How Does a Wood Burning Stove Work?
To many people it seems relatively straightforward when considering how a wood-burning stove works. You burn the fuel, retain the heat, slowly release into your room and the harmful emissions go through your flue pipe. In essence, this is the basics of how a wood-burning stove works but there is a lot more to consider. The look and style of the traditional wood-burning stove may not have changed too much but under the surface the technology has.
Body of the stoveTraditionally the body of a wood-burning stove is made from cast iron due to the fact it can withstand extremely high temperatures for a large period of time. As the stove body heats up this allows the heat to be radiated into the room in a controlled manner over a prolonged period of time. More recently we have seen greater use of steel which does offer some options not applicable to cast-iron.
Cast-iron is seen as more durable
Steel stove bodies can be crafted from one piece of metal meaning no joins
Steel is often more malleable to create contemporary designs
Cast-iron is perfect for radiating heat into a room
While many people still prefer wood-burning stoves with cast-iron bodies, trends do change. You can now acquire traditional and contemporary steel body stoves of all styles and designs. In many ways it has come down to personal preference for consumers. However, many still associate wood-burning stoves with a traditional cast-iron body.
FireboxWhen you consider the extreme heat created with a wood-burning stove, the firebox plays an integral role in the process. This confined area is the perfect means of burning wood in an efficient manner and retaining heat within the body of a stove. It is stating the obvious to suggest that the size of a firebox relates directly to heat output. The structure and way in which a firebox operates is also vital when looking towards reduced emissions.
Firebox dimensions relate directly to heat output
A firebox is created to withstand extreme temperatures
The flickering flames and burning embers in a firebox are integral to the stove experience
Controlling the flow of extremely hot fumes is key to an efficient stove
As with many elements of a modern day wood-burning stove, we automatically assume the process of burning wood and radiating heat has not changed for decades. The reality is that primary, secondary and tertiary airflows are now more commonplace. We will take a look at these in more detail later in this article but technology has certainly developed.
FirebricksFirebricks are placed inside the firebox to ensure that as much heat as possible is retained. These heat resistant bricks simply reflect the heat back into the firebox thereby building up the temperature and heat output. The material (traditionally vermiculite) used to create a modern day firebrick is amazing. It can be extremely hot on one side while cold on the other. It is not only the extreme temperatures which challenge the durability of firebricks but also the variation of temperatures. Over time, constant changes from extreme temperatures to room temperature place pressure on the material. In many ways, a firebox is only as efficient as the firebricks within it.
Reflecting heat back into the firebox helps build up the temperature
Firebricks are made of the vermiculite or heat resistant ceramic materials
A stove is only as sufficient as the firebricks retaining the heat
The efficiency of a cracked firebrick will fall dramatically
It is highly advisable to check your stove thoroughly at least once a year. Firebricks may need replacing due to the extreme temperatures they experience over time. It is a false economy to put off the replacement of a cracked firebrick because its efficiency has already been compromised. The less heat retained within the firebox the less output into your room. Even though a cast-iron stove body is itself extremely durable, the firebricks offer an added degree of protection from the extreme temperatures.
Flue pipeWe live in an era where emissions into the atmosphere are constantly monitored and new regulations introduced on a regular basis. The UK government has been extremely vocal with regards to reducing emissions and stoves have been recognised as a way to do this. Therefore, while we now have primary, secondary and tertiary airflow combustion systems, the flue pipe is still needed to dispose of potentially harmful excess gases.
A flue pipe is made of stainless steel to ensure safety and offer a natural upwards route for the excess gases dispersed into the atmosphere. While many people assume this is simply a pipe there is a lot more to the flue system than you think. The hotter the gases the quicker they will move up the pipe and into the atmosphere. Many flue pipes will be twin wall which effectively act as a type of insulation. A variety of different material will be used to fill the gap between the outer and inner flue pipe wall. This ensures that the gases emitted through the flue Pipe remain as hot as possible creating a draw as the hot air rises, pulling in cold air.
The flue pipe is used to emit excess hot gases into the atmosphere
Often twin walled it is essential to keep the gases as hot as possible
An uninsulated flue pipe can lead to excess tar deposits and issues with draw
The style and type of flue pipe will depend upon the location of the stove
It is advisable to clean your flue pipe on a regular basis to avoid the build-up of tar
Over the years we have seen a significant reduction in the amount of gases emitted into the atmosphere through the flue pipe. Clean burn systems essentially burn and re-burn excess gases to create more heat and reduce eventual emissions. Many people use a flue pipe thermometer to check for the temperature of gases passing through. This is because you can overheat your stove which burns fuel at a far higher rate than normal. This reduces efficiency, heat output and can ultimately lead to various deposits such as tar in your flue pipe.
If a flue pipe is located in for example a chimney breast, then you will likely require a register plate. This is a metal plate with a hole in the middle through which the flue pipe passes. The diameter of the hole is just enough to accommodate the flue pipe. The plate essentially allows excess gases to go up through the flue pipe but prevents them returning through the chimney breast. It also ensures that soot, bricks and other materials cannot fall onto the stove potentially causing damage.
Baffle plateThe individual components of a modern day wood-burning stove are relatively straightforward but put together they create a highly efficient environment in which to burn fuel. The baffle plate is an integral part of this system even though it is best described as a curved deflector. The baffle plate is placed at the top of the fire chamber and, as hot air rises, it is used to deflect heat back into the fire chamber. This constant circulation of heat allows the production of extreme temperatures with heat then radiated into a room over a prolonged period of time. An enormous amount of heat would be lost through the flue pipe without a baffle plate in place.
While there are many areas of a modern day wood-burning stove which experience extremely high temperatures, the position of a baffle plate is one of the most challenging. If you can imagine, the excessive heat created in the firebox automatically rises and is deflected back into the firebox by the baffle plate. As a consequence, over time we have seen baffle plates become cracked and worn at which point they will need to be replaced. Failing to replace a cracked baffle plate will mean more heat is lost through the flue pipe which should have been emitted into the room.
Baffle plates experience extreme temperature changes
The constant heating and cooling down will eventually compromise the material
Baffle plates are made of cast iron, heavy steel, stainless steel or vermiculite
Classed as consumables, baffle plates are not covered your stove body warranty
In the event that you over fire your stove, adding too much fuel, there could be major consequences. Due to the excess heat the baffle plate will struggle to deflect all heat back into the firebox making your stove less efficient. If, as many people tend to do, you are tempted to stack far too much fuel in your firebox this will impact the baffle plate. Aside from the excessive temperatures it will create, the actual flames from over fuelling can burn directly onto the baffle plate. This mixture of heat, changing temperatures and flame exposure will eventually damage the baffle plate.
Primary, secondary and tertiary air supplyWhile it will depend upon the particular style and model of stove, there will be a number of air vents which help control the flow of air during various parts of the combustion process. In order to start your stove in the morning the primary airflow, situated below the firebox, will be required. Opening the primary flow vent will ensure sufficient airflow to light your fuel. As the fire begins to take hold the primary air supply can be reduced. This is an essential part of the process because as the fire burns this draws in more air which is required in the combustion process. It is also essential to restrict airflow once the fire is burning to avoid overheating.
The secondary airflow is, in the opinion of many people, particularly vital when burning wood. It is essential that the firebox retains as much heat as possible to get the maximum heat out of the fuel. So, the secondary air supply system has a short journey before it enters the firebox but this short journey, around the firebox, ensures it is hot before it enters the combustion chamber. As a consequence, because the air is hot already this will maintain the current temperature and build on this as more fuel and gases are burned. This air supply is also channelled across the face of the stove door glass thereby reducing the amount of soot and particles which can be deposited on the glass surface. This in turn ensures a cleaner viewing area which for many people is an important part of the stove experience.
Primary, secondary and tertiary airflow increase heat and reduce emissions
Primary airflow should be reduced when the fuel is burning in order to avoid overheating
Preheated air via the secondary airflow system avoids cooling in the firebox
Tertiary airflow is integral to clean burn systems thereby reducing emissions
To maximise efficiency the primary, secondary and tertiary air flows should be optimised
More and more stoves now take into account a tertiary air supply which is situated in the back of the firebox. This air supply utilises the extreme temperatures within the firebox and will burn off excess gases as they are about to leave the firebox and enter the flue pipe system. While the tertiary air flow system is vital in this world of reduced emissions, the burning of these excess gases creates even more heat.
GlassThe glass in a wood-burning stove door is an essential part of the stove experience, offering the opportunity to watch the flickering flames and the burning embers. As with each element of a wood-burning stove, the glass in a stove door is treated to withstand extreme temperatures. In traditional cast-iron body stoves the glass is often sealed with rope to ensure that all of the heat is retained within the stove.
Viewing the combustion chamber is an integral part of the stove experience
As soon as any cracks appear on your glass it should be replaced
Compromised sealing rope will leak heat and should be replaced
There is a modern day trend towards wider glass viewing areas
While the traditional wood-burning stove incorporates flat square glass there are contemporary stoves with curved glass. This can enhance the look of a stove and, with a trend towards wider viewing areas, the glass is becoming an integral part. If you notice any cracks on the glass or maybe the seal rope has been compromised it is essential they are replaced as soon as possible. Any general wear and tear, or cracks in the glass, which allow the heat to escape in an uncontrolled fashion will reduce efficiency. The less efficient your stove, the more fuel required and the more expensive to run.
ConclusionThe modern day wood-burning stove still has that traditional look but under the surface is the latest in combustion technology. Primary, secondary and tertiary airflow play a major role in extracting as much heat as possible, reducing emissions. Clean burn systems will be the way forward for the UK stove industry as regulations continue to tighten. It is also imperative that any cracks or wear and tear on stove components are addressed and replaced as soon as possible. They too will compromise the efficiency of your machine and make it more expensive to run.