moisture is driven out.. the wood starts to burn.. the first gases are released... the flames in the stove from the last logs help them to ignite and then the secondard gases are released and it all goes again

A word of caution- before opening a stove that's been running low levels of air to reload it.. open the air intake full for a minute. It will burn off the gases I have described above . If you don't do that you CAN get a sheet of flame erupt out of the stove. I know.. I have acheived it

[CountStex]

Thanks for laying this out. It's a method I've only just adapted to after having lived in Denmark for almost 7 years now and using a wood burner in conjunction with a heat pump to warm up your house. I've definitely seen benefits in how quickly the rooms warm up using this technique.

[Zak33]

Thanks for laying this out. It's a method I've only just adapted to after having lived in Denmark for almost 7 years now and using a wood burner in conjunction with a heat pump to warm up your house. I've definitely seen benefits in how quickly the rooms warm up using this technique.

now Denmark and her neighbours understand stoves better than the UK.

It is my understanding that the Norwegians now promote the top down fire to the nation in an attempt to lower emmissions from stoves. Everytime someone lights a stove, it helps lower the emmissions

[kalniel]

OK, but doesn't this fall down (no pun intended) if you use paper as a firestarter? It assumes your lightest material/kindling is still heavy enough to remain where it is when it's burning. If you use paper (rookie, I know) then it'll just fly up and set your chimney on fire as it's burning, so you need to avoid paper, or still have some heavier kindling on the very top to prevent it, right? It'd be more like:

Kindling
Paper
Logs

[Butcher]

If your wood is dry you can light kindling straight off a match (or use a small amount of firelighter), you shouldn't need paper to get a fire going. If your kindling won't catch it's either too big or too wet.

[Zak33]

OK, but doesn't this fall down (no pun intended) if you use paper as a firestarter? It assumes your lightest material/kindling is still heavy enough to remain where it is when it's burning. If you use paper (rookie, I know) then it'll just fly up and set your chimney on fire as it's burning, so you need to avoid paper, or still have some heavier kindling on the very top to prevent it, right? It'd be more like:

Kindling
Paper
Logs

It's OK to use paper if you have a very cold flu... so here's how to do it so it doesn't fly up the chimney.

Lay each sheet out flat and roll it from the corner to make a large paper sausage. Then tie it into a loose knot, like a giant pretzel. It's ok to use paper if you need too - you can lay the paper around the kindling.

But I admit I don't use paper, just kindling. I make feathersticks, and split a lot of dry pine, off cuts from woodwork and pallets etc. Thin driy stuff. But there are times on cold days with cold chimney flue's when it's worth it for speed. The extra heat created by newspaper helps warm the chimney as well as catch the kindling.
As an aside- open a window a little in the room you're lighting the stove in, to equalise the outdoors air pressure with the room pressure to help the chimney flue draw properly until its warned up a little.

[Ttaskmaster]

If your wood is dry you can light kindling straight off a match (or use a small amount of firelighter), you shouldn't need paper to get a fire going. If your kindling won't catch it's either too big or too wet.

Kindling and feathersticks take time to prepare, and I'm usually knackered from shifting all the logs into and from the woodstore... newspapers are free from the commuter people at work, and can be done by a freezing cold wife who doesn't have the patience to geek about with bushcraft skills. She cleans the fireplace out anyway, so it matters little in the end!

[Zak33]

Kindling and feathersticks take time to prepare, and I'm usually knackered from shifting all the logs into and from the woodstore... newspapers are free from the commuter people at work, and can be done by a freezing cold wife who doesn't have the patience to geek about with bushcraft skills. She cleans the fireplace out anyway, so it matters little in the end!

here's how it is in my house.

I enjoy cutting wood, I love splitting logs, and I enjoy fire lighting. Feather sticks are a pleasure, and a treat to make, and I genuinely enjoy cleaning my stove

I know... all very odd. But it's part of who I am now. Cleaning a wood stove, polishing the glass and preparing a fire.. are a relaxing thing for me. Prepping firewood, building an extra log store, and seasoning wood are who I am, along with a love of PC Hardware and shooting guns

[Butcher]

In my house the logs are shifted into the log store by the log delivery man.

I normally keep a stock of logs and kindling by the fire so I can just light one as required. I'll split a batch of kindling when it gets low.

[Ttaskmaster]

I know... all very odd. But it's part of who I am now.

It's not odd at all. I just don't have that kind of time any more. You're lucky if I've swept the wood shavings off the workshop floor this month.
I do have a very nice big woodstore, though!

In my house the logs are shifted into the log store by the log delivery man.

Ours are all too fat to fit down the narrow side passage...

[Zak33]

It's not odd at all. I just don't have that kind of time any more.

I choose to do it instead of watching a film or playing on my PC.

[CountStex]

As an aside- open a window a little in the room you're lighting the stove in, to equalise the outdoors air pressure with the room pressure to help the chimney flue draw properly until its warned up a little.

My mother in law swears by that technique, however I find just leaving the door ajar 4-5cm until the fire lighters have caught the starter wood is enough nine times out of ten. I should point out however we have an oil powered boiler and it uses the same chimney so it's probably pre-warmed most of the time when I start the fire in the morning.

[Ttaskmaster]

I choose to do it instead of watching a film or playing on my PC.

It's the sort of thing I might do while watching a film... but I'd have to vacuum up afterward and I don't trust the dogs to stay away from the sticks!!

[Zak33]

My mother in law swears by that technique, however I find just leaving the door ajar 4-5cm until the fire lighters have caught the starter wood is enough nine times out of ten. I should point out however we have an oil powered boiler and it uses the same chimney so it's probably pre-warmed most of the time when I start the fire in the morning.

she's right and so are you.

When a chimney is cold.. particularly on a damp cold day.. the smoke from a fresh fire climbs the chimney,... cools... cools more.. and then falls back down the chimney and smothers the fire. But worse... it then escapes out of the stoves intake vents and fills the room

I know.. I have done it many times. But having the window open helps a lot. If it's a very cold damp day, ... I expect to ue two matches and I split more kindling than normal, really thin.. so it definately starts and the heat pushes up the chimney.

Oil powered boiler using same chimney... not heard of that but it's very likely you're sorted.

[Butcher]

Tried a top down fire this evening. Worked like a charm. Lit it three hours ago and it's still hot now.

[Zak33]

Tried a top down fire this evening. Worked like a charm. Lit it three hours ago and it's still hot now.

next job it to light it, then after 5 minutes go check the smoke outside... should be minimal

I lit a MAMMOTH one yesterday outdoors to burn a mountain of laurel trunks. It was the ultimate chance to really try the principle, and to ensure minimal smoke hit the horizon.

It worked a treat