For a farm with its own woodland, enough storage space and a heat demand of several tens of kilowatts, a wood chip boiler is usually the cheapest form of wood heating. Pellets are the more convenient choice if space is tight, there is no timber of your own or the heat demand is fairly small. In Germany both systems must meet the limits of the First Federal Immission Control Ordinance (1. BImSchV), and both are funded under the Federal Funding for Efficient Buildings (BEG). Profitability depends on fuel costs, utilisation and your own labour.
Wood chips and pellets compared
| Criterion | Wood chips | Pellets |
|---|---|---|
| Fuel | from your own timber or bought in, variable quality | standardised, uniform quality |
| Energy density | low, a lot of storage volume needed | high, roughly three to four times more compact |
| Fuel cost | low with your own timber | higher, depends on the pellet market |
| Technology | robust feed systems, higher investment | simpler, cheaper at small outputs |
| Labour | harvesting, chipping, storing, ash | delivery by blower truck, little ash |
| Typical output | mostly from around 30 kW | also small outputs |
Under the 1. BImSchV, wood chips count as untreated lump wood (fuel no. 4), while pellets made from untreated wood are a separate fuel (no. 5a). Both may be burned in type-tested, automatically fed boilers.
Pellets have a net calorific value of around 5 kWh per kilogram. The calorific value of wood chips depends heavily on species and moisture; one loose cubic metre (Schüttraummeter) yields roughly 700 to 1,000 kWh. With wood chips, therefore, work with loose cubic metres and moisture content, not just tonnes.
Own timber, storage and drying
The economic advantage of wood chips comes mainly from using thinnings, crown wood, hedge cuttings or small roundwood from your own farm. You then essentially pay for chipping, transport and your own time.
Freshly felled wood often contains around 50 per cent water. Wet chips heat up in the pile, lose substance and grow moulds whose spores irritate the airways during handling. These steps have proven useful:
- Let the timber pre-dry over the summer in an airy spot at the forest edge or on the farm, and only then chip it.
- Store chips under cover and well ventilated, ideally on a solid floor.
- Dry technically if needed, for example with waste heat from a biogas combined heat and power (CHP) unit or with warm air through a ventilated floor.
- Wear respiratory protection when handling dusty or mouldy material.
For many small and medium boilers, a moisture content of up to about 30 per cent makes sense. Larger boilers with moving grates can handle wetter material depending on design. The store should be big enough to cover the heating season, or a large part of it, without constant refilling.
Pellets are delivered loose and blown into a storage room or fabric silo. Carbon monoxide can build up in closed pellet stores. Ventilate the room before entering and follow the manufacturer’s instructions.
Choosing the right boiler size
Boiler output should match the heat load of all connected buildings, not the size of the old oil boiler. On farms, the farmhouse, retirement cottage, workshop, milking parlour or piglet rearing unit add up, but they do not all need full output at the same time.
A first estimate starts from annual heat demand: divide past consumption in kilowatt hours by a realistic number of full-load hours. With an annual heat demand of 150,000 kWh and 1,800 full-load hours, for example, you arrive at around 83 kW. For the actual design, a heating engineer should calculate the heat load according to the relevant standard.
Oversized wood boilers often run at part load, cycle more often, wear faster and emit more dust and carbon monoxide. A sufficiently large buffer tank smooths peaks and extends run times. Where demand is very uneven, a wood boiler for base load combined with a peak load boiler can make sense.
When choosing a system, think about day-to-day operation too. Wood chip boilers are sensitive to oversized pieces, stones and strongly varying moisture, which can block feed augers or impair combustion. Make sure the fuel quality suits the boiler. For wood chips, the standard ISO 17225-4 defines quality classes by size, moisture and ash content; for pellets, the ENplus A1 certificate is widespread. Plan for annual servicing by a specialist, plus regular ash removal and cleaning of the heat exchangers. Good access to the boiler, discharge system and ash container saves a lot of labour later.
Emission limits under the 1. BImSchV
The 1. BImSchV applies to combustion plants that do not need an immission control permit, which covers typical farm boilers. For wood chip and pellet boilers installed after 31 December 2014, the stage 2 limits apply from 4 kW rated heat output:
- dust: 0.02 g/m³
- carbon monoxide: 0.4 g/m³
In addition, section 5(4) of the 1. BImSchV requires water heat storage, for automatically fed boilers at least 20 litres per kilowatt of rated heat output. The requirement does not apply, among other cases, if the boiler meets the limits even at its lowest adjustable output. The chimney sweep measures after commissioning and then at regular intervals. From a rated thermal input of 1 MW, other rules apply, and such sizes need their own permit planning.
Also check the combustion ordinance (Feuerungsverordnung) of your federal state: above certain outputs a separate boiler room is required, and large quantities of fuel must be kept in separate storage rooms.
Funding and profitability
Wood boilers in residential buildings can receive grants under the Federal Funding for Efficient Buildings (BEG), with a base rate of 30 per cent of eligible costs and possible bonuses. Requirements include a sufficiently large buffer tank and stricter emission standards. Some bonuses are being reduced step by step. The guide to funding gives an overview of schemes and deadlines.
Heating in non-residential buildings such as a workshop or farm office can also be funded under the BEG. If the heat is mainly used for business processes, such as drying or heating livestock buildings, other programmes may apply. Clarify the classification before applying, because it determines the funding rate and the responsible body.
To judge profitability, compare the full cost per kilowatt hour of heat: capital costs for boiler, store and feed system, maintenance, electricity for drives, chimney sweep, fuel and your labour. The higher the utilisation, the more the low fuel cost of wood chips counts.
Local heat for neighbours
Many farms sit at the edge of a village or right in its centre. A larger wood chip boiler can also supply neighbouring houses, the school or the village hall via a small heat network. This improves boiler utilisation and creates extra income.
Plan enough backup, such as a second boiler or a peak load boiler, because your customers expect heat during maintenance and breakdowns as well. Network heat losses, pipe lengths and connection density determine profitability. Set out prices, price adjustment, term and liability in a written heat supply contract. How this works with biogas heat is explained in the guide to biogas heat use.
Installers of wood chip and pellet heating systems are listed in the directory under wood energy.
Frequently asked questions
How dry do wood chips need to be?
Freshly felled wood often has a moisture content of around 50 per cent. For storage and for many smaller boilers, a moisture content of no more than about 30 per cent makes sense, otherwise the pile decomposes, heats up and grows mould. Larger boilers can handle wetter fuel depending on design. The manufacturer's specification is decisive.
Does a wood chip boiler need a buffer tank?
Under section 5(4) of the 1. BImSchV, automatically fed boilers generally need at least 20 litres of water heat storage per kilowatt of rated heat output. Exceptions apply, among others, if the boiler meets the limits even at its lowest output. Funding programmes often require larger tanks.
How often is a wood boiler measured?
In Germany the chimney sweep measures new wood boilers from 4 kW after commissioning and then periodically, under the 1. BImSchV once every second calendar year. If limits are exceeded, the boiler must be adjusted and measured again.
Can I sell heat to my neighbours?
Yes, supplying heat to third parties is possible, but it needs a written heat supply contract, enough boiler capacity with backup and clarity on tax treatment. Standard contracts with households are governed by the AVBFernwärmeV, the German regulation on general terms for district heating.
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