Top-down Burn not the Best?

  • Active since 1995, Hearth.com is THE place on the internet for free information and advice about wood stoves, pellet stoves and other energy saving equipment.

    We strive to provide opinions, articles, discussions and history related to Hearth Products and in a more general sense, energy issues.

    We promote the EFFICIENT, RESPONSIBLE, CLEAN and SAFE use of all fuels, whether renewable or fossil.

qwee

Feeling the Heat
Jan 17, 2013
367
Idaho
"......Another emissions-reducing technique recommended in the last few years has been the top-down burn (ignition from the top). One surprising result from this test series was that the top-down burn performed worse than front (bottom) ignition....."

This is a Masonry Heater study in Austria. I wonder if it is the same for wood stoves?
https://www.mha-net.org/docs/v8n2/docs/Austrian Field tests 1993.pdf
 
1993 doc about even earlier testing. It's interesting, but I am not sure there is much correlation to modern EPA stoves.
 
A quick read and came away with the impression that the emissions they mentioned was CO only not particulate which was not measured.
 
  • Like
Reactions: St. Coemgen
I have a harder time with top down starts. Piling the kindling east / west in front of the air inlet works much more consistently. The wood in my stove always seems to burn from front to back more than top to bottom, so having a good fire in the front gets that off to a good start.
 
Reading closely and including the statement immediately after the OP s quote


A more thorough analysis
yields a plausible explanation
for this effect. The top-down
burn results in a longer burn
time, resulting in a lower burn
rate (kg/hr). At rated heat
output (maximum wood load), we
see a positive influence, since
the lower burn rate assures that
there is an adequate oxygen
supply during all phases of the
burn.
With a partial fuel load -
which is usually the case, in
practice and in this test series
- top ignition can result in too
low a burn rate, leading to lower
firebox temperatures and an
adverse effect on the quality of
the burn. A higher burn rate is
more advantageous with partial
loads, and this is achieved with
bottom ignition or reloading onto
a coal bed.”


so top down is better on a cold start with a full load. Not as good with partial load as burn rate is to slow to get temps up for a clean burn before fuel is exhausted. How often are fires lit in masonry heaters and not restarted on a coal bed? Average efficiency of 83% seems a bit high. I still don’t understand how these designs are not maze like creosote factories. The narrative that these are amazingly efficient heaters with no secondary combustors can only be true if the flue exit temps are very low which translates to creosote. Am I missing something?
 
Both our Buck and Kuma stoves start best top down. I'm sure every brand is different as far as what it likes at start up. We put 4 larger 3" splits on the bottom n/s. Then 4 or 5 2" splits e/w, then back n/s with 1" or less kindling and lastly 1" e/w on top. Add one fire starter and were good to go for a few hours.
 
The reason bottom or side burns are better is that they get things going faster. The cold start time until things are up and going is when the most pollution occurs. In the big picture, a wood burner who starts his/her fire from the bottom and gets their firebox hotter will create less pollution then a top-down burner. The top-down burner is keeping the starting fire in the stage where it creates the most pollution - longer.
Top-down starting makes burners think they are creating less pollution - but science says otherwise.

I want to burn in the least polluting way, so I'm going to follow the - get it going and hot ASAP strategy.
 
Last edited:
I like the top down on a cold start. If you leave a space in the middle top to put the paper & kindling, it heats the tubes up quick, then stick another split in the space once it's going. Bottom front starts seem to take longer for the stove to heat up for me, too many places for the flames to go instead of up to the burn tubes.
 
I've switched to top-down based on how quickly I see the chimney stop smoking and have the flue hot. The other thing I prefer is that top-down avoids the bowels of hell fire-bloom that happens when all the wood gets heated up all at once.
 
Maybe top-down starts smoke less, but will smoke for longer period, and bottom-up starts smoke more, but the period of smoking is shorter? For the total amount of smokes a top-down may not be a win.
I never look back after I learned how to do top-down starts. The heavy smoke for a bottom-up start is embarrassing, even if it may not last very long.
 
Top downs with full-loads allow you to not open the stove door as frequently, which would reduce indoor pollution, as hypothesized in another recently in posted article in Hearth.com
 
If I had a masonry heater, I would probably do bottom-up. The goal here is to get the fire hot and burn it quickly and hot. This heats up the labyrinth flue passages quickly and keeps the flue temp hot enough to not accumulate creosote. Considering that the whole system is essentially based on stack robbing of the heat and transferring it to the masonry, this makes sense.
But this is not how I run a woodstove unless it is in the teens outside and I want the stove to stay around 700ºSST. For normal heating with the woodstove, I want a much longer burntime and steady release of heat over many hours. This is in lieu of a thousand pounds of masonry to do that job, so a regulated fire is what works best.
 
The question is: You have the same wood stove in the same house. In one you do a top-down burn and in the other you do side or bottom burn. Let's say the two methods are done over an entire season. Science appears to say that the side/bottom burner is polluting less.
 
The question is: You have the same wood stove in the same house. In one you do a top-down burn and in the other you do side or bottom burn. Let's say the two methods are done over an entire season. Science appears to say that the side/bottom burner is polluting less.
One sample does not make for good science. It would be great to see this testing done with a modern cat and non-cat stove.
 
This is a bit ironic, it's the article that John Gulland picked up on. It converted him to top-down starting for everything from woodstoves to campfires. The article was first published in the masonry heater news.
 
Agreed. We all need to know which method is best. So someone needs to do an official study with wood stoves. Someone who can hook up devices to monitor this stuff. Until then, I'm leaning on the side of side/bottom being the best method. A firebox is a firebox, and those Austrian masonry folk are a pretty reliable source.
 
I’m liking the top down burn in the insert but top down works great in the wood boiler.
 
The question is: You have the same wood stove in the same house. In one you do a top-down burn and in the other you do side or bottom burn. Let's say the two methods are done over an entire season. Science appears to say that the side/bottom burner is polluting less.
That's an anecdote, not science. And bgreen is right. The stoves were quite different in the early 90's. I'm entirely certain that top down starts are way cleaner in my modern EPA stove. Particularly once the tubes heat up, which happens much faster with a top down start.
 
I'm entirely certain that top down starts are way cleaner in my modern EPA stove. Particularly once the tubes heat up, which happens much faster with a top down start.
Same thing for a cat stove, the sooner the cat is active, the better.

John Ackerly has the Green Heat lab done any testing with modern stoves with regard to the best starting methods for achieving the lowest emissions when starting a stove?
 
On one side you have getting the firebox hot to burn as much stuff as possible vs a top fire burning slowly, and burning off gases as it is going along (maybe with the aid of the metal baffles?).

In my mind, it makes more sense that getting the firebox hotter faster will create less pollution over the long run. But, once an idea is ingrained in being better it is hard to think there may be a better method.
 
On one side you have getting the firebox hot to burn as much stuff as possible vs a top fire burning slowly, and burning off gases as it is going along (maybe with the aid of the metal baffles?).

In my mind, it makes more sense that getting the firebox hotter faster will create less pollution over the long run. But, once an idea is ingrained in being better it is hard to think there may be a better method.
Now you are catching on.

There are stainless steel tubes that feed oxygen to the top, or a catalyst on the top, on the way out of the flue that are the primary pollution controls. Once they are working, the burn is clean. A bottom up fire kicks out so much smoke before the controls become active, that it's not even close. Heating up the whole stove is not what's making the stove hit the EPA required numbers.

For my part, this is not ingrained, it's observed. And completely obvious from the stoves engineering to hit the EPA numbers. Circa 2020, not 1993.
 
  • Like
Reactions: ct_administrator
I wonder how much species of wood has to do with emissions on startup? I always start with pine, spruce, or bone dry birch.

I always start bottom up, I can't get a quick or clean start with top down. I lay 3 splits north south at the base about 3 inches apart, put birch bark and a cup of wood pellets in that space between, then place 3-4 splits on top east west and ensure there is about an inch gap between these. Then light the base. The birch bark lights quick and the pellets add rapid heat to get the fire going, the flames travel up and between the second layer and quickly to the secondary tubes, lighting the entire stack in the process.

I'll have to get a video of this, that way I can measure the time it takes from initial lighting to secondary ignition. But I definitely don't get a lot of smoke from the chimney on startup, and it's gone pretty quick.
 
The one downside to the 74 is it is not very deep....15 or 16 inches I believe. With the firebox being wider, I cut all my firewood for e/w loading but have wondered since a lot of folks here load n/s if this is a factor in the top down burn being better. I know a top down burn takes longer to catch all the wood thus creating a longer overall burn. But when I get home after being gone all day I want the stove up to temp as fast as possible. It just seems forever loaded e/w for a top down to get the stove up to temp. Does n/s loading increase how fast a top down burn ignites all the firewood or is it about the same?
 
Now you are catching on.

There are stainless steel tubes that feed oxygen to the top, or a catalyst on the top, on the way out of the flue that are the primary pollution controls. Once they are working, the burn is clean. A bottom up fire kicks out so much smoke before the controls become active, that it's not even close. Heating up the whole stove is not what's making the stove hit the EPA required numbers.

For my part, this is not ingrained, it's observed. And completely obvious from the stoves engineering to hit the EPA numbers. Circa 2020, not 1993.
Yes but those tubes aren't going to be activated until the fire gets going. The small top down fire probably isn't going to bring these tubes into play - the bottom fire is more likely to get these tubes up and going because it gets the fire going faster.