Multi fuel stoves are a popular way to help you get more heat out of burning a number of different types of fuel in your home such as wood and coal.
The efficiency of open fireplaces can be as little as 10 or 20%, while multi fuel stoves can have efficiency ratings of over 70%, meaning that they can produce far more heat to warm your home compared to open fireplaces.
Multi fuel stoves can extract much more heat from every piece of fuel used, so how do multi fuel stoves work?
Multi fuel stoves work by creating a controlled environment in which different types of fuel can be burnt to produce heat. The air supply to the fire can be managed to help burn the fuel more efficiently compared to open fireplaces, and by keeping waste gases within the stove for longer and at hotter temperatures, even more heat can be produced through secondary combustion. The metal shell of a multi fuel stove helps to radiate heat out into the room for prolonged periods of time, even after the fire has subsided.
Multi fuel stoves are fairly simplistic appliances as they don’t have any electrics or mechanical moving parts, but are cleverly designed to maximize efficiency where the most amount of heat is extracted from every piece of fuel consumed.
I’ve explained how multi fuel stoves work in much more detail below, using our own multi fuel stove as an example throughout the article to help showcase how they work.
How Multi Fuel Stoves Work
To explain how multi fuel stoves work, they should first be compared to what many people choose to replace: having fires in open fireplaces.
Although open fires provide an unobstructed view of a fire inside your home, along with all of the great crackling and popping sounds, there are a few downsides to having a fire in an open fireplace:
- They are highly inefficient in many cases, with the majority of the heat produced being lost up the chimney rather than being used to heat the room.
- The fire can only be controlled by adjusting the amount of fuel on the fire, or by using a damper (if you have one).
- The waste gases from burning the fuel, which could be burnt to produce even more heat, are also lost up the chimney.
Multi fuel stoves can be installed within existing fireplaces, subject to there being sufficient room within the fireplace, among other considerations. I’ve explained how our multi fuel stove was installed in our existing fireplace here.
In one of our fireplaces we’ve installed a multi fuel stove help produce more heat when burning our supply of wood.
Here’s what our multi fuel stove looks like for reference:
How Do Multi Fuel Stoves Work?
A multi fuel stoves works by taking the problems of having a fire in an open fireplace and improving upon them to help produce more heat.
Open fireplaces fires aren’t very efficient because they can’t be easily controlled. A fire needs both fuel and oxygen to survive. We can add more fuel to an open fire but we can’t control the air supply because it’s all open.
A multi fuel stove therefore creates a controlled environment where the fire can be controlled by being able to adjust both the airflow and the amount of fuel.
Multi fuel stoves are completely sealed systems. When the door to the stove is closed, air can only get to the fire through the air vents. When the vents are completely closed, there’s no oxygen supply to the fire and it will eventually go out.
The only way out of the stove is through the flue, which is typically located above or behind the stove, and vents waste gases and smoke from your home during a fire.
Multi fuel stoves differ to wood burning stoves because they are able to burn types of solid fuel other than wood, such as coal (You can read the complete list of differences between multi fuel stoves and wood burning stoves here).
As such, there are typically two main air vents on a multi fuel stove, one that serves air to below the fire, and one that serves air to above the fire (known as the primary and secondary airflows respectively).
Wood burns best with a source of air from above the fire and coal burns most efficiently with a supply of air from below, which is why you’ll typically find multiple air vents on a multi fuel stove to allow it to be able to switch between burning different types of fuel.
The diagram below shows how our particular model of multi fuel stove works, highlighting where the air comes in, and where waste byproducts from a fire are removed from your home.
On our multi fuel stove, fresh air is supplied to the fire either through the vent located on the front of the stove, or through the vent located underneath. Both of these vents can be manually adjusted to control the amount of air entering the stove from either above or below the fire.
When burning wood (which needs a source of air from above the fire to burn efficiently), the secondary air vent located underneath the stove becomes the main air vent for controlling the fire.
The secondary air vent can also be located on the front of the stove at the top, but on our particular model of multi fuel stove it’s located on the base of the stove. This helps to warm up the air as it travels around the stove to the top of the fire.
Depending on the particular situation, the vent that supplies air to below the fire can either be left partially open or fully closed when burning wood.
When burning coal (which requires a source of oxygen from below the fire to burn efficiently), the primary air vent located on the front of the stove becomes the main air vent for controlling the fire.
When burning coal, the secondary vent is typically left partially open for the duration of the fire, and never fully closed.
Airflow through the secondary air vent can help provide air for both secondary combustion and air wash processes, and so keeping the secondary vent open when burning coal can be important.
When the fuel is burnt in a multi fuel stove, it releases a number of waste gases including Carbon Dioxide. These gases can be burnt to provide even more heat, and in many cases the heat generated through secondary combustion of these gases can be more than the heat produced from simply burning the fuel.
One of the main reasons that multi fuel stoves are more efficient and can provide more heat to a room compared to open fireplaces fires, is this secondary burn process.
Burning waste gases from a fire requires higher temperatures and pressures. A multi fuel stove provides both of these requirements thanks to creating a controlled environment.
Multi fuel stoves are designed to prevent waste gases from leaving the stove too quickly. At the top of the firebox inside a multi fuel stove you’ll typically find what’s known as a baffle: a metal plate that keeps byproducts from the fire inside the stove for longer.
Waste gases have to leave the firebox via a small gap located at the top of the firebox.
By slowing down the waste gases, more time is provided to allow for secondary combustion to occur. A fresh supply of oxygen to above the fire, typically from the secondary air vent, also helps the secondary burn process to be able to burn the fuel more efficiently.
Therefore, a combination of a controlled environment where high temperatures can be reached, a fresh supply of oxygen and more time spent inside the firebox for gases, helps multi fuel stoves to produce large amounts of heat.
A multi fuel stove creates an enclosed environment in which a fire burns, and so the fire needs to be viewed through a glass door be enjoyed.
As a result, soot can build up on the inside of the glass through the duration of a fire, and in circumstances where the wood is wet or there is too little oxygen bring supplied to the fire, the glass can quickly blacken over time. This leads to being unable to see the fire as clearly and requiring the glass to be periodically cleaned.
Many multi fuel stoves, including ours, have air wash systems that help prevent soot from settling on the glass.
Air wash systems work by providing a supply of cooler air down the inside of the glass, therefore providing a barrier between the fire and the glass.
Airflow for the air wash is supplied through its own vent, or from one of the other vents on the stove. The airflow is typically from the secondary vent generally located on the front of the stove at the top, or located underneath.
The air wash system on our multi fuel stove is fed air from the secondary vent located underneath the stove. The air wash works very well in keeping the glass door clear, and as a result we rarely need to use any form of cleaning product to clean the glass.
How Multi Fuel Stoves Work – The Components
To further explain how multi fuel stoves work, I’ve gone into more detail below to explain each component of a multi fuel stove.
Multi fuel stoves are essentially box-like appliances with legs that sit on a hearth, with a door on the front and a vent pipe located either on top or at the back.
Many multi fuel stoves are made from steel or cast iron, as they are both very good conductors of heat and help to radiate the heat from the fire into the room. The body of our own multi fuel stove is made from steel.
The firebox is the area inside a multi fuel stove where a fire is built, lit and controlled.
At the base of a firebox inside a multi fuel stove you’ll typically find a metal grate, while the sides will be made from other fireproof material.
At the top of the firebox is the baffle plate, which helps to keep waste gases from leaving the stove too quickly to aid in secondary combustion.
The flue is typically located above or behind the baffle plate, and air must travel around the baffle to leave the stove via the flue.
On our multi fuel stove, the flue goes straight out of the top of the stove and up to the top of the chimney via a stainless steel flue liner.
The flue is capped off at the base of the chimney to prevent any other airflow from entering the system.
To be able to burn different types of fuel, a multi fuel stove has a grate at the base of the firebox to allow air to get the fire from below. A multi fuel stove therefore needs an ash tray located in it’s own compartment below the firebox to catch any ashes that fall through the grate.
The door to the stove is lined with a heat resistant seal to ensure that all air going into the stove is through the air vents.
The air vents control the supply of air to the fire. Multi fuel stoves will generally have multiple air vents to be able to feed air to the fire from above or below, depending on which type of fuel is being burnt.
Our multi fuel stove has two air vents, located on the front of the stove near the bottom, and underneath.
The vent underneath the stove is controlled with a handle that sticks out the front of the stove, and can be adjusted by pushing the handle in and out to close and open the vent.
This air vent, known as the secondary vent, supplies air to the top of the fire when burning wood, and provides air for secondary combustion and air wash.
The air vent on the front of the stove, known as the primary vent, supplies air to the fire from below through the ash pan compartment.
This vent can be opened and closed by rotating the slider wheel.