FIVE-MINUTE GUIDE TO Wood burning stove Biomass Electric MVHR Solid fuel

113 Five Minute Guide to Biomass Heating 116 Interior Timber Cladding: How to Get the Look 122 A Buyer’s Guide to Rainwater Goods 125 How to Create a Room Outdoors 130 10 Things you Need to Know About Garage Conversions

Inspiration, design ideas, planning and product advice to create the perfect brief for your home

The last few years have been a bit of a roller-coaster ride for biomass: there’s been a huge increase in the number of woodburning stoves, a bit of an increase in boiler sales, followed by a downturn as the renewable heat incentive (RHI) tariff tumbled.

The term biomass, in this context, refers to any organic material that can be used as fuel. Generally, we are talking about wood. Straw, oil seed rape and miscanthus are all used as fuel, but the huge majority of biomass is wood and that is what we will be talking about here.

As a fuel, wood is available in three forms: logs, chips and pellets. There are technical differences but for homeowner purposes wood chip tends to exclude itself as the boilers available are mostly sized at 50kW and above. There are a couple of smaller options but even 30kW of heat is still around three times more than the typical new build will need.

For these purposes we will be focusing on logs and pellets. The main difference is that pellet boilers can be automated and log burning appliances and/or boilers will always be manual. Ground source heat pump

loaded at least once per day, in some cases three or four times a day, and the ash cleaned out daily. A manually fed wood pellet boiler will typically be loaded once each week (in winter) and the ash can emptied maybe twice each year. An automatically fed wood pellet boiler will have the bulk store loaded once or twice a year, direct from the delivery lorry, with the ash can still emptied once or twice a year.

Some people enjoy loading logs and others prefer the convenience of pellets. In both cases, maintenance comprises an annual service, as it does with a gas or oil boiler.

hoW do they Work?

Set a piece of wood on fire and it will give off heat. The drier the wood the more heat it gives off. Put that bit of wood into a steel box and it is possible to capture more of the heat and use it in a more controlled way. By doing that we are introducing efficiency and the term to look out for is ‘gasification’. A gasification boiler is one that burns the fuel as a batch in a two-step process. The wood is heated until it gives off gas. The gas is passed back through the combustion chamber and used to burn the residual material, which in turn heats the next batch of wood to gasify. This technology used to be restricted to pellet boilers but is now also available in log boilers (such as the Walltherm from Wallnöfer).

types of fuel

You might think that logs and pellets, both being wood, are technically largely the same. Logs are big and pellets are very small. Kiln dried logs will have around 20% moisture content and pellets around 8%. The combustion chamber in a pellet boiler will be not much bigger than a pint pot; it will be self-igniting, self-regulating in terms of heat output, self-extinguishing and self-cleaning. A domestic-scale log boiler is none of those things — but the output of any appliance will only be as good as the fuel that you put in. A gasifying log boiler using good quality fuel will still achieve over 90% efficiency; a gasifying pellet boiler around 93% or 94%.

Moisture content in wood pellets will be around 8% — it is governed by compliance with ENplus regulations. The ENplus A1 standard (which is what you will want to buy) ensures it is a good quality fuel that will deliver at least 4.8kWh/ kg. The moisture content of kiln dried logs will be around 20% which will give a heat output of around 3kWh/kg, although this will vary — oak has a much higher heat output than birch, for instance. But logs tend to be sold by volume rather than weight, in crates or bulk bags. One supplier offers a big crate of 1.6m3, which could weigh about one tonne, for £240. The current price of wood pellets is about £280 per tonne. That gives a unit price for logs of 8p/kWh and 5.8p/kWh for pellets. The difference is the moisture content.

Some people will want to harvest their own wood, in which case it is likely to be dried naturally. Depending on the species, the time of year the wood is collected and whether the tree was

Self-builder Richard Kidd chose a biomass boiler for his 250m2 detached four bedroom family home in West Sussex. “The main reason was the sustainability factor,” he says. “I wanted to build a home that would use a low carbon fuel but still provide me with reliability at a low cost.”

Installed in two days by McS-registered Intelligent Heating Solutions, the 15kW wood pellet Windhager BioWIN 2 biomass boiler is housed in a standalone plant room and provides all the heating and domestic hot water requirements for the house’s annual heat demand of 17,500kWh. Under the RHI scheme, the family receives £1,250 a year for seven years; the annual cost of 4.5 tonnes of wood pellets is around £1,125.