Best Practice Guide Wood Burning Technologies for Irish Consumers
Objectives of this Guide? Whether installing a wood boiler in a new or existing dwelling, the initial capital investment will generally be greater than for a conventional boiler. Modern wood burning technologies might be relatively new to you and it is possible that you may have already gathered some useful information with regard to specific products and systems.
Technology Overview For domestic installations there are three common types of boilers: Wood pellet boilers Wood chip boilers Wood log / briquette boilers Combination boilers are also available which can burn one or more of the above fuels. Wood logs have always been used in Ireland as a fuel source.
Pellet store and feed mechanism Wood pellet boiler installation Wood pellet boiler cutaway Wood chip boilers Wood chip boilers are very similar to wood pellet boilers; in fact, many boilers can burn both fuels. Typically wood chip boilers are found in larger scale installations, with wood pellet the preferred option for smaller domestic installations. Chip boilers are generally automatically igniting and continue to operate without manual intervention. Again, self cleaning options are available.
A specific type of wood log boiler, a gasification boiler, has very high efficiencies and is readily available in Ireland. Combination boilers are also available which offer a “dual fuel” burning option, for example wood log and wood chip / wood log and wood pellet.
Fuels What are wood fuels? Today, wood heating is a cost competitive, highly comfortable and environmentally sustainable alternative to fossil fuel. Refined wood fuels are available in a variety of forms including logs, briquettes, chips and pellets. The most efficient way to use wood fuels is in a closed system. Home produced wood fuels are independent of international energy prices and offer Irish consumers a secure, indigenous and carbon neutral source of energy.
Wood chipper in action Typically wood chips used in domestic boilers should possess the following characteristics: A uniform size 8-15 mm Maximum 30% moisture content (the dryer is better) High energy and low ash content Free of dust and fungal spores Wood chips are generally more common for non-domestic heating applications. Another consideration regarding wood chip storage is the feeding mechanism for bringing the wood chips from the fuel store to the boiler.
Wood briquettes Produced from compressed sawdust (similar to wood pellets) they are usually round in shape (similar to a wood log but with a higher energy content and more uniform in shape). Briquettes can measure 25100mm in diameter and between 10-200mm in length. Due to their large size, they are not suitable for fully automatic feed in small boilers.
Clockwise from top left: Wood logs – stacked and covered; Fabric wood pellet store; Outdoor wood pellet storage (plastic example); Outdoor wood pellet storage (metal example).
Below are two examples which detail the calculations for identical situations using different boiler types: Example 2 - Wood pellet boiler Wood pellet boiler - 20 kW output, 90% efficiency, running for 1,750 hours per year (typical) kWh required = 20 / 0.9 x 1,750 = 38,888 kWh per year (note divide the kW output of the boiler by the efficiency to get the actual fuel input energy required) Wood pellet energy content = approximately 4.8 kWh/kg @ 8% moisture content.
Pellet marker used in Austria. Moisture content Pellets Chips Logs Briquettes 8% 30% or less 40–60% fresh 8% dried Ash content 0.5% 1-3% 0.5 % 0.5% Dimensions 6-8mm diameter 10-20mm length 3-45mm (variable) Variable 65mm diameter 25-200mm length Density 652 kg/m3 214 kg/m3 (variable) 350 kg/m3 (slightly variable) 450-550 kg/m3 Specific volume 1.53 m3/tonne 4.67 m3/tonne 2.86 m3/tonne 1.82-2.22 m3/tonne Energy content 4.8 kWh/kg 3.4 kWh/kg 4.
The Irish Wood Fuel Quality Assurance Scheme (WFQA) The Wood Fuel Quality Assurance Scheme (WFQA) is an Irish wood fuel quality scheme. The main objective of the WFQA is to facilitate and ensure best practice operations throughout the wood fuel supply sector in Ireland.
Feasibility and System Design If you are considering installing a wood boiler it is recommended that you examine the possibilities from the point of view of cost and technical perspective. This does not need to be a complex process, the information below will help. If it is deemed feasible to go ahead with a wood system, it also needs to be designed so that it integrates with other systems in the dwelling.
The ash pan needs to be emptied on a regular basis. The frequency will depend on the boiler type, the fuel and also the amount of time the boiler is used. Wood ash can be used in the garden but care must be taken that the ash contains no toxic elements. Controls – control of any boiler system is crucial to the efficient operation of the system. A wood boiler is no different – many higher quality boilers come with integrated control panels that are able to enhance the efficiency of the system.
Wood boiler (left) and buffer tank (right) The size of the buffer cylinder depends on the capacity of the boiler. It is recommended that for every kiloWatt (kW) of boiler power output you have 55 litres of buffer cylinder storage. For example for a 15 kW wood log gasification boiler, the recommended buffer tank size would be approximately 15 kW x 55 = 825 litres. Manufacturers / suppliers may recommend a variation on these numbers depending on the system.
Burn back Protection Measures Interruption design – pellet drop Twin-auger design with rotary pocket feeder Boiler showing melting tube(in blue) Pressure Relief Valves All heating systems incorporate pressure relief valves. In the event of a very high pressure build up in a sealed system due to a malfunction, the pressure relief valve will open up to relieve excess pressure and prevent damage to the system.
or logs are subject to variations in the fuel, e.g. different woods have different net energy contents requiring different air fuel ratios. The net energy content of the wood may therefore vary over time with different batches of deliveries. Lambda control overcomes the problem of fuel energy content fluctuations. A lambda sensor measures the oxygen content of the flue gas and makes adjustments to the air supply rate based on the readings it gets.
to ensure the water temperature is safe for people to use. This is not exclusive to hot water systems heated by wood, it is important for all hot water systems. Thermostats and Timers The control of the boiler on/off times and the temperature in each of the rooms is very important. In order to read room or zone temperatures and to allow the homeowner to set them, a thermostat is required. A dwelling will usually have one “master” thermostat per zone.
Delivery and Access As with all solid and liquid fuels, where on-site storage is normally required, wood systems require storage areas for the fuels. Deliveries can range from self-collection of bags to bulk delivery by lorry. Bagged pellets do not usually cause any delivery or access problems in the case of self collection.
Procurement and Purchase of a Wood Boiler At this stage you will have hopefully decided which type of boiler best suits your needs, and you will have a basic knowledge of how the system might be configured. The process of sourcing and buying a system is the next step. High efficiency makes economic sense Wood boilers use the latest technology to control the amount of fuel and air released to the burner.
difficulties may be uncovered once the installation is started, for example connecting to an existing heat distribution system. Problems such as this will be mainly related to the installation costs so it would still be recommended that boiler costs and associated equipment are quoted separately leaving only variable costs associated with the installation. As with any purchase, there will be options as to which manufacturer’s product you select. There will be equivalent boilers within a quality range.
Boiler Installation Installing the boiler correctly is hugely important. A correctly designed system still needs to be installed in a way that allows the design to be fulfilled in practice. For this reason, engage a suitably qualified professional to carry out the work.
Operation and Maintenance Once installed it is important to operate the boiler correctly to get the maximum benefit from the system and ensure trouble free operation into the future. Your system should be supplied with an operation manual in English that comprehensively covers the operation and maintenance of your boiler. This should always be the main reference point.
Maintenance Most reputable boiler suppliers offer service contracts to ensure an optimum operation and long life of the boiler. Some boilers have an automatic system alarm that is sent to a service engineer via SMS messaging on a mobile phone. Quality boiler suppliers report very low failure rates and most problems are solved within minutes, remotely without callout. Boilers should be serviced on a regular basis, normally once per year.
Sustainable Energy Authority of Ireland Wilton Park House, Wilton Place, Dublin 2, Ireland. t +353 1 808 2100 f +353 1 808 2002 The Sustainable Energy Authority of Ireland is financed by Ireland’s EU Structural Funds Programme co-funded by the Irish Government and the European Union e info@seai.ie w www.seai.
