Wood burners have become very popular over the past few years both for a design contribution to the living space, and as a way of trying to offset the house fuel costs. There are many different styles and types of wood burners that offer the consumer many different advantages, but the age old adage “you get what you pay for” works very well in our industry. Buy a cheap stove, and pay using a lot of fuel, buy a better stove and save fuel.
Great play is had with how good a manufacturer’s clean glass system work, this is to enable the user to view the fire while it’s burning, as one of the main reasons people like an open fire is that they can seefire burning. However as most people that have a basic wood burner, they only see the fire for a few hours once the fire is alight.
The better the stove, the cleaner the glass will stay, the cleaner the chimney will be, the less fuel you will burn. In some cases you will only ever clean the glass as the edges are getting dirty, but the view of the fire is still there. To explain this I have given some parameters on how stoves use technology to achieve their combustion.
Primary Burn Stove
A Primary Burn stove only uses cold air from the room for the fire to burn. This in theory is little different to an open fire expect that now it’s in a metal box so there is some form of heat recovery. Most of the fuel goes up the chimney as waste gases (smoke). You can identify these units as the manufacturers promote the “Air wash” to help keep the glass clean. In my experience, this does not work, and after only a few hours the glass will start to go oily. By the end of an 8 hour burn the fire is only a glow behind the black glass
This technology will burn around 4-5 logs an hour to give 5kw of heat
Clean Glass Stoves
A clean glass stove uses “pre-heated air” to form the air wash. This is done by the air intakes in the stove take the air from the room and pass these around the fire chamber to take the heat from the fire. This means the stove can burn at lower temperatures and the glass will stay cleaner for longer. Some Clean Glass stoves even have double glazed windows, this is to increase the temperature of the combustion and also to help stop the smoke condensation on the glass
This technology will burn 2-3 logs an hour to give 5kw of heat
Clean Burn Stoves
Clean burn is a technical term used in Europe for stoves that are primarily designed to burn the smoke. This means that all air going into the appliance must be pre-heated, and the air flows worked out to give the optimum burning, so very little in volatile matter (smoke) goes up the flue. Some companies will say they have Clean Burn technology and an Air Wash to keep the glass clean. These two technologies cannot be mixed because the Air Wash will cool the combustion air down meaning the fire won’t burn as clean. In Europe a bench mark qualification is the DIN to verify that the appliance will burn clean. DIN is an efficiency standard developed in Germanyand you can read more about it on the Institute website.
This technology will burn as little as 1 log every 2 hours to give 5 Kw of heat
There are two ways in which a stove can transfer its heat. Radiant and Convection. Convection will heat a space evenly, and the end user will have more of a chance to heat multiple interconnected spaces. A radiant heater is a room heater and less efficient in its heat transfer.
The 2 stoves mentioned below, are both clean burn stoves to the new European standard which is due to come in, in April 2016.
XEOOS PUR x5 Twin Fire
This is a 5 kw stove that burns the fire downward. Once the stove is up to temperature the user pushes the handle down to the twin fire setting and the closure plate in the top of the fire chamber is shut off opening the flue to the bottom chamber. The flue draft then pulls the smoke through the fire bed to be burnt below (Giving the fire the Twin Fire name) this temperature can exceed over 1200 degrees Celsius (a normal open fire burning wood is around 350 degree Celsius) . The heat is then recovered from the stove as the gases travel to the flue leaving the flue gas temperature around 185 degrees Celsius.
The stove then recovers the heat by convection and helps to transfer the heat around the space.
This stove is available in Europe, America, Australia, Japan and China and has a heat efficiency of 95%. This stove can do as little as 1 log every 2 hours to give 5 Kw per hour.
See the video for more information on this stove.
The Tonwerk Eye is a storage stove. The idea of this appliance is that you load the fire up with 3 x 2kg logs, these burn to the maximum and last around 2 hours, putting 80% of the heat into the storage stones. Through this burning time the fire is giving off 3kw of heat by radiant heat. Then, once the fire goes out, the stoves carries on giving a steady low heat for over a further 6 hours. This stove is ideal for a low energy house or passive house as these types of home need a slow and gentle heat. If there is a heat recovery system in a low energy house, then with this stove you only need two fires a day to heat the house to a comfortable level.
DIY Doctor is grateful for the specialist knowledge of Stuart J Vialls, the Sales and Technical Director for Anglia Fireplaces and Design Ltd in Cambridge who provided the information, pictures and videos in this post. They were the winners of the Homebuilding and Renovating Product Excellence awards in 2011.