The 10 commandments of wood burning stoves

04 Mar.,2023


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Burning wood to keep warm is an age-old tradition in many parts of the world. In Nordic countries especially, the topic leaves no one indifferent, whether it’s owners and users of wood burning stoves or fireplace inserts, or opponents who are worried about the significant emissions caused by wood burning when it’s either done wrong or with outdated equipment. But even the most modern wood burning technology will run into trouble if the combustible material is of bad quality, for example if the logs are too humid or if the user burns things the stove was never intended to burn.

There are many good wood burning tips available, including in the user manual that comes with your new, clean-burning stove or insert.

New wood burning stoves, that is to say the ones that have been on the market from 1998 onwards, are called clean-burning because they operate with a superior combustion principle, based on supplying air at two different stages – as opposed to old-fashioned stoves, which provide air at only one stage. This two-step air supply makes it possible to separate the combustion into two stages: the vaporization and partial combustion of volatile compounds with the first air intake; and the combustion of the remaining volatile compounds with the second air intake. This second air supply comes into the combustion chamber at high speed, at the exact spot where it’s needed, and is also pre-heated. This ensures an optimal mix of air and volatile compounds, as well as a high combustion temperature. In this way, everything is in place for a rapid and efficient combustion, and the ensuing reduced emissions of volatile compounds – both gas and particles.

That being said, the operator (you) still has a large impact on the combustion process in a wood stove. The building the stove is located in will also have an impact. Wood burning stoves in Norway, for example, generally operate with a passive air supply: that is to say that the air is not pushed into the combustion chamber by a fan, but rather sucked in by the draft generated in the chimney by a temperature difference that in turns creates a density difference. In addition to this, very few stoves in Norway are equipped with automatic air supply controllers, which means that regulating the air supply is up to the operator (you). Anyone can heat a house with wood, but it’s a greater challenge to heat a house with wood in a way that minimises the impact on nature, climate and health, and maximises the heat output.

This means that a large responsibility lies on the shoulders of the owner and operator, who are usually the same person. In these times of high energy prices in most of Europe, we decided to provide you with detailed advice on how to achieve the best results, including explanations for why the advice is actually good advice.

You can reduce emissions from your wood burning and get more heat out of every log by respecting the following 10 commandments.

1. You shall not be content with outdated technology: Switch your old wood burning stove or fireplace insert (bought before 1998) with one-stage air supply for one that is clean-burning. Even with the most careful operator and the best combustible material, the emissions from an old stove or insert will be too high, because the combustion principle it follows is outdated. If you have an open fireplace used for anything else than ambiance a couple of times a year, get a fireplace insert installed. Open fireplaces contribute only minimally to heating the house (sometimes they actually cause a net loss) and have high emissions. Antique wood stoves (from before 1940) are often beautiful to look at and worth preserving, but they are best left as a beautiful object to have on display but not use.

2. You shall take into account the amount of heat needed, the type of residence and the chimney: When you switch to a new stove, make sure it is suitable to your heating needs (measured in kW) and to your residence, including the chimney. The characteristics of the chimney (height, cross section, insulation, airtightness) are all important because they determine how much draft will be generated to provide your stove with the air it needs. When you light up a fire in your stove, the draft gets created by the chimney, and the difference in temperature and hence density between the room air and the outside air – as you probably know, hot air rises. As the temperature of the exhaust smoke increases, draft will improve because of the increased temperature difference between the smoke and the outside air. To regulate the draft further, you can adjust the aperture of the stove’s air vent (there can be more than one, to regulate the supplies of primary air and secondary air separately). The operator’s adjustment of this air vent is therefore very important, especially immediately after lighting up, and in the period that follows. An open fireplace has no air vent, and eventually sucks far mor air than needed, which drastically reduces energy efficiency and causes high emissions.

3. You shall not use other combustibles than firewood: Wood burning stoves are built and optimised to burn firewood. Other types of combustibles can create significant challenges with respect to controlling the combustion process, which can result at best in higher emissions, and at worst in a chimney fire. Different types of firewood have different properties. Hardwood, like birch, have a higher density than softer (more porous) varieties like spruce. The energy density of hard firewood is correspondingly higher. It follows that the volume of wood used has to be reduced. Bark has a different composition than the wood it protects, and gives higher emissions of certain compounds. Try to avoid inserting many thin logs with lots of bark at the same time in the stove. Paraffin firelighter logs should also be avoided in wood burning stoves, since the paraffin they contain can melt and flow down into the air regulating mechanism. Paraffin is also a fossil fuel, which means burning it contributes to global warming. Compressed combustibles like wood briquettes are not logs. They have completely different physical properties compared to actual firewood logs, and wood burning stoves are not intended or indeed type-approved for such fuels. Of course, wood briquettes and many other types of materials will burn if you place them in the stove, but because of their different composition and physical properties, they can contribute to extreme emissions or dangerously high temperatures. And don’t burn rubbish in the wood stove.

4. You shall light up in an efficient and environmentally friendly way: When lighting up, pile the logs horizontally, preferably in two layers and in such a way that they are not touching the oven walls or the glass pane. Also make sure that the air intake is not obstructed. The largest logs should go at the bottom, and the smaller ones on top. At the very top, you can pile some wood chips or birch bark for lighting up. Alternatively, you can use fire starters. Avoid newspapers. When everything is ready, light up the fire from the top. This prevents the entire log pile to catch fire at the same time. Instead, only the top part will burn at first, which will quickly give a small flame with a sufficiently high local temperature to ensure an optimal start to the combustion process. The flames will slowly spread downwards and activate the bottom layer of logs. If you activate the whole log pile at the same time by lighting up at the bottom, it will take significantly longer to reach the temperatures required to achieve a clean combustion, and the emissions will be higher.

5. You shall adjust the air supply when you light up: Adjusting the air supply when lighting up is important because the stove is cold at first, and the draft is limited. When you light up, let the stove door be open just slightly, until a good flame has taken hold at the top of your log pile. Make sure the cooking hood and bathroom fan are turned off. When you close the stove’s door, control of the air supply is delegated to the air vents, which should be open at their maximum for a little while, to ensure a sufficient air supply. After a few minutes, the draft in the chimney is strong enough, and you can regulate the air supply by reducing the air vent’s aperture. At this point, you want to provide less primary air and more secondary air. If you neglect to regulate your air supply, you will end up burning more wood than necessary and increase the temperature inside your chimney. A high temperature inside the chimney means more lost heat (you are essentially “warming the outdoors”), which is uneconomic. In the worst case, it can even result in a chimney fire, if the temperature reaches very high levels and there has been a lot of residue build-up in the chimney.

6. You shall not close the air supply completely: Adjusting the air supply after the stove is hot is not necessary, as long as you regularly add small amounts of wood to keep the combustion going. The wood stove will achieve a thermal balance with its surroundings, and as long as its heating effect (measured in kW) is sufficient for your home, you should not have to touch the air vents. If it becomes too hot in the room, add less wood at a time, and increase the amount of time between each insertion of wood. A log placed on glowing embers will light up easily. Closing the air supply completely is a bad idea. Without a proper air supply, the emissions of volatile compounds skyrockets. Inserting large amounts of wood and closing the air supply in the hope of having a long-lasting burn is therefore not recommended.

7. You shall adjust the amount of wood used when the stove is hot: Add one log at a time, horizontally. Place it on the glowing embers, preferably right in the middle, so that it touches neither the walls of the stove nor the glass pane. The interval between each wood insertion depends on the size of the log and the amount of heat you wish to generate. Here, you can go by trial and error, since heat requirements vary from person to person, and also depend on the building and the outside temperature. When you open the stove door, avoid doing it too quickly, since this creates a vortex that sucks smoke and unburnt volatile compounds into the room. Avoid messing around with the fire too much, since this takes time and releases particles that can end up in the room. Be efficient when you add a log so that the door is open for the shortest amount of time possible when the stove is hot.

8. You shall allow the wood burning to end properly: Let the air vent be open until all the fuel is completely burnt. If there are bits of charcoal left, you can keep these for the next fire. But do remove the ashes. When the stove is not in operation, close the air vents to avoid heat loss through the chimney.

9. You shall ensure proper maintenance: All wood burning stoves need a little maintenance to function optimally. Remove the ashes between every fire. Check the gaskets regularly; air leaks can significantly impair the combustion process, by adding air (and too much of it) in the wrong places. The chimney should be swept regularly to prevent chimney fires. The interval at which this should be done depends on how often the stove or fireplace insert is used. It also depends on the technology and the way the stove or insert is operated. The surfaces inside the stove or insert and the stove pipe should be cleaned once in a while. Bad combustion can cause residue to build up over time, which both reduces the heat transfer to the room and increases the risk of chimney fire. Significant residue build-up can also increase flow resistance, which will result in poorer draft.

10. You shall not heat the outdoors: Evaporation of water in insufficiently dried wood, heating of excess air, higher temperature than necessary in the chimney, emissions of unburnt volatile compounds and too much charcoal in the ashes: all these factors can contribute to reducing the efficiency of your stove or insert, which means heating ends up costing you more. Firewood should be dry when used. That is to say that it should not contain more than 20% water. Do not dry fresh logs indoors (that can be detrimental to your health) but logs that are already dry can be placed near the hot stove before use, to remove a bit more moisture from them. Having the air vents open more than necessary means you’ll need to heat up more cold air than is strictly necessary for the combustion process. This increases the amount of smoke and results in more heat being released outside, via the chimney. If you add lots of wood, especially when the stove is already heated up, the temperature in the chimney will rise and lots of heat will go to waste. High emissions of unburnt volatile compounds mean also a correspondingly high level of wasted energy – the same goes for having lots of unburnt charcoal in the ashes. Bad combustion over an extended period of time causes residue to build up on the heat exchanging surfaces of the stove and in the stove pipe. This build-up reduces the heat transfer to the room and increases the temperature in the chimney. Again, more wasted heat.

As you can see from reading these commandments, there are quite a few factors to take into account when lighting up your wood burning stove or fireplace insert. The key is to be aware of the consequences of doing things the wrong way. Everyone wants lower emissions and the largest amount of heating per log. So, give your contribution to a better environment and climate, and to healthier air quality and get more bang for your firewood buck by the same token. All you need to do is follow these rules. And remember: by heating with wood on cold days, you contribute to reducing electricity consumption when it’s at its very highest, which provides much-needed relief to the electricity grid, and contributes to lower electricity prices – also for you.

Updated 5. December 2022.

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