Tel. 01206 432 999
Biomass Boiler and Stove

Wood Burning Stoves

Windhager Boiler

This section is titled "Wood Burning Stoves" but what exactly is a Wood Burning Stove?

There are predominently two types of Wood Burning Stove, "Dry Back" or "Wet Back" and we are going to concentrate on the Wet Back (Back Boiler). If you are interested in the Dry Back, alot of the information is still valid.

These appliances are available for Domestic and Commercial, although our experience extends into the Domestic Market, although the information should apply to other markets too.

The Wood Burner is in the simplest form, a chamber which wood can be burnt and a chimney to take the products of combustion safely into the atmosphere. The design is however much more complex, with controls to control the combustion air, baffles to direct the products of combustion and advnaced features such as air wash systems.

Air Wash Systems are designed to keep hot gases flowing over the inside of the viewing glass and increase the time required between cleaning.

Wood Burning Stoves are an open flued appliance and requires the chimney system to be compliant to Part J of the building regulations, with purpose provided ventilation for combustion.

They are decorative items and are used for primary or secondary heating, the storage for the fuel should be suitable for the intended use.

The Stove can be positioned within a chimney breast or stand alone, depending upon the preference of the user. Typically, the wood burner is situated in the main living area, heating the room directly and the remaining part of the property indirectly via radiator or underfloor heating.

Particular attention should be paid to the possible routing of the flue pipe and chimney system, there are strict guidlines for the design, construction and materials used. Various British Standards and Building Regulations apply, these must be adhered to ensure performance, reliability and safety of your Stove.

The Wood Burner varies depending upon the model and manufacturer. Generally, the appliance will feature two air controls for primary or secondary combustion air.

If you are using Wood as your fuel, primary air should only be used for lighting the appliance and once the fire is established, primary air should be closed completely. Secondary Air is used to control the rate at which combustion takes place within the Firebox.

The other key consideration is fuel storage as a substantial area could be needed depending upon the heat requirement of the property and how often fuel deliveries are required or possible.

If you wish for your Wood Burner to be repaired, replaced or serviced, simply call us on 01206 432 999.

Stages of Wood Burner Installation

Are you Interested in Heat Lab Ltd improving the efficiency of your Wood Burner or integrating it into a heating system?

We are glad you are still reading and hope to inform you of the options available from ourselves. With wood burners, the biggest single effect on efficiency is fuel and that is closely followed by how the Stove is operated.

A Wood Burning Stove can be used for heating or domestic hot water, shown below are typical options used for both applications and also a few tips on keeping a Stove efficient;

All of these products are available from ourselves and have a thorough description in their relevant section of this website or just click on the description above.

The Technical Side of Wood Burning Stoves

Wood Burning Stoves - Wet or Dry

Wood Burners are uncontrolled and safety is a main concern as the heat can not be stopped, unlike most other heating appliances.

Stoves are generally used for secondary heating and are often used for decorative purposes. Available as either a "Dry Back" or "Wet Back", meaning they can have a back boiler for heating a central heating system (Wet Back) or be stand alone to heat the local area only (Dry Back). This follows the options that are also available for Biomass Pellet Boilers.

Both types of appliances are available from a multitude of manufacturers, costs vary, various kw ratings are available and personal preference or previous experience often dictates buying of the appliance.

This section will explain the key differences between installation options for Wood Burners and will also show schematics for their conventional installations.

Wood Burner Domestic Hot Water Only Schematic

Wood Burning Stove - Domestic Hot Water Only

All Wood Burner Installation must be open vented and can not be pressurised, unless specified by the manufacturer. The simplest installation is for Domestic Hot Water only but you will note that there must be atleast one radiator on the system, the heat leak radiator.

The heat leak radiator must be on a gravity circuit, not rely on circulation from an electric pump and feature no thermostatic control.

The gravity circulation works on the principle that hot water rises as it becomes less dense when heated and when cooled, travels back to the Stove. With gravity circulation, the flow rates are much lower and require pipework that is larger in diameter than fully pumped systems.

As can be seen by the plumbing schematic to the left for the heat leak radiator (which isn't generally recommended), there are a greater number of external components and as such, the space required for the installation is greatly increased. The inclusion of additional boilers, wood burning stoves and solar thermal not only possible, it is recommended.

The Heat Leak Radiator must be atleast 20% of the Wood Burning Stove heat output to water.

Wood Burners are generally used with open vented type hot water cylinders as they are inherently safer, the system relys on the height difference between the cold water storage tank and the hot water outlet. This type of installation is simple and effective, although pressure and/or water flow can be limited.

Also available are Thermal Stores, which can provide much better hot water performance if correctly installed and again, we would recommend referring to the links above for further information.

Wood Burner Domestic Hot Water and Heating Schematic

Wood Burning Stove - Domestic Hot Water and Heating

The addition of space heating is relatively straightforward. In addition to the requirements for Domestic Hot Water, we now require a wet distribution system and this is normally in the form of radiators, although underfloor heating can be used with appropriate overheat controls.

The heating system is generally pumped, although the domestic hot water and heat leak radiator remain on a gravity circuit. The system also features a "injector tee" and this increases the water flow through the gravity circuit by creating a venturi within the return pipework.

The general recommendation is to size the domestic hot water cylinder to the requirements of the property, although it must be atleast 117 litres in volume and the motorised valve must be of the normally open type, so the valve opens in the event of a power cut.

The manufacturer of the Wood Burning Stove must always be consulted to ensure the appliance is suitable for requirements and the instructions followed regarding installation.

Biomass with Buffer Schematic

Wood Burner - Thermal Store

The thermal store can be installed in a variety of ways with a Wood Burner, Pumped, Pump Assisted and Gravity.

The schemtic to the left shows a pump assisted wood burner and is probably the most complicated in appliaction.

The pump assisted Thermal Store installation is normally selected because sufficient gravity circulation is not possible but some gravity circulation will occur, hence is pump is required to "assist".

In the event of a power cut, as some gravity circulation is possible, we must protect the Thermal Store from overheating with a "Quench Coil" and the Stove requires a dedicated heat leak radiator, again atleast 20% of the heat output of the wood burning stove.

Some very detailed information is given in the section concerning Thermal Stores and this information can be found here;

The storage volume of the Buffer Tank and Thermal Store is dictated by the Heat Output of the Biomass Boiler, with the Thermal Store we must however consider other factors such as Domestic Hot Water requirements and "Heating Volume".

The general recommendation of sizing the Thermal Store is based on the suggestion by Hetas, we use 25 litres per kilowatt of heat output for the heating volume and an additional volume for Domestic Hot Water. Therefore, if the appliance has a heat output to water of 10kw, the "Heating Volume" should be 250 litres and this would be in addition to the "Domestic Hot Water Volume" which varies with requirements, although 30% of the heating volume would be a general guide. Therefore for our example here, the total volume would be 325 litres.

If the manufacturer of the biomass appliance suggest a different volume for their appliance, their guidelines should be followed. Using a Thermal Store that is too small will result in overheating and a lack of Domestic Hot Water. A Thermal Store that is "too large" may become unresponsive to demand if not controlled correctly and as explained in the relevant section, Domestic Hot Water could also be a problem.

Combustion - Wood

Using heat for comfort is not a recent innovation, humans have been using fire for thousands of years but we now have a very thorough understanding of the science involved. The chemistry does depend upon the fuel used but all domestic or commercial boilers rely on the combustion triangle.

Combustion Triangle

The Combustion Triangle comprises of the three conditions to create fire, or combustion. If you take any of the three conditions away, the chemical process can not be supported and the combustion stops.

C6H10O5 + 6O2 = 6CO2 + 5H2O

The above chemical equation is representing a example of what is happening with combustion using wood but this is a generalisation, the chemical make up of wood will vary.

Logs used for fuel will burn differently depending on the tree which the wood was originated and makes a substantial difference to the energy content of the logs, hardwoods such as Oak generally contain a greater amount of energy and release the energy over a longer period.

Other aspects that can influence the wood is the environment which the tree grew (the tree can absorb elements from the air and ground), the age of the tree and even the different part of the tree result in different burning characteristics. The bark for instance burns for a much longer period and contain a greater amount of energy, it can contain Sulphur which when burnt produces Sulphur Dioxide, which can be very descructive to the environment and the chimney system.

A poor characteristic of logs is they tend to be high in moisture, typically around 20%.

It is very important that the logs stay as dry as possible, so storing the logs in a dry, ventilated and covered area is crutical. Moisture effects the energy that is available from the wood to heat our properties and we need to understand that before wood can burn it must be free from moisture.

Bag of Biomass Pellets

Typically, a greater amount of heat will be delivered per kg with kiln dried wood.

It is generally accepted that wood has approximately 4100 kilowatt hours of heat per 1000kg, when moisture levels are around 20%. This energy will increase per kg of wood when moisture decreases to around 4800 at 10% moisture.

Our delivery of 1000kg of logs could contain 30% moisture if not "fully seasoned" or to view it another way, 300kg (litres) of our expensive fuel is actually water!

We should always buy fully seasoned wood with less than 20% moisture.

To release the energy of the wood, the moisture within the wood must be evaporated and this change of state (water to vapour) uses energy which would otherwise be used for heating our property.

This water vapour is in addition to the water vapour released from the chemical relation taking place as part of the combustion process and the evaporation process also lowers the combustion temperature, which will result in tarring of the appliance. The water vapour that is generated by the combustion process is also slightly acidic and will attack materials that it comes into contact, causing corrosion within the flue system and/or appliance.

To raise 300kg of water 90 Deg C (above boiling point from ambient) takes 113130kj or 31.42kWhr - over 7kg of fuel.

Match with Volatile Gases

Drying - Pyrolysis - Combustion - Reduction

To fully appreciate how wood is burnt within the appliance, we must understand the four main stages that take place before wood is fully burnt.


This is the first stage required for combustion and is simply the wood being lifted to a temperature above boiling point, this causes the water to evaporate and travel up the chimney system.


This is the second stage and is effectively the wood decomposing, releasing volatile gases to support combustion. One of these gases is likely to be Carbon Monoxide, which is harmful to humans and is potentially lethal.


It is the volatile gases that are released during the pyrolysis stage that are mixing with the air and due to heat, cause combustion


This stage requires carbon dioxide and water vapour to convert the hot charcoals from carbon to hydrogen and carbon monoxide. These gases are again volatile and add to the combustion process.

When wooden pellets or logs burn, you are actually seeing the gases above the wood combusting and not the wood "burning".

At any given moment, all four stages are occurring within our firebox and keeping the fuel as dry as possible with sufficient air for combustion, will keep the combustion process as optimal as possible.

Solid Fuel Terminal Smoking

Pictured to the left is light grey smoke on starting of a solid fuel appliance and is not to be confused with water vapour. The light grey smoke is as a result of incomplete combustion, it is primarily caused by a lack of heat within the firebox or lack of combustion air, this condition is should be avoided as it is inefficient and is likely to be producing large amounts of Carbon Monoxide.

If the smoke is black and continues after start up, the appliance will very likely be operating improperly and will requires immediate attention.

Any water vapour that is visible from a Solid Fuel Flue Terminal is present as a result of combustion and evaporation, this water vapour must not condense within the appliance or chimney system. If condensation is not seen, is does not mean than water vapour is not present, it is just condensing over a wider area in the atmosphere and is less visible, once the products of combustion are below the dewpoint of the atmospheric conditions, some form of condense will occur.

It is important to ensure the chimney system remains above 100 Deg C to ensure that condensation does not occur within the system.

Regretably, this is a form of heat loss and does effect our efficiency. However, to ensure the chimney system remains functional and does not build up volatile matter (tarring), the chimney system must remain above 100 Deg C along the full length. If tarring does occur, the chimney system is at rick of a chimney fire.

This change of state is using latent heat from our products of combustion and it is energy not being used to heat our property or domestic hot water, which is obviously not good for our boiler efficiency.


As we have discovered in the Combustion Triangle above, we require Oxygen to support Combustion and with open flued appliances, the air within the heated room is used and an air vent in the outside wall allows fresh air (possibly cold) to enter from outside.

This ventilation, although critical for combustion, is actually cooling the room we are trying to heat and therefore prudent positioning of the air vent is crutical to reduce draughts through the living space.

Oil Boiler Flue Terminal

If we are using an existing open flued appliance, the combustion air to enter the boiler must have a permanent, adequately sized air vent installed in an external wall within the same area the appliance is located.

The size of this air vent is calculated according to the air permeability of the building, rated output of the appliance in kilowatts and if a draught stabiliser is fitted.

Below is an example of such a ventilation calculation on a 1940's property with high air permeability, no draught stabiliser and a rated heat output of 10 kilowatts.

10 (Rated Output of Boiler in kw) - 5 (adventitous air) x 5.5 (cm2 per kw) = 27.5cm2 Vent or Larger would need to be used.

It is clear that a vent to the outside air of any size has a potential to cause draughts and careful siting must be considered to ensure problems are avoided.

Inadequate ventilation is potentially a very serious situation.

Firstly a lack of ventilation will not allow complete combustion to occur and carbon monoxide will be formed, potentially entering the room the appliance is located. Secondly, a lack of ventilation will also deteriorate the performance of a open flued chimney system and the lack of draught will likely not clear the products of combustion (which are also likely to be very high in Carbon Monoxide).

Carbon Monoxide is a killer.

Carbon Monoxide is formed whenever there is incomplete combustion and is present in the flue system of even the best performing boiler.

Routine maintenance is required to ensure that only minimal levels of Carbon Monoxide are formed, check all seals/gaskets in the boiler are in good condition and that the flue system is operating correctly to ensure that these products of combustion do not enter the premises.

The situation of products of combustion entering the room is known as "spilling". If encountered, an appliance should not be used, the area well ventilated and an engineer called prior to entering the property.


The commissioning of a Condensing Boiler is critical if it is to perform efficiently, effectively and most importantly, safely.

Listed below are some key factors which Heat Lab Ltd verify before the Boiler is commissioned;

Wood Burner Primary and Secondary Air

Lighting Procedure

Esbe Loading Valve

Back End Protection

As mentioned previously, there are various strategies available to optimise Condensing Boilers and they concentrate on reducing the temperature of the water flowing through the plate heat exchanger, this does NOT apply to Wood Burning Stoves.

It is important to maintain the boiler return temperature OVER a certain temperature, typically 60 Deg C and to do this we use a loading valve, or backend protection.

The Esbe loading valve pictured to the right is a simple mechanical valve that ensures the water temperature returning to the wood burning stove is above 60 Deg C. These valves can also "Laddomat" or similar but their purpose remains unaltered.

The loading valve alters the water flow rate which is being delivered to the Thermal Store as a result of the differing amount of mixing that can occur through the temperature range and this can have an adverse effect of system performance.

Wood Burner Esbe Loading Valve Schematic

To the left is a heating schematic featuring a "Load Valve" and also noted are various typical temperatures that can be experienced with a Wood Burner and Thermal Store.

The temperature returning from the heating system is 45 Deg C and is lower than the minimum of 60 Deg C which we require for our Wood Burning Appliance. The Esbe valve therefore mixes some of the 70 Deg C Flow Temperature with the 45 Deg C returning from the heating system and thermal store, the result is a minimum of 60 Deg C.

This reduces the potential of condensation occuring within the appliance if the water jacket temperature is too low and also results in a useful increase of flow temperature.

However, it is not without problems and we suggest you refer to the links below for further information;

Tel. 01206 432 999