There’s plenty to learn about wood pellet heating, and that’s why we’ve put together this A-Z of Wood Pellet Heating.
A – Automatic ignition
Most wood pellet boilers for domestic use have automatic ignition. This means that when heat is not needed the boiler can turn itself off. It can self ignite when there is a need for heat. These are service items and will wear out over time e.g. 3 years, and are easily replaced during a service visit.
Hoppers that do not have automatic ignition have a low burn or stoking mode where the embers of the pellets are kept alight. This does mean that extra fuel is used, but the benefits may be a cheaper boiler.
A – Ash, Ash chamber and maintenance
All wood pellet boilers produce ash. The smoke containing the ash goes from the pellet burning chamber or bowl through the heat exchangers before being removed through the flue.
With no automation the boiler owner is required to clean the burner bowl every 2-4 weeks and also clean the heat exchangers by rodding or a semi automatic rodding device. The flue also needs ash emptying on a monthly basis and sweeping annually.
Some boilers do contain automation that reduces the need for ash cleaning.
B – Biomass
Biomass is strictly the weight of living biological organisms within an ecosystem. It has been adopted as a collective name for fuel that has been grown e.g. wood, miscanthus, straw, pine nut kernels, palm oil.
For homes wanting the Renewable Heat Incentive biomass alternatives are wood gasification and wood pellet boilers. Larger systems (above 45 kW) that may include a wider variety of fuels will have to be approved by Ofgem.
B – Burn Chamber
A burn chamber is where the pellets are fed and burnt. Some burn chambers are round and some are open. Some are supported by automatic ignition and self cleaning.
C – Carbon Emissions
Carbon dioxide emissions are part of the cause of global warming. Space and water heating in homes makes up 70 percent of all emissions from the home in the UK. If you are trying to reduce your carbon emissions this is the place to start. Either use less heat or use heat that has been produced from a low carbon source such as wood pellets!
D – Density of Wood Pellets
Wood pellets are very dense as they have been compressed. Compared to wood chip this means that you need less storage (perhaps 3 times) for wood pellets.
You know how many wood pellets you need to buy and what space you need to store. The transport cost is also lower.
D – Dusty wood pellets
Good wood pellets are not dusty. Buying your wood pellets from a trusted source that conforms to wood pellet quality schemes ensures (E.g. Hetas, or Woodsure) that you do not get the increased maintenance problems that may result.
E – Energy of wood pellets
Wood pellets produce about 4800 kW per tonne. If the average house needs 20,000 kWhs a year then 4-5 tonnes of wood pellets will be sufficient.
Heating oil generally delivers 10 kWhs per litre. The same house would need 2000 litres of heating oil.
F – Fuel supply
Wood pellets are an alternative to gas and oil as a fuel to heat a property. Currently wood pellets are cheaper to purchase than either gas or oil and come with the knowledge that there are lower carbon emissions that result.
The fuel is normally supplied in small bags (easy to carry e.g. 10kg or 15 kg), 1 tonne bags, or blown from a lorry into a large hopper or silo.
You can purchase wood pellets from a number of places. The best place to start is often the manufacturer where they will either organise themselves or have a directory of partners that they work through.
Your installer will have good experience of using wood pellets and they may also know how best to organise deliveries.
You can buy pellets at country stores in bags and this ensures that you would not run out.
F – Flue
All wood pellet boilers have a flue to take the gases of combustion out of the building.
Flues can be traditional chimneys from existing fireplaces. These suit attractive boilers such as MCZ or other similar Italian boilers. Your chimney should be checked for suitability with respect to the building regulations. This may mean that a liner has to be installed.
If you have not got an existing chimney or flue it is common to install twin wall flues that probably go on an outside wall and up to above the height of the eves. Internal flues can also be installed, but will need additional measures to ensure that they are not near to any combustible materials.
The flue needs to be sited so that it does not blow emissions through a neighbours window – and that it is sufficiently high to create enough draught to take the gases away from the boiler. A height of 4.5m is recommended as a minimum unless a suitable calculation can be provided.
G – Grants
Wood pellets are a renewable and low carbon fuel. Increased adoption of them will help the UK to reduce its carbon emissions and reduce the UK’s impact on global warming.
The Renewable Heat Incentive is an incentive rather than a grant and is a payment based on heat need per annum rather than a grant to install. For further information please see our RHI page.
For domestic installations the government has introduce Renewable Premium Payments. This will be £950 for wood pellets and available for installations until March 31 2012.
H – Heat Exchange
When pellets burn they create heat. This is circulated within the boiler through tubes (which may be self cleaning) that in turn heat water that is circulating.
Heat exchange is used to provide heat to rooms through radiators or underfloor heating.
Heat exchange is used to make domestic hot water through passing the hot water from the boiler through a tube running through a hot water tank. This tube heats the water in the hot water tank.
H – Hetas
Hetas is the solid fuel body and an accredited assessor for MCS accreditation and competent installers.
“Competent” installers should have been through training to install and had an inspection of an installation.
Many installers will therefore refer to their Hetas training and accreditation in the form of a badge on their van or paperwork.
It should be noted that Hetas are not the only source of training and accreditation. There are a number of MCS accreditation bodies who each have their own competent person scheme. Details are currently emerging (October 2011) as to what these are.
If in doubt do ask your installer.
H – Hopper
A hopper is used to store wood pellets. This may be internally ie part of the boiler, or externally as a separate fuel store.
If you are tight for space then a boiler with an internal hopper may be for you. You would have to manually feed pellets into the hopper probably 2 or 3 times a week when it is really cold.
If you have lots of space you can have a large fuel store and benefit from bulk discounts and hand feeding your boiler. Your installation costs will be higher.
I – Ignition
A traditional biomass boilers might be a wood boiler with a back burner. This is lit with matches and either paper or similar fire starting catalyst.
Most multi-fuel and wood boilers also use this method.
Many wood pellet boilers have automatic ignition linked to a thermostat or boiler programme. This saves you time and inconvenience.
J – Building Regulations Part J
Any installation of a wood pellet boiler must comply with Building Regulations Part J. As part of MCS accreditation installers can sign up to a competent persons scheme that means that they can sign off an installation. Equally Hetas courses for biomass can be used for a sign off without MCS accreditation – your installation will be perfectly valid with this, but you will not be eligible for the RHI (Renewable Heat Incentive).
If your installer cannot sign off your installation they can contact building regulations and pay a fee (perhaps £150) for an inspection and sign off.
Key issues raised by building regulations are:
The hearth – sufficient distance from the edge of the hearth to the boiler
The flue – size and siting
Ventilation of the room
Combustible materials and proximity to either the flue or the boiler
K – Kilowatts (kW) Kilowatt hours (kWh’s)
Kilowatts (kW) is a measurement of heat and is used to rate how much heat a boiler can produce. E.g. a 15 kW boiler. This is common to all boiler systems, although some of the old ones will be in BTU’s.
Each property needs an amount of heat during one year. An average insulated house may need 20,000 kilowatt hours from a wood pellet boiler.
Wood pellets provide about 4.8 kWs of heat from 1kg. The average house may therefore need just over 4 tonnes of wood pellets a year before you take into account the efficiency of the boiler.
L – Lambda Sensor
A Lambda sensor is an electronic device that measures oxygen. It is used within wood pellet boilers to adjust the air/fuel mixture to achieve the best burn possible.
M – Cubic Meters (m3)
Wood pellets need to be stored. By volume 1 tonne takes up 1.65 m3 of space. The average house that is well insulated may need between 4 and 5 tonnes of wood pellets in a year. You can therefore work out how large your storage space needs to be taking into consideration bulk discounts and available space.
N – NOx
NOx emissions are of concern because they are associated with the increased acidity of aerosol particles, cloud water and precipitation (acid rain), causing damage to aquatic and terrestrial ecosystems, as well as being responsible for the corrosion of building materials and affecting human health. NOx can act as a fertiliser in soils but, in excess quantities, leads to eutrophication (an overabundance in nutrients leading to oxygen depletion) in many environments. Furthermore, NOx emissions are of crucial importance in the formation of photochemical smog and oxidants, especially ozone (O3), in the lower atmosphere. (Coal online)
A DECC study found that NOx levels from UK produced wood pellets ranged from 0.13 kg per mWh and were similar to both oil and gas NOx emissions.
The advantage of wood pellets is that through their compression and clean burning there is a lower transport cost and this reduces NOx emissions.
O – Organic compounds
Wood pellets are made to European standards and there are wood pellet schemes in the UK.
Wood Pellets are made from non contaminated wood ie there are no coatings such as paint. This ensures that there are no emissions from halogenated organic compounds. Wood pellets are tested in a lab for emissions. If you are unsure do check the site of your supplier or ask them about emissions testing.
If you burn wood with coatings then you will be releasing harmful emissions into the environment.
P – Pelletising
Wood pellets are made from sawdust. This may be from the milling process in which case the sawdust will be “wet” and need drying. Dry sawdust can be sourced from mature non contaminated wood waste.
The sawdust is compressed mechanically through a metal ring dye that compresses the dust into the wood pellet and produces heat. This heat causes the wood to produce lignans which acts as a natural glue binding the sawdust together.
Wood pellets are most commonly made in 6mm width. This relates to the metal ring dye and not to the length of the pellets.
Q – Quality and Net Calorific Value (Q)
Wood pellet quality is often discussed in relation to the net calorific value (q). 4.6-4.8 kW per kg is often given as the range that you may find. Fuel suppliers do compete and try to offer fuel that is of a higher net calorific value (q) such as 4.8 kW. This does affect the cost of your fuel bill and it is sensible to calculate the cost of the fuel against its energy content in order to get the best value.
R – Renewable Energy Association
All MCS accredited suppliers of wood pellet boiler installations have to sign up to the REAL Assurance scheme for customers. This provides the customer with confidence to purchase a micro renewable device such as a wood pellet boiler from a wood pellet installer.
S – Sustainable
Are wood pellets a sustainable fuel? Clearly if everyone in the UK wanted to use them all at once and at the moment, then no. However this is not the case. Wood pellets are made from wood waste that came from sustainably managed woodland or forest.
” FSC (Forest Stewardship Council) accreditation certifies that the biomass used in wood pellets (or for other uses) came from sustainably-managed forests. FSC is not the only such accreditation scheme in the global forestry market. PEFC(Programme for the Endorsement of Forest Certification schemes) is another. You will often see the phrase “FSC or equivalent” to denote wood pellets from sustainable sources.
Just as lack of quality accreditation does not mean that a product or service lacks quality, simply that its quality has not been verified independently, so lack of FSC or equivalent accreditation does not mean that a wood pellet is from unsustainable sources, just that the sustainability of its sources has not been verified indepedently. Many smaller producers will not be able to manage the cost and bureaucracy of accreditation of one sort or another. They may be using forestry and timber-mill by-products that are the most sustainable inputs to pellet production, as they would otherwise have been wasted, but be unable to provide traceability on these by-products to sustainably- or unsustainably-managed forests. In this context, that should not matter. You should not dismiss wood pellets that are not FSC or equivalent certified, but you may want to ask more questions about their provenance.” (Forever Fuels)
T – Thermostatically controlled heat
Wood pellets are an automated fuel. This means that fuel gets fed into the burn chamber as it is needed and tops up the heat in a property as needed.
Thermostats are placed in different rooms as per gas or oil heating to regulate the heat. Thermostatic valves are also put on radiators.
This means you can have different heating zones and heat different areas of the property to different temperatures easily.
If you wanted to choose wood gasification as an option the fuel cannot be fed automatically on a small scale. Therefore wood is burnt and heats up a lot of water. This is then released automatically on demand to the house.
U – Under floor heating
Wood pellets boilers produce hot water. Using switches you can divert the hot water to where it is needed on demand. Underfloor heating is on a different heating circuit to radiators (e.g. upstairs) and needs to be controlled separately.
Underfloor heating is very popular and it is as easy to use with wood pellet heating as with gas or oil.
V – Verses
In the days where it was only gas verses oil comparisons were quite easy to show.
Wood pellet comparisons (and indeed heat pumps, solar thermal etc) have broadened the choice of consumers and in doing so have made comparisons more complex.
Do you want a comparison based on environmental performance? Cost? Ease of use? Maintenance costs?
More difficult is trying to predict the future and future price rises and availability of all fuels.
There are no guarantees. Hopefully there is some information in this A-Z that helps give you confidence that wood pellets and wood pellet boilers can be an alternative. They can be cheaper to run and installed to a higher standard. The installation can be offset with the RHI (see above), and provide effective, automated, and no hassle, heating.
W – Wood
Wood is the only ingredient of wood pellets. Although any wood can be made into pellets it is normally the softwood that is used. This is partly as it is easier to pellet as it is softer, partly it is easier to grow, and often it grows more quickly.
Many people refer to wood as “biomass”. It is a term that is not that helpful. Biomass is simply biological material from living or recently living organisms such as trees. Therefore when “biomass” is discussed on a large scale it could mean palm oil, pine nut kernels, straw, or anything that burns.
X – “X marks the spot”
When you want to install a wood pellet boiler it takes a little thought as there are a number of variables.
- Where can you put the flue?
- Where is there space for the boiler?
- Where do we want the boiler and which type do we want?
- Can I meet all building regulations part J?
- Am I in a conservation area (or area of outstanding natural beauty)?
- Do I have a listed building?
- Can I meet all regulations cost effectively?
If you are having difficulty with any of these issues do not hesitate to get in contact through the form to the right or on the contact page
Y – Yearly demand for wood pellets
Most people want to compare costs based on 1 year. Unfortunately we have had a few years of volatile temperatures.
In an average year with an average house you will need 20,500 kWh’s. Last year this would have been more as 2010/11 was one of the coldest winters on record. During this time you would have used oil, gas or wood pellets to keep you warm. No fuel is better or worse at doing the job.
With gas you do pay on demand. Therefore you do not pay the summer tariffs for the majority of your gas, but the tariff when it is coldest.
Oil can be ordered and depending on the size of your hopper you can try to get through the winter, but if you get cut short then there is a wait for delivery and the price will have risen considerably.
Wood pellets are like oil in that they have to be delivered. Fortunately that is where the comparison ends. Your storage is unrestricted if you have space enough. Either to store bags or a hopper. ie you can order enough for the winter and keep some in reserve.
Pellet suppliers are keen to look after their customers and keep their own costs down. Therefore it is not uncommon for them to ring you up if they are making deliveries in the area and “top up” your supplies.
Z – Zero Carbon Home
In order to combat global warming many governments have put in place schemes that mix taxes an incentives to help individuals and organisations to reduce their carbon.
In the UK there has been a debate about what constitutes a “zero carbon” home.
When you bury down into the figures, not just on heating, but on construction and living (e.g. food, transport etc) it is very difficult to get within a reasonable distance of this figure not just on cost grounds, but being practical as well.
A good read is “one tonne life” a project about reducing carbon emissions down to one tonne from 7.5 tonnes.