Biomass Stoves

Biomass stoves are the modern equivalent of a traditional log fire, providing a warm and cosy atmosphere to enjoy on a cold winter’s evening but with none of the smoke, dirt and emissions of a coal or log fire. However, biomass stoves only qualify for Domestic Renewable Heat Incentive tariff payments if they are designed to operate using biomass wood pellets only and they are also fitted with an integrated boiler.  Biomass stoves are generally designed to heat just a single room and so are normally used in conjunction with another domestic heating system.

According to the Energy Saving Trust® automated biomass stoves with an output of 5 – 7 kilowatts should cost between £2,000 and £4,000 including installation.  Biomass stoves are ideal if you are considering modernising a small but old house without a central heating system but where there is a functioning chimney.

Modern biomass stoves use compacted wood pellets

Instead of installing a wood burner which uses wooden sticks or logs, fully automated, modern biomass stoves are available which are much more sophisticated than ordinary wood burning stoves.  Some modern biomass stoves have automatic ignition, automatic pellet feeding systems from an internal hopper and more efficient methods of supplying air to the burners. These systems can include Lambda controls, more commonly built into biomass boilers, which are used to modulate air flow throughout the burning process in order to improve efficiency. Some advanced biomass stoves even offer remote control access via a mobile phone, with thermostatic readings accessible on a smartphone device.

Fuel hoppers in biomass stoves can typically hold 2-3 days’ biomass fuel supply which are loaded manually from bags of wood pellets or alternatively, more expensive versions offer automatic fuel supply from a storage facility. In common with wood burning stoves, some biomass stoves can provide both radiant and convective heat for a room or they can incorporate an integrated back boiler to provide hot water – a prerequisite for complying with Domestic RHI criteria and receiving RHI tariff payments.

Biomass stoves with integrated back boilers operate in a similar way to traditional open fires with back boilers and are capable of providing all the heating and hot water for a small house. They do, however provide a significant proportion of the heat to the room in which the stove is installed.  That is, unless it is an inset version where the biomass stove is insulated during installation, in which case more heat is directed to provide hot water.

Biomass stoves – Existing chimneys and flue liners

Traditional brick chimneys may be okay for an open wood, log or coal fire but wood burning stoves must have either a liner in an existing chimney or a new purpose made flue. This is because closed wood burning stoves produce more concentrated smoke than an open fire and more condensation can be produced when exhaust gases come into contact with the inside of a cold chimney.  Relying on a standard chimney can result in residual tars soaking into the surface of the bricks or worse, running back into the wood burning stove where they can cause a fire hazard.

An existing chimney must be completely sealed along its full length to avoid gas escaping and also to prevent rainwater ingress at its top outlet. Suitable products include double-skin flexible stainless steel flue liners, a pumped refractory concrete lining or a clay lining to the chimney’s internal brickwork. Internal flue liners should also be the same size as the stove outlet to prevent any build-up of gases, condensation and other potentially harmful emissions.

Where a biomass stove or biomass boiler is to be installed in a property without an existing chimney, a new pre-fabricated flue must be installed and in certain cases this may require planning permission. In any case, whether an existing chimney is to be re-lined or a new flue installed, Building Regulations may apply so you should contact your local authority’s Building Control department – though your installer may well do this for you. Alternatively, if the flue liner is installed as part of the biomass stove installation by a registered competent engineer who has been approved under a scheme such as HETAS, the engineer should be able to self-certify an installation.

Biomass stoves and Domestic RHI

Biomass stoves which use biomass compacted wood pellet fuel and have an integrated back boiler are eligible renewable heat technology systems for the Domestic Renewable Heat Incentive scheme.  This means that householders who install a biomass boiler may apply for Domestic RHI tariff payments of 12.2 pence per kilowatt hour of renewable heat energy generated and used in their homes.  Domestic RHI tariff payments are paid quarterly for seven years after the installation of a biomass stove.

Biomass fuel – compacted wood pellets

Biomass wood pellets for use in biomass stoves generally consist of 100% untreated sawdust and wood shavings which have been compressed under high pressure to form small, uniformly sized cylindrical shapes about 30 mm long and 6 mm in diameter. Normally supplied in 10kg – 15kg bags, they provide a very convenient fuel with very little ash residue when burnt – less than 0.5% of the original weight of the pellets.  They are often a by-product of saw milling, furniture manufacture or tree surgery and are held together by lignin which occurs naturally in wood. Biomass wood pellets are much denser than natural wood and have a very low moisture content which means that they provide compact source of combustible energy with a high calorific value.

To satisfy sustainability criteria set by the Domestic RHI, biomass wood pellets must be purchased from an accredited supplier included on the RHI Biomass Suppliers List and this will ensure that the biomass pellet fuel meets all accepted standards of production and sustainability. If a wood burning stove is installed and you intend to burn wooden sticks or logs it is important that all fuel is properly dried first. This is to avoid a potentially harmful creation of waste products such as tars, creosotes and particularly carbon monoxide in the flue gases which is hazardous to health. However, only biomass stoves which are designed and manufactured to use biomass wood pellets only and no fossil fuel such as logs and wooden sticks, will comply with Domestic Renewable Heat Incentive criteria and qualify for RHI tariff payments.

Biomass stoves - Biomass fuel - Smoke control exempt logo

Stoves displaying this symbol are approved by Defra to burn wood in Smoke Control Areas as they are exempt from the Clean Air Act of 1993.

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