Tel. 01206 432 999
Biomass Boiler and Stove

Biomass Pellet Boilers

Windhager Boiler

This section is titled "Biomass Pellet Boilers" but what exactly is a Biomass Boiler?

There are various types of Biomass Boiler but we are going to concentrate on the Wood Pellet Boilers and Stoves. If you are interested in using logs as fuel, please refer to the section for Wood Burning Stoves above.

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

The Biomass Boiler is generally a fan assisted open flued appliance and as such, requires the chimney system to be compliant to Part J of the building regulations, with purpose provided ventilation for combustion.

Biomass Boilers are generally quite substantial as the appliance also includes the fuel hopper, a storage section for the fuel large enough to give several hours of continued operation with requiring any attention from the user and a very good illustration is provided to the left.

As a result, biomass boilers are often sited in their own plantroom or enclosure. Unless designed for external use, the biomass appliance should never be used externally or in unheated areas, such as barns or garages.

If the appliance must be installed in one of these unheated areas, the manufacturer of the appliance should be consulted and if possible, an enclosure with adequate ventilation constructed to Part L of the building regulations to minimise unwanted heat loss.

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 Biomass Boiler.

The Biomass Pellet Boiler can be very simple or very complicated, depending upon the model and manufacturer. Generally, the greater the complication, the greater the efficiency and level of automisation.

Biomass Pellet Boilers can re-fuel and self cleaning automatically, if required.

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 boiler to be repaired, replaced or serviced, simply call us on 01206 432 999.

However, are you Interested in Heat Lab Ltd improving the efficiency of your Biomass Boiler?

We are glad you are still reading and are interested in energy efficiency, our passion. Biomass Boilers are not as tolerant to installation errors, heating control problems and fuel quality, all elements must be correct for optimal efficiency.

The efficiency can be improved by ensuring the some key factors are taken into consideration;

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 Biomass Boilers

Pellet Stoves or Boilers

Pellet Stoves are generally used for secondary heating and are an alternative to the traditional Wood Burning Stove. Like a wood burner, they are generally available as a "Dry Back", meaning they are be stand alone to heat the local area only. Another common description is an "Air" appliance.

The more popular option is a Pellet Boiler and this can be linking into a wet heating system, to heat the Domestic Hot Water and provide the required space heating.

A Biomass Stove is simply Biomass Boiler with a decorative appearance.

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 Biomass Boilers and will also show schematics for their conventional installations.

Biomass Heat Leak Schematic

Biomass Boiler - Heat Leak Radiator

The heat leak radiator installation is probably the simplest but also the worst. Unless the boiler manufacturer stipulates that it is possible, this configuration will result in shorter running periods and increased number of ignitions, both causing efficiency and reliability problems.

Depending upon the exact model, they can be installed open vented or on a sealed (pressurised) system, a Buffer Tank or Thermal Store is generally required and the distribution system can be either a "S", "Y" or "W" plan, with or without a Domestic Hot Water Cylinder.

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.

A Heat Leak Radiator is generally not recommended with Biomass Boilers.

Biomass Boilers are often used with open vented type not water cylinders, 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 Unvented Cylinders or 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.

Biomass with Buffer Schematic

Biomass Boiler - Buffer Tank

The reason why a Buffer Tank or Thermal Store is used is to decrease the number of ignitions required for an average heating season. Some very detailed information is given in the section concerning Buffer Tanks 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.

The general recommendation of sizing the Buffer Tank is suggested by Hetas, we use 25 litres per kilowatt of heat output. Therefore, if the appliance has a heat output to water of 10kw, the buffer tank volume should be 250 litres and this would be in addition to the cylinder required for domestic hot water, so additional plantroom space would be required.

The requirement for a Buffer Tank and indirect Hot Water Cylinder does require additional plantroom space.

A common problem is a lack of heating system flow temperature with Buffer Tanks, particularly with Biomass Boilers. If you are experiencing this problem, you may be interested in this solution;

If the manufacturer of the biomass appliance suggest a different buffer tank volume for their appliance, their guidelines should be followed. Using a Buffer Tank that is too small will result in poor efficiency due to short cycling and will cause reliability concerns. A Buffer Tank that is "too large" will be good for reliability and efficiency but the system may become unresponsive to demand if not controlled correctly.

Biomass with Buffer Schematic

Biomass Boiler - Thermal Store

The reason why a Thermal Store is used is the same as a Buffer Tank, to decrease the number of ignitions required for an average heating season and the plantroom space is potentially reduced, as only one cylinder is now required. 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.

The Thermal Store does potentially require less plantroom space.

A common problem is a lack of heating system flow temperature with Buffer Tanks and Thermal Stores, particularly with Biomass Boilers. If you are experiencing this problem, you may be interested in this solution;

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 poor efficiency due to short cycling, reliability concerns and a lack of Domestic Hot Water. A Thermal Store that is "too large" will be good for reliability and efficiency but the system 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 - Biomass Pellets

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 biomass pellets but this is a generalisation, the chemical make up of wood will vary.

Wood Pellets are generally made from waste wood from Inductrial processes, such as making wooden furniture or flooring. The tree which the wood was derived makes a huge difference to the energy content of the biomass pellets, 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 biomass pellets are 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 good characteristic of Biomass Pellets is they tend to be low in moisture, typically less than 10%.

It is a very important the biomass pellets stay as dry as possible, so storing the fuel in a dry, 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, Biomass Pellet manufacturers quote an energy content of 4800 kWhr/tonne

This is energy is only available if the moisture level is below the figures quoted, typically less than 10% and has not become damp due to storage deficiencies.

Our delivery of 1000kg of wood pellets should contain 4800 kilowatt hours of heat, based upon 10% moisture or to view it another way, 100kg (litres) of our expensive fuel is actually water!

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 100kg of water 90 Deg C (above boiling point from ambient) takes 37710kj or 10.47kWhr - over 2kg 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 due to the control we have over combustion, can keep the combustion process as optimal as possible.

Biomass Boiler Terminal Smoking

Pictured to the left is light grey smoke on starting of a Biomass Boiler 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 Biomass Boiler 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.

Poor Biomass Bioler Terminal Position

Poor Combustion

Every aspect of the system should be considered for reasons of poor combustion, pictured to the right is a flue terminal with insufficient clearance above the roofline and would be highly likely to cause problems with inconsistant draught.

The type of terminal can also have an effect on the chimney system performance and should be selected depending on the installation criteria. It should also be noted that this particular installation has trees local to the terminal and pressure problems could therefore be experienced, a shielded terminal may help in these circumstances.

Flue suppliers provide installation specific designs and calculations to confirm the Chimney will perform as required.


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 to position the appliance in a non-living area but within the heated envelope.

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, a draught stabiliser and a rated heat output of 25 kilowatts.

25 (Rated Output of Boiler in kw) - 5 (adventitous air) x 8.5 (cm2 per kw) = 170cm2
AND 5 (adventitous air) x 3.0 (cm2 per kw) = 15cm2.
A total of 185cm2 (170cm2 + 15cm2) 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;

Danfoss Oil Boiler Oil Pump

It is important to select a heat output of the boiler that is suitable for the property in which it is installed, with biomass boilers, this is potentially altered by altering the pellet feed quality and combustion fan speed. It is important that the manufacturers instructions are followed and that the minimum power output comfortably maintains the chimney temperature over 100 Deg C.

If the pellet "recipe" is altered, it is essential to undertake the flue analysis to ensure the products of combustion are as desired.

It is important that the boiler remains on high fire through the commissioning process, the boiler does generally have the ability to modulate and will be firing according to demand.

Generally Biomass Boilers are more efficient at the lower heat output sustainable whilst maintaining the chimney temperature of over 100 Deg C.

Once the commissioning process is complete and all factors have been passed, the installation is now ready for use.

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 Biomass Boilers.

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 biomass boiler is above 60 Deg C. These valves can also be electronic on sophisticated appliances but their purpose remains unaltered.

The loading valve alters the water flow rate which is being delivered to the buffer tank 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.

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 Biomass Boiler and Buffer Tank.

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 Biomass 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 buffer tank, 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