Introduction to biomass boilers

    Heating

What is a biomass boiler?

Biomass boilers are very similar to conventional gas boilers that you will be familiar with, providing you with space heating and hot water for the entire home, but instead of using gas (or oil) to produce the heat, they combust sustainably sourced wood pellets.

Using wood in place of fossil fuels helps to prevent long-term climate change, since the carbon dioxide released during the combustion was actually absorbed while the tree was growing, so they are essentially carbon neutral.

Each year, approximately 8.5 million tonnes of wood goes into landfill in the UK; this waste wood could be used in either biomass boilers (if converted into the pellets) or burned in wood burning stoves. This would not only provide heat and hot water, but in doing so, it would also ease the pressure on landfill capacity.

How does a biomass boiler work?

A biomass boiler works in a very similar way to conventional boilers, combusting fuel to produce heat that is then used to heat water. Biomass boilers are normally substantially bigger than their fossil fuel-burning brothers though, for a number of reasons. Firstly since they are burning wood pellets as opposed to gas, the boiler needs to be larger to hold the larger volume of fuel.

In addition, you may wish to install an automatic feed hopper on your biomass boiler, which will require additional room. This hopper stores a large volume of the wood pellets that are then automatically fed into the boiler as required, meaning that the boiler needs to be refuelled very infrequently.

It is also a good idea to have a store of the wood pellets at your property so you can keep producing heat if for some reason there is an issue with your fuel supplier. Ideally this should be close to where the fuel is delivered to your home, to minimise the distance you have to carry it.

Biomass pellet

Most residential biomass boilers can also run on logs as well as the wood chips, so if these are in plentiful supply or if you can source them cheaply or even for free, it will dramatically reduce the operational running cost of your biomass boiler.

Every four weeks or so, the biomass boiler will need to be emptied of the ash. This can be put straight onto a compost heap to help fertilise the soil.

Biomass boilers are designed to work all year round; however you may choose to turn them off in the summer. They can be coupled with solar heating or an electric shower, providing you with your hot water for washing only, during the warmer summer months.

How does biomass measure up against traditional fuels?

Biomass boilers measure up very favourably in terms of running costs vs. natural gas, heating oil and especially electricity. The numbers can all be seen in the table below.

Figures courtesy of Biomass Energy Centre

Fuel Type Price per Unit kWh per unit Pence per kWh
Wood Chips £100 / tonne 3,500kWh / tonne 2.9p / kWh
Wood Pellets £200 / tonne 4,800kWh / tonne 4.2p / kWh
Natural Gas 4.8p / kWh 1 4.8p / kWh
Heating oil 60p / litre 10kWh / litre 6.0p / kWh
Electricity 14.5p / kWh 1 13.4p / kWh

Standalone boilers

A biomass boiler might simply be too big for your home, but smaller standalone wood burning stoves are also available, which are normally used to heat one room by burning logs or waste wood. These wood burning stoves can be fitted with a back boiler that uses the heat produced when the wood is combusted to heat water, that can then be used for either space heating elsewhere in the home or for hot water only.

Both standalone wood burning stoves and biomass boilers will need a vent, designed specifically for wood fuel appliances, with sufficient air movement for proper operation of the stove. Your existing chimney can be fitted with a lined flue, which is relatively inexpensive.

Can I get a free biomass boiler?

Under the Domestic Renewable Heat Incentive scheme, you will be eligible for payments towards the cost of installing the technology.  These are quarterly, and over seven years, so you will still have to find the money to cover the upfront costs. How much funding you will receive depends on how energy efficient your home was before you installed your biomass boiler. You will start by having an EPC survey, and then payment rates are calculated by multiplying the ‘heat demand figure’ on your report by the current rate for biomass boilers. This means that some models will eventually be paid for fully by RHI payments, but many – especially top-end models – will not be covered completely. Find more information here.

Remember a carbon monoxide detector

It is really important when burning any type of hydrocarbon fuel (natural gas, coal, biomass) that you install a carbon monoxide detector in your home. In theory if all the fuel is 100% burned you produce heat, water and carbon dioxide, but in reality not all of the fuel burns. This means sometimes harmful gases like carbon monoxide can be emitted, which can be deadly. As long as you have a working carbon monoxide detector, you will be able to make full use of all the benefits a biomass boiler can bring.

Benefits

Limitations

Cost

Installing a biomass boiler

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    Biomass boilers vs conventional gas boilers

    Heating

Comparing the cost of biomass boilers with conventional boilers

Biomass boilers start at about £7,000 for a 12kW domestic version, which is sufficient to provide heat and hot water for a 4-bed house. A comparably sized gas boiler will only cost around £2500 to install. A bigger biomass boiler with an auto-feed hopper may cost closer to £12,000.

However, biomass boilers burn biomass fuel, which is considered renewable. Therefore, provided that the boiler is MCS-accredited, you will be eligible for the Renewable Heat Incentive that will pay you for every kilowatt-hour of heat produced.

In terms of the cost of fuel, the average price of wood pellets are around 4.2p/kWh which is very much in line with mains gas, while oil costs a little more at 6p/kWh. However, the price of wood pellets is likely to become more attractive going forward since gas prices have continued to rise in recent years, and this trend looks to continue. Biomass boilers are completely independent of the fluctuating import prices of foreign fuels such as gas and oil.

Also, if you are lucky enough to have a free supply of wood, then you can heat your home at zero cost.

Comparing the efficiency of biomass boilers with conventional boilers

Biomass boilers run at an efficiency of 89 – 91% (Trianco Greenflame – 91%; Angus Orlingo 500 – 92%), while the top-rated gas boilers similarly run between 88 – 91% efficiency (Baxi Duo-tec HEA 91%; Valiant Ecotec Plus series over 91%); therefore biomass boilers are comparable to conventional gas boilers.

Comparing biomass boiler maintenance with conventional boilers

If you decide to opt for a biomass boiler, you will have to feed it fuel from time to time. For example the Baxi Bioflo will need to be refuelled every 3 -4 days. Bigger biomass boilers with auto-feed hoppers will only need to be refuelled every couple of weeks. In addition, ash builds up as part of the combustion process (wood combustion produces about 0.5% – 1.5% by weight of ash) which will need to be emptied too.

A gas boiler is very much an ‘install and forget’ technology, so it is the clear winner here.

Comparing the size of biomass with conventional boilers

Biomass boilers are much bigger than conventional boilers, since they will have to have some element of an auto-feed option. In the case of the Baxi Bioflo, it is 1.2m high, while a conventional gas boiler is half the size.

The other thing is that gas tends to come from the mains gas supply via a gas pipe. A biomass boiler will need to be situated relatively close to a fuel supply, which obviously means you are going to need a decent-sized storage facility that can be kept dry and contained so there is no fire risk.

Comparing the environmental credentials of biomass boilers and conventional boilers

Biomass boilers run on biomass fuel. Biomass fuel tends to be derived from quick-growing trees and fuel grasses. Trees absorb carbon dioxide as part of photosynthesis, which is the process that allows them to produce sugars required for growth. When the trees are felled and then burnt, they release the same level of carbon dioxide that they took in, meaning that biomass is arguably a carbon-neutral fuel.

It can be argued that gas is also derived from the breakdown of vegetation, but this process takes millions of years, and we are currently using the gas at such a rate that we are getting through our supplies much quicker than it is being replenished; it is therefore not sustainable. As a result gas and other fossil fuels are not considered to be renewable, since the time scale for the process to occur is simply too long.

As a result, burning biomass is not considered particularly harmful to the environment, whereas burning gas is considered to have negative consequences.

The other thing to bear in mind is that growing crops for fuel is only okay if the land that the crops are being grown on has not historically been used to grow food. Growing fuel crops at the expense of food crops is obviously an unsustainable practise.

The final verdict – biomass versus conventional boilers

MCS-approved pellet fired or gasification biomass boilers are comparable with the most efficient conventional gas boilers and, through the Renewable Heat Incentive, they actually produce a very healthy return (provided the heat demand on your EPC is sufficient). Therefore provided you have the space to house the fuel and the boiler itself and you are happy that you are going to have to ‘feed and clean’ the boiler on a regular basis, then a biomass boiler is certainly worth considering.

Obviously they are very expensive to buy upfront, so this too is something you have to bear in mind. If you have access to cheap finance then installing a biomass boiler could actually be a no-brainer!

Benefits

Limitations

Cost

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