Overdraft or too little draft

Flodorf

New Member
Oct 18, 2020
11
Netherlands
Dear Hearth forum users,

I have found many great tipa here on the forums, but one thing that still puzzles me is whether or not my wood stove has too much draft or too little.

What is the typical behavior of a fire with too little draft?

The situation is as follows. When I light the stove, using the top down method, the kindling generally starts burning rapidly. Primary vent is fully openend. However, it takes ages before the bigger logs catch on fire. When the fire reaches the primary air inlet, the logs will start burning quite fiercely here, while the remaining logs are still unlid.

Once I feel like the fire burns well, lots of flames, I close down the primary and try to run it on secondary. However, the big flames tend to die down and all I end up with is small flames, nothing that looks like a 'lazy' fire.

If I open the primary again, the fire might start raging near the vents again and get very hot.

I am not sure if I should increase the draft. I don't have damper to control the chimney, just metal I can break away to increase the opening from the stove to chimney. I can't replace this, so I am hesitant to experiment.

I have a Barbas 270 unilux (european brand). It is connected directly to outside air, so no problems with air supply.

Generally my glass gets a little dirty, easy to clean, but never clean.
 

begreen

Mooderator
Staff member
Nov 18, 2005
84,790
South Puget Sound, WA
Assuming that the stove has been installed properly according to the manual's requirements for flue diameter and height, the most likely issue is that the wood is not fully seasoned and damp in the core. Try mixing in some construction cutoffs of wall studs in order to mix in some known dry wood and see if that improves the fire.
 
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Flodorf

New Member
Oct 18, 2020
11
Netherlands
Assuming that the stove has been installed properly according to the manual's requirements for flue diameter and height, the most likely issue is that the wood is not fully seasoned and damp in the core. Try mixing in some construction cutoffs of wall studs in order to mix in some known dry wood and see if that improves the fire.
Thanks for the reply. I forgot to mention I am using seasoned wood with the right dryness (<20%). I even got a measuring device to make sure I got proper wood.

It should be properly installed. They put in chimney lining before installing the stove, so the flu is practically brand new.
 

begreen

Mooderator
Staff member
Nov 18, 2005
84,790
South Puget Sound, WA
How is moisture checked? Is the wood being tested after being resplit, on the freshly exposed face of the wood?

Is the house very well sealed? If so. try opening a nearby window a couple of centimeters and see if that improves performance.
 

begreen

Mooderator
Staff member
Nov 18, 2005
84,790
South Puget Sound, WA
Does the 270 spill out smoke when the stove door is opened?

Does it have a 180mm flue pipe all the way up to the chimney cap? Approximately what is the length of this flue pipe?
 

Flodorf

New Member
Oct 18, 2020
11
Netherlands
Thanks for the tip. I believe that it should not matter, as the stove is directly connected to the outside via a pipe. I will try doing so regardless, to make sure!

Part of my question remains, what does a fire look like with too much draft and with too little draft?
 

Flodorf

New Member
Oct 18, 2020
11
Netherlands
Does the 270 spill out smoke when the stove door is opened?

Does it have a 180mm flue pipe all the way up to the chimney cap? Approximately what is the length of this flue pipe?
When I poke the smouldering coals during a refill and they start smoking it will come out. When I light the fire with a starter, that smoke usually doesn't come out

The flue pipe is 180mm all the way up to the top. Its approximately 8-9m. Part of the reason i am worried to get overdraft is because the flue is quite tall.
 

begreen

Mooderator
Staff member
Nov 18, 2005
84,790
South Puget Sound, WA
If overdraft was an issue you would be seeing the opposite effect. The fire would be hard to control, igniting all the wood at once. The fire would be very strong and wood consumption would be high. There would be little smoke spillage if any.
 
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Flodorf

New Member
Oct 18, 2020
11
Netherlands
If overdraft was an issue you would be seeing the opposite effect. The fire would be hard to control, igniting all the wood at once. The fire would be very strong and wood consumption would be high. There would be little smoke spillage if any.
Thanks! That was something I only guessed, and you confirmed it :). At least I have gotten a little wiser ;-)
 

begreen

Mooderator
Staff member
Nov 18, 2005
84,790
South Puget Sound, WA
Sometimes the house or the chimney location can affect draft. I would try the stove with some dry pine mixed in with the hardwood to see if that improves burning. If not, the manual shows how to modify the baffle to improve burning.
 

Flodorf

New Member
Oct 18, 2020
11
Netherlands
Sometimes the house or the chimney location can affect draft. I would try the stove with some dry pine mixed in with the hardwood to see if that improves burning. If not, the manual shows how to modify the baffle to improve burning.
I do have a question that popped into my head right now. I read somewhere that overdraft can cool a fire too much for clean combust. I have hard time understanding how very strong fire can be cooler than a lazy fire. Can you explain? My initial thought was that a cool fire was small and whimpy.
 

moresnow

Minister of Fire
Jan 13, 2015
1,377
Iowa
Sooo? Second film clip is with primary air completely closed. Correct?

Having blue flame with restricted primary air is not abnormal in many stoves. i have zero knowledge of your make and model however!
Any reason for having such a small load in the stove? Maybe that's all you need for heat?

Any chance you know of any Noteboom family in your area?

Welcome to Hearth
 

Flodorf

New Member
Oct 18, 2020
11
Netherlands
Sooo? Second film clip is with primary air completely closed. Correct?

Having blue flame with restricted primary air is not abnormal in many stoves. i have zero knowledge of your make and model however!
Any reason for having such a small load in the stove? Maybe that's all you need for heat?

Any chance you know of any Noteboom family in your area?

Welcome to Hearth
Its a 12 kW stove which is about 40,000 btu/hr I believe? The manual recommends to start with 4 logs of about 1kg each.

There's a couple of Notebooms around here, but I don't know any personally. ;-)
 
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begreen

Mooderator
Staff member
Nov 18, 2005
84,790
South Puget Sound, WA
I read somewhere that overdraft can cool a fire too much for clean combust. I have hard time understanding how very strong fire can be cooler than a lazy fire.
By reducing the primary, the vacuum in the firebox created by the draft pulls more air through the secondary air feed which helps the wood gases burn more completely with a hotter fire.

My efforts tonight using well seasoned birch.


One is with primaries open and the other is with primaries fully closed. Why are the flames so blue?
Blue flames = hotter fire. It looks good.
 

EbS-P

Feeling the Heat
Jan 19, 2019
361
SE North Carolina
I have run the same stove in two different locations. One is 9 m of liner the other just over 4. I am quite certain that the 9 m install overdrafts it was ( it was -20C out and the wind was howling), and and I need to run the primary air more open or the secondaries get blown out more or less. On the 4 m install once I get the stove top over 200c and logs burning well I can shut the primary air way down faster than I could in the 9 m install.
The 9 m install still spills some smoke on reload be cause it’s a rear vented Jotul F400.

As for blue flame. That is the color of complete combustion with very little or no residual unburnt hydrocarbons. Blue is hot and clean. Wet wood never burns blue for me. I have found the species of wood I burn can effect the color.

try cutting down the amount of air through the outside air intake (tape or an inline damper??). See what that does to the burn. That would probably give you the best indication of you are over drafting.

Evan
 

Flodorf

New Member
Oct 18, 2020
11
Netherlands
Perhaps an easy question with an easy answer. Can dirty windows occur with overdraft? I have a really hard time keeping them clean, even with loading to the max load for the stove. Using well seasoned wood. The windows are not black or hard to clean, most comes off with a wet towel.
 

Corey

Minister of Fire
Nov 19, 2005
2,500
Midwest
Everyone probably has a slightly different definition, but to me, 'overdraft' means that when you get a fire going, you can't close the air down enough to control the stove. Even with the air controls 'closed', too much fresh air is getting pulled in and the stove is burning too hot. So if you feel you can control a well-burning fire and you can 'dial it down' to get the heat you want, then you likely don't have overdraft. Undedraft usually means smoke spilling out the door at all times and the fire burns lazy and smoky and won't really 'take off' and burn no matter how much you open the air. With 9m / almost 30 feet of flue you shouldn't be near underdraft conditions but could be close to overdraft. Though overall, as long as you can control the fire, it's not an issue.

To the original message, I would say "welcome to top-down burning!" A lot of people talk wonderful praise about it, but it seems like one of those things you need to get 'just right' for it to work. To me, it sounds like you may be making too big of a jump in wood size between your kindling and logs. You first need some fairly fine kindling that will take a match and light. Below that, sticks/twigs or bits of wood, I would say no bigger than 10-13mm in diameter, 3/8 to 1/2 inch. Next some small branch sized wood maybe 2-3cm diameter... up to 1-1/2 or 2 inch. Then finally onto your splits/logs. ...and you need a good amount of all these layers, or at the very least, an increasing 'pyramid' of wood. Lighting a small hand full of kindling / twigs / branches and expecting it to effectively light a whole stove full of wood doesn't work either. It all needs to be very dry, and ideally with no or minimal bark in the first three layers, and bonus points if you can stick an additional size gradation in there somewhere!

Closing down the air too much and too early is another issue that seems more prominent with top down. Those initial layers light off and it 'looks' like you have a nice fire going, so the urge is to close down the air. But the stove is still pretty cool and there is no good coal bed, and the fire dies out pretty quickly. So the initial stages need very small air adjustments which can really damp out the flames because there is no real 'thermal mass' behind them. This could also be a bit tricky with your tall flue as a very small bit of hot air will make a strong draft and those small twigs can really flare up and burn hot/fast, but damping down the air makes them go out fast, too!

Hope this helps!
 
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Flodorf

New Member
Oct 18, 2020
11
Netherlands
Perhaps someone can describe a lazy fire, well-burning fire and uncontrolled fire?

To me a large, raging fire is when I have the primaries opened and the stove fills with lots of flames, sometimes even getting white and shooting out underneath the logs as if its rocket fuel. Also, all the metal starts ticking and squeaking.
With overdraft, would the fire be similar to this with primaries closed?

When I close the primaries, I tend to get a fire with small, blueish flames. These must be hot (otherwise they would not be blue), but compared to the heat with primaries fully opened, it is a lot less hot. Closing down the secondaries even a little tends to make the fire die down.

Our cats tend try to find spots around the stove with primaries closed, but if I open the primaries, even for them its too hot. :p

The trouble I have is getting feedback on my fire. The instruction online () is a similar wood stove to mine, but I haven't managed to achieve this type of burning yet, where all the logs are burning steadily. (Around 3:25 in the video). I suppose it helps the wood stove in the example has only one window compared to the 3 of mine.

I am pretty sure the combustion is usually clean, because I can't see smoke coming from the chimney when the fire is burning blueish.
 

Flodorf

New Member
Oct 18, 2020
11
Netherlands
Does it have a 180mm flue pipe all the way up to the chimney cap? Approximately what is the length of this flue pipe?
Well, I recently found out that our installer used a smaller diameter pipe (I suppose 150 mm), because the 180 mm did not fit in our chimney. He seems to think its fine...

So to answer your question, the 150 mm diameter fluepipe is almost 9 meters long all the way to the cap. I have no idea how he fit the 150mm to the 180m diameter exit on the stove.

EDIT: Apparently he consulted the manufacturer, so it should be fine (?)
 
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