Wood Pellet Boiler Chimneys and Flues

Chimneys like those above, are almost always higher than the ridge of the roof that they are located on.

The reasons for this include:

  • Dispersing the particles that are in the smoke emitted from the fire
  • Giving the open fire a good draft for burning

Older buildings, like those above, can also be listed and in a conservation area. Adding a new flue to the roof can often be a planning issue. In conservation areas it can be difficult to place the flue at all without altering the view of the roof line as above.

Adding a steel flue to the view above would not look so good. A good approach is to see whether it is possible to use one of the available chimneys within the building for a wood pellet boiler.

In larger homes the answer can be to move the wood pellet boiler “outside” into an outbuilding or into its own container. The flue can be out of site and away from view or damaging the listed building. If the boiler is far enough away from the main building then the flue can be shorter. The fan in the boiler enables the good burning conditions to take place.

The above compact boiler by Wood Energy Solutions is an effective boiler that can rated at 15 or 25kW.

In smaller homes there is less room available. Boilers do have to go in a space within the home and use an available chimney. Commonly this may mean using a Wood Pellet Boiler chimney within a kitchen area, using a chimney at an end of a house and siting outside, or sometimes siting within a living area.

Living area boilers are characterized by being more compact. The boilers have top loading hoppers and they are similar in size to oil boilers or older style gas boilers.

Boilers such as the LP14 by Extraflame are white goods and look similar to other kitchen or utility room items. Within compact ranges outputs of between 10 and 30 kW are typical.

Other boilers are more decorative and have a front glass view of the pellets burning.

This Suite boiler from MCZ can comes with a ceramic finish.

Extraflame are the largest manufacturer in Italy and produce over 100,000 units per year of pellet stoves and boilers for Europe. Italy is the largest market for pellet stoves and boilers in Europe.

The above Duchessa is a popular model.

The living area boilers with cladding and a view of the flame are rated between 9 and 24kW. This is ample for most homes e.g. 4 bedroom detached.

If you are interested in further details on how a wood pellet boiler may be able to work with your home or building then do contact us using the form on the right hand side.

Log Burning Boilers or Wood Pellet Boilers?

If you are considering “biomass” as a heating system the most obvious choices are between log burning boilers and wood pellet boilers.

For some the clear advantages of log boilers are:

  1. Cost of installation
  2. Cost of fuel
  3. Sustainable use of wood

For others the clear advantages of pellets are:

  1. Automation of heating
  2. Higher efficiency
  3. Pellets are a waste product
  4. Space saving

Cost of Boiler

Log boilers such as the WBS active are MCS accredited and burn wood up to 90% efficiency. The boilers start at at a little over £1000 (20kW WBS) and this is a lot cheaper than the Pelleburn wood pellet boiler that starts at £3500.

Cost of Extra Water Heating

Wood pellet boilers modulate. This means that they can adjust to produce the  amount of hot water that you need to store for heating. Log boilers need to heat water when they are burning otherwise there is nowhere for the heat to go. This is often in the range of 30-50l per kW.

For instance a 20kW log boiler will need to heat 600-1000l of hot water.

You could choose an accumulation tank or a thermal store to do this.

An accumulation tank or “buffer store” can used to store the heat. They are well insulated and once heated can distribute the heat without the need for the boiler to be “on”. A 1000l buffer store is nearly 1 meter diameter by 2 meters tall. This does mean that you need to find space for this. The buffer store will be an extra £1000+ and there are extra plumbing costs in installation.

A thermal store can incorporate a hot water tank and heating hot water in one tank. A “tank in tank”. Assuming you need hot water for baths and showers, then you will need a hot water tank. Some houses already have these, some do not. Others are old and need replacing. A thermal store is a good solution if there is a need for a new hot water tank.

Wood pellet boilers do also benefit from the use of accumulation however they can burn efficiently for shorter periods of time and so need less water to heat. On smaller systems it is possible to work without one, however your boiler may need to come on more often. Using an accumulation tank may prolong the life of your parts.

In summary – log boilers need more accumulation than pellet boilers, and this will increase costs of installation. This will vary depending on the installation.

Space Needs

Wood pellet boilers that are completely automated generally infers a large pellet store that can be filled. This can take up a lot of space e.g. 10m3. This has to be dry and purpose built. Whilst there are pellet stores that can be outside for most homes the choice is between a top loading boiler (ie no storage) e.g. an MCZ Musa , an indoor pellet hopper e.g. Pelleburn or a purpose built hopper. This means that the storage needed varies in size depending on the location and budget. For those tight on space a 1.5m by 1m space is generally more than enough.

For logs the space is larger for the smallest boilers. The boiler is larger, needs more space for a larger flue, and accumulation tank. This is typically 2.5m2

Cost of Fuel

Often the most immediately available fuel is more expensive on the internet. Pellets do range in price, but are frequently £200 per tonne or 4.5p per kWh. (Oil prices were 65p per liter (6.5p per kwh in March and are currently 53p per liter, 5.3p per kWh)

Logs vary more in price and this also varies considerably regionally. For those with a ready supply there is the cost of cutting logs ie your time, or alternatively you can get fuel delivered often at £90-100 per tonne 

This makes the cost of logs about half the price of pellets.

Efficiency and Sustainability

A WBS log boiler can be 90% efficient and a pelleburn pellet boiler can be 94% efficient. Some of the efficiency difference can be put down to pellets being dryer and only pulp, ie no bark.

If your logs are from your immediate surroundings then there is no transport or significant processing costs. In addition some pellets have been artificially dried. Whilst not significant, pellets are transported and there is a manufacturing cost that involves drying wood pulp.

If your logs are the result of forestry or woodland management and the wood is offcuts ie cannot be used for other things, then it is sustainable. This is also true for pellets e.g. if as part of making paper there is sawdust and pulp left over then this waste product can make pellets.

There is a strong argument for logs in terms of sustainability and transport. Unfortunately this does not work for many people due to the availability of local woodland management for our homes and logs are not economic to transport.

Renewable Heat Incentive

Currently the non domestic Renewable Heat Incentive pays the same incentive for log boilers as pellet boilers.

A 30kW log boiler May get RHI payments of  30 (Size of boiler) X 8.3 (higher tariff) X 1314 hours = £3271 per annum.

For an installation of logs or pellets costing between £7-10,000 this would payback in under 3 years before you take into consideration the lower cost of fuel.

A Possible Compromise

If you are finding it difficult to decide whether you want logs or pellets. For many the logs do appeal but there is the extra work involved and will you be happy doing this?

The WBS range of log boilers all come ready to fit a pellet burner ie you can convert your boiler over at a later stage. The pp and pelle ranges of burner are easy to fit and with some minor adjustments to your settings you can be burning pellets.

For more information do get in touch with us using the form on the right hand side or by phoning 01225 580 401

Assessing a House for Wood Pellet Heating

Assessing a House for Wood Pellet Heating is a part of our service where we assess requirements, and properties, to help you choose the right system or boiler for your heating and lifestyle choices.

For this we asked Tom Jacques, an architect and RIBA Client Adviser based in the South West for some comments.

Tom responded with 3 articles the first of which is below.

Part 1   Assessing a House and its location for wood pellet heating

Many houses can benefit from wood pellet heating, but before you can install you do have to consider:

1.Is the house listed, in a conservation area, or smokeless zone? If so there are limitations placed on you. E.g.

  • You may have to consider Listed building Consent for flues or internal alterations
  • You may have to “hide” the flue, use an existing building for your fuel store or seek planning consent
  • There is a more limited list of boilers to choose from

2.Is there reasonable access for pellets to be delivered?

  • Some deliveries are “blown” through a tube to a maximum of 20m from the lorry.
  • Other deliveries are in 1 tonne bags which are moved by pellet truck.
  • Alternatively you can have 15kg bags (67 make up 1 tonne, and this is more expensive)

3.Has the house got a chimney which you think could be used?

  • It will have to be checked, and may need a flexible liner to make it safe.
  • If you have thatch you have to be more careful and you may have to check your plans with your insurance company and building control, but many situations can be dealt with.

4.If you are planning to use the chimney then the boiler should be located nearby, especially if your chimney is in a living area. This will affect which boiler you choose. For instance MCZ produce styled boilers for living areas.

5.If you have no appropriately located or serviceable chimney then you will need a new flue. There are building regulations concerning the construction of these. However as a rough guide you do need to disperse the smoke you produce, and this will need to be on your roof or to roof height. For this reason a back wall can be preferable.

6.Is there room for a fuel store? Fuel stores can be as large as 3 x 3m or smaller at 1.5 x 1.5m. Adjustable in height between 1.5 and 2.2m. They can feed a boiler using a screw feed if it is close, or by suction pump if it is up to 8 metres away.

7.How much heat do you need? You will need to have a survey to confirm your heat need, but you can approximate by calculating the volume of the house and if it is well insulated divide by 30. If it is not, divide by a lower number e.g. 25. A survey uses a version of SAP that looks at both the volume of the house and assesses the fabric of the building e.g. windows, insulation etc. This is then adjusted for temperature using “degree days”, a measure of climate in your area. Typically this between 10 and 20 kW for a 4 bedroom house depending on volume and construction. This then helps you choose the boiler that will do the best job for you.

8.The draw of air into a room with a boiler needs to be sufficient for the boiler to burn. Typically air bricks or vents are used to ensure that this is possible. For this reason “middle” rooms may be problematic as you have no direct access to the fresh air outside.

9.Biomass boilers are heavy, some are also quite large. All boiler manufacturers have specifications and show size and weight of their boilers. You will have to locate a boiler on a hearth. If there is not one it will need to be constructed. We also have to get the boiler into the room so doorways and access is important.

Tom Jacques

Tom Jacques is an architect and RIBA Client Adviser based in the South West.

Alongside interpreting,defining and meeting clients spatial and aesthetic requirements, Tom prioritises environmental design alongside an understanding of building physics, the use of appropriate technology, and the performance of buildings and their systems in use.

He embraces different approaches in different circumstances, and works where possible with specialists in their fields. Many of his projects involve work with listed buildings and in sensitive settings, and where planning and listed buildings consent is required.

He is not a specialist in any particular product or technology, nor a building services engineer, but is able to offer advice and assistance on projects where required.

If you wish to contact Tom, please contact us and we will pass your comments on to him.