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Rural Heating - The Facts

Energy prices are rising and for most households, energy bills represent a substantial and growing proportion of domestic expenditure. For the majority of U.K. homes, the most economical and convenient form of heating is still natural gas (see energy cost comparison table below). When burnt at the point of use, natural gas is an extremely clean and efficient fuel, with modern condensing boilers achieving efficiencies of well over 90%. However, for the rural community the situation is different, with many rural properties not being on the natural gas grid, the options available are more limited and careful consideration of a number of factors is required in order to make the most appropriate choice.

The main heating fuels available to rural households are oil (usually kerosene), electricity, propane (bulk tank), propane (cylinders), solid fuel, and wood/ biomass. Within these fuel types, wood / biomass are the only fuels that can be regarded as carbon neutral, and on current trends, can represent and economical and effective way of heating rural properties.

In the UK and continental Europe, log fired wood boilers still represent the most popular and simplest method of burning carbon neutral fuel, in addition to this, the supply chain for wood is well established and reliable and many users of this fuel are self sufficient and manage or own woodland.

Recent fuel cost comparisons (June 2008) show that hardwood logs are commercially available in small delivered loads on a national basis from locally based regional suppliers, and have an average cost of approximately £70-80 per stacked cubic metre. This has a weight of 350-500 kilo and will vary considerably due to moisture content and type of wood, the level of seasoning varies from each supplier and this is a key factor in the weight, performance and cost of the logs. Agood supplier should be able to give an approximate moisture content for the wood and the cost of the wood will be higher the longer is has been seasoned and the lower the moisture content. An average value for the heating capability of normal seasoned hardwood (energy density) is 3-4 kWh/kg. Kiln dried hardwood logs may be as high as 4.5 kWh/kg, and this wood will be premium priced but will offer the highest heat output per kg. Wood at an energy density of 4 kWh/kg will result in an average energy yield of 1700 kWh per cubic metre of well seasoned, stacked hardwood. This provides an energy cost of approx. 4.1 pence per kWh, which is well below the present cost of bulk deliveries of heating oil.

This is however still an expensive method of buying wood for a large (boiler) user, and is more suited to smaller volume users with stoves and wood fired cookers. Bulk buying of unprocessed wood is far cheaper, with typical prices of less than 25% of the above example. This would yield an energy cost of below 1 pence per kWh but would require work to cut, split and stack the wood and an adequate timescale must be allowed for seasoning. This requires space and forward planning but is possibly the cheapest way to heat a rural home in the UK. It also means that the user is in control of the seasoning of their wood and can therefore ensure that this is correctly carried out. Farmers and landowners who manage suitably sized areas of woodland have even greater potential to reduce energy costs and this make wood-firing a highly attractive choice to these groups.

Additionally, because of the simplicity of the design, cleaning and inspection of the Perge MC series boilers can be easily carried out by the end user and removes the annual servicing costs associated with oil and gas fired boilers.

Approximate Fuel Cost Comparison - UK

Prices as of June 2008. Significant regional variations exist and these prices are for guidance only.

Consideration of Wood-firing as a Primary Central Heating Heat source

A wood-fired boiler is an effective means of heating and offers the benefits of being a carbon neutral heat source and of isolating the user from issues of security of energy supply. The fuel can be stored in large quantities and if procured and processed efficiently is one of the lowest cost heating fuels available.

To evaluate wood-firing effectively as a method of heating, it is important to be aware of and understand a number of issues including comparative fuel costs, property type and location, available space, type of lifestyle, user convenience, security of supply and heating system design.

Factors to Consider in Deciding on Wood-firing as a Heat Source

Advantages

• Carbon neutral fuel (if managed sustainably)
• Cost effective especially if bought in bulk
• Can be stored to provide security of supply
• Environmentally clean
• No toxic ashes
• Low capital cost
• Low / Zero maintenance costs
• Extremely reliable
• Long boiler service life
• Versatile and can be linked with other heat sources
• Reduced VAT on boiler and installation labour
• Can be backed up by alternative energy sources

Disadvantages

• Large volume of wood required to heat a typical rural property
• Storage space required under cover
• Must be correctly seasoned for best results
• Manual handling and stoking of fuel
• Commercially available logs can be expensive
• Not easy to synchronise stoking with usage of heating (see system design below)

When considering the above factors, the suitability of the property must be included. The most suitable property will have good access and covered log storage space. The ideal location for a wood fired boiler within a property is in an out house or annexe with space to store wood for immediate use and good access. In addition to the above factors, consideration of a suitable system design will make a huge contribution to the user friendliness and running costs of a wood fired heating system. It is worth noting that using wood as a primary central heating fuel is a much greater commitment than operating one or two wood burning stoves, due to the much larger volume of wood required.

Wood fired Heating System Design

Effective and user friendly utilisation of wood firing for central heating depends hugely on the layout and design of the system. The system should be robust, safe and where possible include a back up fuel / heat source that can be used if required. The following is an outline of two of the systems that can be used for wood fired central heating.

Conventional Gravity Hot Water and Pumped Radiator System

Suitable wood fired boilers or stoves can be directly plumbed in to the heating system in a similar method to solid fuel boilers, or can be used with an accumulator. Some, more sophisticated wood fired boilers are only suitable for use with an accumulator and this depends on the boiler type and manufacturer. The Perge MC series boilers can be used with or without an accumulator.

Without an accumulator, the preferred system in the UK is the traditional solid fuel system, with a gravity domestic hot water circuit from the boiler to an indirect hot water storage cylinder, and a pumped radiator circuit. This system should be open vented with a suitable header tank at least 2 metres above the highest radiator or cylinder, and with a 22mm open vent pipe to the header tank, and an open cold water feed pipe from the header tank. No valves should be fitted in the primary gravity circuit, thereby allowing failsafe venting via the vent pipe, and re-filling of the system from the header tank. The radiator circuit will usually be pumped and will utilise the other top and bottom boiler tappings (if present) to feed the radiator circuit via. a suitable circulator pump. Use of the other pair of boiler tappings will help prevent operation of the pumped radiator circuit from robbing hot water from the domestic hot water circuit. With a two tapping boiler, an injector tee can be used to prevent the radiator circuit from robbing flow from the hot water circuit when the pump is running.

The hot water storage cylinder should be an approved insulated cylinder of minimum size 450mm x 1200mm high (160 litres capacity) with a coil suitable for gravity circulation. It is recommended that a heat bleed radiator is fitted within the gravity circuit to help sink heat from the boiler when it is idling without the heating pump running, this radiator would normally be located in a bathroom adjacent to the cylinder cupboard.

An additional system safety feature for this type of solid fuel system is the addition of an overheat thermostat fitted to the gravity flow pipe from the boiler. This would be set to 90-95 degrees centigrade and must have a permanent live mains feed. This thermostat should switch on the central heating pump in the even of an overheat situation and will safely disperse the excess heat around the radiator circuit.

A further fail safe safety feature is present on all Perge MC series boilers in the form of a “quench coil”. This is a close wound copper coil within the heat exchanger of the boiler, which can be plumbed in to the cold water mains via. a thermostatic valve. If the valve detects an over heat of the boiler, the valve opens and floods the coil with cold water at mains pressure which would exit to a suitable drain. This removes excess heat from the boiler heat exchanger immediately and cools down the system. This fail safe system is widely used on the continent and provides very effective additional safety feature which would still function in the event of a power failure.

The heating circuit / system described is cost effective to install and would usually be present when an existing solid fuel appliance is being remover and replaced. Whilst it provides an effective means of operating a wood fired boiler, it has two main disadvantages. The first is that firing of the boiler is required whenever heat is needed from the radiators, this can be inconvenient, if for example, the radiators are required to come on first thing in the morning. . The second disadvantage is that when low heat or no heat is required from the radiators, the boiler is idling (and possibly still producing more heat than the hot water cylinder can absorb), this can result in unwanted heat to disperse and results in the wood burning inefficiently at low temperatures. Low temperature smouldering of wood results in more tar formation and flue emissions and higher usage of fuel.

Use of an Accumulator (Buffer Tank)

An accumulator or buffer tank is a vessel designed to store a volume of heating water, the vessel should be well insulated to reduce heat loss. The size of the vessel depends on the system performance required and the size of the heating load. Typically, the ideal accumulator will be large enough to store sufficient heat to provide one full day of heating and hot water requirements for the property being heated. The amount of heat that can be stored (measured in kWh) by an accumulator also depends on the temperature range of the output of the accumulator. This is the difference between the maximum temperature and the lowest usable temperature of the water at the outlet of the accumulator. A maximum water temperature of 85 degrees is practical and a typical minimum water temperature for heating radiators is 45 degrees C. This gives a temperature differential of 40 degrees C and this allows the maximum heat storage capacity of a given size of accumulator to be calculated. With a temperature differential of 40 degrees C, a 1,500 litre accumulator will store approximately 70 kWh of heat energy, enough to provide full heating and hot water requirements for a reasonably well insulated 4 bedroom detached house.

The correct size of accumulator would need to be calculated using information including the size of the house, the usage of the heating required and the heat loss of the house. This is similar to the process used for sizing a boiler, however unlike a boiler, the calculation is based on the quantity of heat required over a given period rather than the rate of producing heat in the case of a boiler only.

The benefits in using an accumulator are substantial. Because the heat is stored, the boiler can be stoked at a convenient time, and the heating timed independently with a timer / programmer when required. For example, the boiler can be stoked in the evening, and the heating timed to come on first thing in the morning and again late afternoon. In winter, the boiler can then be relit in the evening to repeat the process. In Spring and Autumn, it may be possible to reduce boiler firing to once every two days, and in Summer, for hot water only, the boiler may be fired even less frequently.

Because the boiler can be fired at maximum firing rate, the combustion temperatures are higher and the boiler works more efficiently, with significant fuel savings (up to 50%). There is also a dramatic reduction in tar and creosote deposits in the boiler and flue, as more of these compounds are burnt up in the combustion process.

An accumulator can be used to link up the wood fired boiler with additional back up sources of heating, and provides a simple and more reliable means of linking up different heat sources than some other systems.

Additional heat sources can include oil, natural gas or propane boilers, or by the use of night time tariff electricity to charge the accumulator by immersion heating elements. The accumulator provides an easy method of harnessing off peak electricity to heat a property with conventional central heating and has the benefit of a low initial installation cost and running costs that are lower than oil based on current energy costs (June 2008). Use of night time tariff electricity is a cost effective and low capital cost means of providing a back up to a wood fired boiler or other renewable energy heating system. Recently in the UK, night time tariff electricity has been an unfashionable and over looked means of heating mainly because of the experiences of people who have previously used storage heaters, where it is difficult to prevent heat leakage during the day and there is often insufficient heat left in the evening from the previous night time charge. With an accumulator this is not an issue, as the insulation on the accumulator is capable of retaining the heat from an overnight charge for several days. This hot water can then be used for central heating or hot water when required and controlled with a conventional heating programmer.

Revised June 2008