Domestic Wood Heaters

For dwellings there are two generally available technologies that utilise wood fuels.

Stoves are available as room heaters only or with room heating and centralised water heating capabilities. A less common form provides room heating and ducted air heating to adjacent rooms.

Boilers are generally located in a dedicated boiler room as with a conventional boiler and are used as centralised systems for both room heating and domestic hot water heating.

Modern wood boilers offer the warmth and comfort of traditional wood boilers but are highly efficient, clean burning and often fully automatic, saving you time and money.

Wood boilers come in many different formats. Chip, pellet and log versions are available.

Wood Chip Boiler with Remote Chip Storage and Feeding Mechanism	Wood Pellet Boiler with Adjacent Fuel Storage	Log Boiler

Wood pellet boilers

These can be considered the most popular form of domestic wood boiler as they are fully automatic in operation. Fuel handling is convenient and requires a relatively small storage volume due to the high energy density of the pellets. Storage can be adjacent to the boiler itself or in separate fuel store. In both cases a feeding mechanism (screw / auger) is used to transport the pellets from the store to the boiler. Pellet boilers are generally lit automatically and continue to operate without manual intervention. Higher spec boilers are self-cleaning so you can forget the cleaning chore of traditional solid fuel heating systems. Some pellet boilers can also operate using corn which is similar in shape and has a similar energy content.

Wood chip boilers

These are essentially the same as wood pellet boilers. The main difference is the feeding mechanism. A stirrer is required in the store to ensure a constant supply of wood chips as they are not as uniform in terms of shape and tend to form bridges unless a mechanism is in place to prevent this. The storage volume also needs to be larger than for chips because the energy density is less than for that of pellets. Chip boilers are generally lit automatically and continue to operate without manual intervention. Higher specification boilers are self-cleaning so you can forget the cleaning chore of traditional solid fuel heating systems.

Wood log boilers

Modern log boilers also have very high efficiencies. These are not generally as convenient in use as the fuel logs normally need to be loaded manually. However, a certain amount of storage in an integral chamber limits the frequency with which reloading of logs is required. Log systems work best where heat is required over longer periods of time as the boilers are not as suited to frequent start up and shut down sequences. Some systems use a large hot water storage tank or buffer in order that the boiler can be lit once, the heat stored and subsequently used over a number of days, before the boiler needs to be restarted. Log boilers can be an advantage where you have your own supply of wood as the running costs can be very low while providing you with a degree of self-sufficiency.

Gasification boilers maximise fuel efficiency to achieve, as close as possible, complete combustion of any fuel. Wood log gasification boilers are now commonly available in the Irish marketplace.

Ash

With all types of wood boiler, some ash is produced (usually less than 1% of fuel input). The ash pan needs to be emptied bi-weekly, or less frequently, depending on service. Ash can be disposed of in the garden and is beneficial to plant growth.

Fuel / Storage

Pellets can be purchased in bagged or bulk form. For bulk purchases, a dry covered storage area is required. Generally bulk prices are more competitive than those for bags. Wood chips are generally sourced locally. It is important that the fuel used (quality, size, moisture content) is suitable for the appliance; your supplier will provide you with these details. For pellets, bulk delivery can be arranged to your home where the delivery truck uses a blower to deliver the fuel into your shed, storage room or outdoor silo. 2 tonnes of pellets will deliver the same amount of heat as 1000 litres of heating oil.

General

The latest wood boilers offer remote control and monitoring, accessible by mobile phone. It is important that there is adequate ventilation and that a clean air source is supplied to the boiler, as the combustion process uses oxygen (in the same way as any fuel fired appliance). For correct operation and maximum efficiency a hot water buffer store should be supplied with the system.

High efficiency makes economic sense
Wood boilers use the latest technology to control the amount of fuel and air released to the burner. Typical costs of boiler and installation range from €10,000 for 15kW home boiler to a €20,000 for a 100kW boiler for a large building. The lower cost of the fuel competes well with similar oil systems.

Operation and maintenance
Wood boilers operate just like a conventional central heating system and work in conjunction with radiators or under-floor heating. You can control the heating system and regulate the temperature from a panel control. Most reputable boiler suppliers offer service contracts to ensure an optimum operation and long life of the boiler. Many 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.

Your checklist for selecting a wood boiler

• Efficiency of 85% or higher
• CO2 Emissions less than 200 mg/m³
• After sales service contract
• Automatic cleaning
• Good quality burn back protection

Planning for a wood boiler

• Fuel Storage for 3-4 tonne of wood pellets per year (allow up to 50% more for wood chips due to lower energy density)
• Pellet Storage space of 2 X 3m floor area & 2.25m height (for domestic 15kW boiler)
• Boiler room size of 2m X 2m X 2.5m height
• Select a trained professional installer & supplier

Everyone loves a wood fire, it really turns a house into a home. But open fires and old-fashioned stoves can be polluting, inefficient (typically only 20-30% efficient) and inconvenient. Modern wood pellet stoves offer the warmth and comfort of wood heating but are highly efficient, clean burning and totally automatic, saving you time and money.

Stove types

The most common form of stove is used for heating the room in which it is located. A key benefit of a stove located in this way is that they are very efficient due to heat being released directly into the room, i.e. casing losses are emitted directly as useful heat. A number of different types are available including

Free standing - stove is located away from the wall or chimney with a flue connected to the chimney.

Insert - stove is housed within the chimney breast or wall with a flush front.

Styles

Traditional - e.g. metal with old fashioned look (normally log burning);

Modern - typically upright style with metal / ceramic casing;

Contemporary

Heating distribution arrangements

Stoves are normally used to heat the space in which they are located. Heat is emitted directly to the room without the need for a primary heat distribution system. Variants on this include:

Stove with centralised water heating. These stoves look like room only heaters but have a water circuit and pump incorporated allowing them to work in the same way as a conventional boiler. The water circuit can be used to heat radiators in another room or used for domestic hot water purposes. For smaller and/or low energy dwellings a stove with a water heating circuit is a practical means of providing all space heating and domestic hot water without the need for additional primary heating systems.
Stove with ducted air units for supplying heat to other room(s). These are less commonly used but can provide a way of heating more rooms within the same dwelling.
Many modern stoves use a fan to provide forced convection over the heat exchange units and gently blow air into the room.

Controls

The type and level of control varies considerably.

Limited Manual Controls. Basic log burning stoves generally control the rate of burn and hence the heat output via a manually operated shutter on the fresh air supply vent. Wood logs are simply placed into the burning chamber. When heat is no longer required, no further fuel is added and the fire burns out naturally.

Electronic Controls. Pellet stoves use electronic controls to closely control all aspects of the stove operation. These include automatic on-off timing, air ratios, output, thermostatic and automatic shut-down in the event of unstable operation. A room temperature sensor is normally mounted on the stove to enable room temperature control.

Remote Control. Many pellet stoves now offer remote control units in order that the main functions can be activated from anywhere in the room, e.g. output & on-off operation. Some manufacturers offer the ability to control basic functions such a on-off and temperature settings using text messages from a mobile phone.

Fuels

Stoves use two types of fuel: Wood pellets or wood logs. Wood pellets offer the convenience of automatic operation.

Further information on wood fuels can be found in the wood fuels section.

Wood pellets	Wood logs

Fuel storage

Pellet stoves normally have a storage hopper located at the top of the stove. Pellets are loaded into the hopper from bags (approx 15kg). Storage is usually sufficient to operate the stove for 1 - 2 days under typical operating conditions.

Wood pellet stove with storage hopper

Some manufacturers also provide larger remote pellet stores with a blower or suction system that delivers pellets to the stove hopper thus eliminating the need for manual loading.

System sizing

As with all heating systems, stoves should be sized to give a heat output appropriate for the space being heated under design heating conditions unless the stove is to be used as a supplementary / secondary heating system in a single room where sizing becomes less important.

Publications

Domestic wood burning stoves Consumer Guide

Domestic wood burning boilers Consumer Guide

Wood Pellet Stoves Consumer Leaflet

Home owners Installation Guide for Stoves

Home Owners Guide to Installing a Biomass Boiler

SEAI list of known Wood Fuel Suppliers

Austrian pellet stove and boiler Technology - White Coal Green Energy Conference 2003 - Karl Heinz Lesch

Wood as a Renewable Energy Source - COFORD Connects Publication 2002

Financial Support for Ireland's Sustainable Heat Markets - Bioenergy 2007 Presentation - Pearse Buckley SEAI

Austrian pellet stove and boiler Technology - Karl Heinz Lesch [from White Coal Green Energy Conference 2003]

Case Study: Boiler at Domestic Houses in Kinsale

Wood as a Renewable Source of Energy - [COFORD Connects Publication 2002]

Benefits of Bioenergy, overview of the benefits of bioenergy including wood fuels - (IEA Bioenergy)