Case study of a wood gasification log boiler in a 6 bedroom house in Wrington, North Somerset
I recently had a 40kW Angus Orligno 200 wood gasification log boiler installed in my house along with a 2700l AKVA accumulator tank. This is linked into my existing heating system and I am open vented on my domestic hot water and pressurized on my central heating. I have my gas condensing boiler that was my primary heating source as my back up heating.
I have the choice of either batch burning or keeping the boiler running all the time. In the late spring through to the early autumn I will batch burn one full loading chamber of wood. This would equte to just over a full wheelbarrow of wood. The boiler will be in full gasification mode and this will last approximately 4 hours and raise the temperature in the accumulator tank by approximately 50°C. This will then provide domestic hot water for approximately 4 to 7 days for a family of four.
In the late autumn or the early spring I will batch burn once every other day to make allowances for central heating and domestic hot water. In the winter I will batch burn once every day to make allowances for central heating and domestic hot water.
I can, if I prefer, keep the boiler running continually if needs be from the autumn through to the spring by loading the boiler twice a day. A typical burn will last 8 to 12 hours when the boiler is allowed to modulate between gasification and slumber mode.
I use two 2 port diverter valves and a 3-way switch to make an instantaneous switch back to the gas condensing boiler. It is not desirable to have the secondary heat source linked directly to the accumulator tank as this will have too slow a response time due to the volume of water. The gas condensing boiler only heats the existing heating circuit of approximately 400 litres.
It is essential to have dry wood and I use a wood moisture meter to check that the wood is between 15 to 20% humidity. This is critical because the right humidity allows the wood to give off the wood gases through pyrolysis. When the boiler fan comes on it drives the wood gases through the ember layer in the loading chamber and you get a jet flame coming into the combustion chamber at the bottom. The temperature has risen from 500°C in the loading chamber to 1000°C in the combustion chamber. The lit combustion gases then travel to the back of the combustion chamber and up a series of eight heat exchanger tubes. It is here that the heat is transferred to the water jacket and this hot water goes to the accumulator tank via the primary flow from the boiler. The accumulator tank is highly insulated and if it is left only loses 2°C a day when in my garage.
I can burn all types of hard wood and soft wood including wood with nails or screws in and my boiler can take a log up to 10” diameter and 500mm long.
I estimate an annual usage of 12 tonnes per annum and working on well sourced wood at £50/tonne I estimate a saving over my annual gas bill of £1600 per year of approximately £1000 per annum.
The installation cost around £10,000 so on fuel savings the product would have a pay back of 10 years. However the DECC are shortly to bring in grants under the Renewable Heat Incentive to be announced July 2011 for commencement October 2012. It is imperative that the product and the installer are MCS approved under the Microgeneration Certification Scheme to receive the grant. All our log boilers are MCS accredited along with our wood pellet boiler.
I estimate that I will receive a payment of £1000 per annum paid tax free over 20 years as I reduce my dependency on fossil fuels for my domestic heating. This would reduce the pay back to 5 years. I would estimate that the payback for this type of product will be between 2 to 8 years taking into consideration the fuel savings and the grants.
I use the installation for product demonstrations and installer training and if interested please contact and we would be happy to organise a product demonstration accordingly.