Always the best fuel is the least fuel and this is down to insulation. Enough insulation and you don’t need a central heating system. This is the basis of the PassivhausSee more on the Passivhaus standard. The PassivHaus Institute has pioneered a standard for low energy buildings. It includes very low energy usage and ways of achieving this. The word is derived from the idea of buildings which are fundamentally low energy and passive solar heated rather than using extra gadgets to heat them. See Passivhaus for the UK branch of the organisation. system. With enough insulation the heat naturally generated within a house (lights, cooking, body heat etc.) is enough. When possible go for Passivhaus standard. See the EPBDEnergy Performance of Buildings Directive. This is the EU directive that all new buildings shall be "Nearly Zero-Energy Buildings’ from 2021. target.
The nuclear uncertainty
Energy sources can generally fall into two types: renewables and non-renewables. Where you decide to put nuclear energy is a bit of crystal ball gazing because whereas nuclear fission is looking quite limited due to the amount of high quality uranium ore available, and the state of the market, nuclear fusion may, just, by the skin of its teeth, come to the rescue in time. Then we would all have limitless safe energy. Don’t hold your breath.
The two problems with nuclear fission are that accidents can be catastrophic, and no-one has yet found a secure way of dealing with the waste products, except possibly the Finns. See for instance the recent National Audit Office report on the appalling state of Sellafield. (There is also the insidious link between nuclear power generation and nuclear weapons – though it can be argued that this is more a matter of politics than technology)
So what are the choices for energy? (Solar, wind and hydro are dealt with under Energy Harvesting)
Always it makes sense, if possible, to go for better insulation and air tightnessA measure of how leaky a building is to air. In other words, how draughty it might be. There are now standard fan pressure tests to check how air tight a house is and the Building Regulations have minimum standards for all new houses (L1A – Conservation of fuel and power in new dwellings (England)). A much higher degree of air tightness is covered by the Passivhaus standard rather than burning more fuel. For instance a Passivhaus is so well insulated that it needs no central heating system or stove.
However if you go for a wood burning stove it is important to get straight whether you want a stove because –
- it is a cosy thing to have on a cold winter’s night
- it is a serious form of total heating
- it is a bit of a backup to a central heating system
- you have access to lots of free timber
- it seems like a green alternative
These factors will create very different outcomes.
Potentially, wood burning stoves are one of the greenest forms of domestic heating in the UK in terms of energy and materials sustainability. This is because they are almost CO2 neutral (in the long run) and there is a very considerable amount of timber goes to waste anyway. Sulphur emissions are almost zero but smoke pollution can be very high if stoves are not well designed or not run correctly.
However there is a big question mark over the CO2 emissions for the first few decades that timber is felled and burned. Presuming that the woodland where the timber came from is replanted with trees, these trees will start to absorb CO2 from the atmosphere as soon as they start growing but it is going to take them anything between 30 and maybe 100 or more years to get to the size where they have absorbed all the CO2 which was given off the moment the previous trees were burnt. So in the very long run they are almost carbon neutral but in the shorter term there is a lag time and the environment takes a large hit. (Compare this with using timber to build with, where the carbon in the timber can be locked in place indefinitely.)
Practically, there are also several possible drawbacks to their use
- buying and storing logs, pellets and chips, along with stoking, management and temperature control can take up a lot of time and space.
- as soon as insulation levels are seriously improved in a house the issues with woodburning stoves as room emitters become, first of all, how to burn them low enough efficiently without going out and secondly how to get most of the heat into a central heating system, and then a hot water store or masonry store in the case of a kakkelovn. Log burning works best and is cleanest when the stove is burning rapidly but then there may be excess heat into the room and this needs capturing for later.
- smoke emissions, The legislation around the Clean Air Act does allow for dark smoke to be emitted providing ‘that the alleged emission was solely due to the lighting up of a furnace which was cold and that all practicable steps had been taken to prevent or minimise the emission of dark smoke’. The Log Pile Website has good information on emissions and also on the safety aspects of burning wood.
how clean is wood burning?
Arguably one of the greenest methods for heating a rural house in the UK (including domestic hot water) is a combination of solar thermal collectors (for summer) and wood pellet or chip (for winter). Logs come a close second if they are dry and burn cleanly. They are also very low in CO2 emissions; (but see the notes above), something better than a tenth of most other fuels.
However burning wood in an urban setting will almost inevitably lead to dangerous levels of air pollution if it is practiced on a large scale rather than the occasional ‘romantic’ evening for lighting the stove. Already major air pollution problems are showing up in North American and German small towns where wood burning is reviving as a tradition. Denmark is getting rid of its older generation of wood burning stoves on the basis of pollution (and interestingly they are turning up in the UK).
The Clean Air Act covers this problem in many urban areas. It is possible to get very clean burning stoves with secondary or even tertiary combustion and catalysts. Many of these are what is known as ‘exempt appliances’ and Defra have a web site covering which stoves and appliances are exempt for which types of fuels. In many cases stoves need a small modification to the air inlet before you can burn logs on them.
Most manufacturers insist on logs being ‘seasoned’ or air dried. According to the graph on the Forestry Commission web site Wood as fuel: a guide to choosing and drying logs, ‘air dried’ means they have a moisture content of a 33% or lower ( compared with green wood which may be up around 60%). This is rather a peculiar statement because 33% is nowhere near 20% which is what proper air drying of timber achieves. Most of the approved stove manufacturers insist on 20% as a condition of achieving a clean burn. Well seasoned wood (at about 20% moisture) will give out about twice the energy of green timber when burnt.
However, burning of timber is not always carried out as cleanly as it could be and particulate emissions, when present can compare very badly with gas boilers:
- although this table from the US is part metric and part imperial, it does demonstrate the huge difference in pollution output between various fuels, especially around the comparison with oil and gas boilers (what they call furnaces)
is there enough timber?
The UK undoubtedly has a considerable amount of timber which is under-utilized or goes to waste (see Woodfuel Resource in Britain – Main Report page 75). However the question of long term sustainability is very complicated for a number of reasons. The whole subject has parallels with the debate about using vegetable oil as a bio-fuel (substitute for diesel oil). This initially seemed like an obvious way forward until the implications of using a food crop as an engine fuel on a global scale became apparent……see more on the issues of burning timber
The following issues are important:
- How do you decide how much of the nation’s timber is only fit for burning? This leads to two further questions-
- How much timber is there anyway? There are big gaps in the statistics because a good deal of UK timber is grown in small, under managed woodlands for which figures are not reliably available. Also a large volume is disposed of in a small scale ad hoc manner in the form of local sales, factory offcuts, tree surgeons’ waste etc. Bear in mind also that a large proportion of timber waste comes from manufacturing offcuts. Information from the Biomass Energy Centre indicates that in England alone there are about 3 million tons p.a. of timber available (at 35% moisture content) from under managed woodlands and as this is not being collected at present it is not in competition with other existing markets. Assuming some very ‘ball park’ figures, this would equate to maybe 10-12 TWH of heat. Given that a fairly well insulated medium sized house (say 120 m²) might use 12,000 kWhkilowatt hour. This is a unit to measure an amount of energy. If you run your 30 kW gas boiler for 2 hours a day you use 60 kWh per day of space and water heating per year then going on for a million extra houses could use wood for their main source of heating. This assumes that all the spare wood gets used which, of course would never be the case.
- What are the competing uses, such as manufacturing timber particle boards like MDFMedium Density Fibreboard or OSBOriented Strand Board. It may be better to use timber waste for this rather than constantly importing plywood. Greenpeace are campaigning for a shift from ply to OSB in cases where the present supply of ply is unsustainable. Although most homegrown softwood timber in the UK is of fairly low quality (compared with say, Baltic timber which grows more slowly and to a higher quality) there is a strong possibility that modern computer controlled techniques for comb jointing and laminating timber will mean that lower quality timber will be in demand for joinery rather than for burning.
- Added to this there is the possibility of importing wood for use as a fuel and this could theoretically be a carbon neutral activity if ships could run on wood pellets or hydrolysis based fuels! Transport by ship tends to be a relatively low energy method anyway.
- How do you decide whether more or less agricultural land should be used for forestry in the future? There is the potential for considerably more woodland in the UK if meat production were reduced. This could help to lower nitrogen, methane and ethylene pollution at the same time. Ghent in Belgium has recently declared a ‘vegetarian day’ one day a week because of concerns about the extremely intensive use of land in the country.
- There is pressure from conservationists to do less ‘tidying up’ after forestry operations because fallen rotting timber is the very basis for much of a healthy woodland eco system. This conflicts with the idea of clearing everything up and burning it.
- How likely are the owners of the smaller woodlands to make more timber available for fuel?
There are a few useful web sites dealing with the issues of fuel wood supply –
- The Biomass Energy Centre who have a good deal of information on decision making with systems, fuel, delivery, supply etc
- The Forest Research Woodfuel Resource site is an academic study of the subject
- The Woodfuel Strategy and Implementation Plan for England by the Forestry Commission
There is also the question of what sort of timber species to plant in the UK in the future and to what extent woodland management should be geared partially towards fuel production. The last 150 years or so of UK planting policy has been an almost unmitigated disaster. Oak for sailing ships was planted just as iron ships arrived, larch and Douglas fir were planted for pit props just as steel started to take over, poplar was planted for the matchstick industry just as gas lighters were invented and sycamore was planted in large amounts everywhere for reasons no one can remember or understand.
And then there was sitka spruce!
At some point sensible management seemed to go out the window and what took over was management of woodland by keepers for pheasant shooting and sending all the bits of timber off to Sweden for paper making.
At present it is difficult to find any woodland in the UK which is not basically managed for pheasant shooting. Try to find a wood without the blue plastic feeding bins for pheasants. Even if you are not against the wholesale slaughter of one of the most beautiful birds on the planet and even if you are not bothered that many of those birds are wounded rather than killed by pellets from badly aiming bankers then it is difficult to avoid the conclusion that most of the management of woodland is skewed to one end only.
whole house heating
If you seriously want to have a stove as your primary source of heating (and possibly cooking) then you need to have a heating engineer or experienced plumber do calculations to establish the optimum size. In the case of stoves which combine central heating with being a decorative focal point in a room then the ratio of how much heat comes directly into the room compared with how much goes into heating the water becomes very important.
For instance you may find you need to run the stove flat out in order to heat bath water and radiator water in a hurry). Say this took 5 kWkilowatt - a measure of how fast energy is flowing. e.g. electricity might flow through an electric kettle element at the rate of 2 kW (3kW for bath water and 2kW for radiators). If the stove was designed to split its heat output 50/50 between the boiler and the room heating then you would also get 5 kW of heat into the room where the boiler was situated. This would make nonsense of energy efficiency because even a large living room in a well insulated house would need less than 1kW of heat during freezing weather conditions so the extra 4kW would have to be wasted out through an open window in order to prevent over heating (or you could run around the house opening and closing doors to spread the heat). What happens in spring, autumn and summer becomes a joke. We have become so used to well regulated gas and oil boilers with sophisticated thermostatic controls that designing a log burning system down to low energy standards is extremely difficult.
single room heating
If this is what you want then you probably need one of the smallest models of free standing stove on the market, usually made of decorated cast iron and with a self cleaning window in the door. The self cleaning bit is achieved by the incoming combustion air ‘washing’ down over the inside of the window. Also it should have secondary and possibly tertiary burn so that little smoke, creosote and soot is produced. The reason it should not be too large is that it will mainly be heating the room it is in (unless you keep all the internal doors in the house open to spread the heat around) and if the room is well insulated it will probably need less than a couple of kW. to heat it, even in very cold weather. If the room is poorly insulated or draughty then you will need to seek advice on how big a stove to get.
choosing the type of stove
The problem with log burning stoves is that they tend to need constant supervision to make sure that the correct amount of fuel is loaded and that the air supply is properly adjusted. If this isn’t done they are either going out, overheating or burning smokily. (compared with wood pellet and woodchip stoves which are easy to control). There are four main reasons you might consider using a wood burning stove and they all lead to different types of stove, especially if the uses are combined. The basic functions are:
- Space heatingthe heating of the rooms as opposed to the heating of the domestic hot water. See Space heating and Domestic hot water only – basically a fire box with a door for loading fuel and emptying ash.
- Central heating (including domestic hot water) – ditto but with a water jacket around it
- Cooking – the fire box with hobs and an oven
- Focal point / Charm factor – any of the above but usually with a window to see the flames
A lot of stoves in the past were aimed at drafty, poorly insulated houses and were designed to kick out as much heat as possible. With a well insulated house the challenge is rather to be able to control a stove so that it burns cleanly and reliably on very low output. A really well insulated house such as a PassivHaus would overheat with any stove except under the severest weather conditions. Very few wood stoves are designed with eco houses in mind, especially when it comes to combining room heating with water heating or cooking. See below. Probably the first decision is whether you want the stove to be the main source of heat in the house or whether it simply augments an existing central heating system and gets lit on cold winter nights
pellet and chip
Both pellet and chip are almost carbon neutral and can be burned in boilers in a controlled way so as to be efficient and very low in pollution (particularly low in sulphur). They can both be automated so that fuel is automatically fed in as required. Pellets can be delivered by tanker and blown into the storage area or come in tonne lots either as a 1 tonne bag or in 50kg bags. The ash is minimal and usually only needs cleaning out once a month or so.
With both chip and pellet it is important to be able to get local supplies of fuel of the right quality. With chip there have been a number of cases of the material being too moist when delivered and this can lead to poor combustion and also auger jams. With pellets it important that they do not break down and form dust while being delivered or pumped into a hopper or store. Dust can be an explosion hazard and any ductwork needs designing and fitting properly to avoid risk.
Although wood pellet boilers (rather than stoves) tend to be expensive, the fuel costs have remained low compared with other fuels. See the Biomass Energy Centre for comparisons. Recently there have appeared on the market pellet stoves which give out 15% of their heat through a glass door into the room and the rest into a water jacket. This sort of ratio becomes important in a well insulated house because rooms can easily overheat if a large proportion op the heat goes into the room rather than heat exchanger
Another recent development is the balanced fluepipe to conduct gas, typically ventilation air or boiler exhaust. see FlueA combined flue, usually serving a gas heater or boiler, which handles both incoming combustion air and outgoing waste gas. pellet boiler (with combi option) which can be placed against an outside wall and does not need a conventional chimney or flue. See the Ariterm Arrow flue channel system imported by BioNordic, which can be used with several of the Ariterm models.
Ariterm are also about to put a zero emissions pellet stove on the UK market.see BioNordic's press release
Unfortunately the UK has lagged behind in stove design and most are imported from Northern Europe, Italy and Austria. Fuel supplies are patchy here (but increasing quickly) and so it is important to check this out for your area. A considerable amount of information is available on the Logpile web site. The Biomass Energy Centre web site (part of the Forestry Commission) gives some comparison of fuel costs per kWh including pellet, chip, gas, oil and electricity. (but note that these are not quite the same as delivered heat costs) It is also important, for wood chip, to make sure that the fuel is available with the correct moisture content as there have been several cases of boilers not operating correctly due to the fuel being too damp and clogging in the auger feed.
The supply of both chip and pellet determines the amount of storage needed for minimum delivery quantities. The storage should be adjacent to the boiler so that an auger feed can be used to transport the fuel over.
see also Timber as a fuel
The burning of dry timber logs in the UK can make good sense for reasons similar to chip and pellet and modern stoves with air washed glazed doors and secondary and possibly tertiary burn can be extremely clean and efficient. The cost of the fuel has so far remained fairly low. The CO2 output is also extremely low.
However log burning does have certain peculiarities which the other two fuels don’t –
- the fuel is often inconsistent in terms of size of logs, calorific valuethe amount of heat energy contained in something. For instance wood logs contain about 4200 kWh per tonne (at a moisture content of 20%) and moisture content and it is difficult to tell how much fuel you are getting in a load unless you know both the weight and the moisture content.
How to buy logs for fuel
Weight for weight, all the timbers in the UK have very similar calorific values. See the Forestry Commission site on this. It works out at a bit over 5000kWh/oven dry Tonne or about 4200 kWh/Tonne at 20% moisture. This means that softwoods are bulkier than hardwoods but give off the same amount of heat for a given weight. This is why you should always buy timber by weight. The other factor is the amount of moisture content in the timber. When timber is felled it may have 40% or 50% moisture content whereas after it has been seasoned it will go down to 20% or less. Damp wood at 40% moisture content only gives out half the heat of timber at 20% because so much energy is used in boiling off the water. More information here.
Current prices (as of February 2016).
Working out log prices is surprisingly difficult because of the very loose terminology used by most suppliers. They talk about builder’s bags as if there was one standard size. In fact there are many sizes. They fail to give volumes or weights and almost always refuse to declare the moisture content.
Imagine buying a ‘large container full’ of petrol that was ‘high quality’ or ‘very powerful’. There are actually standards for firewood produced by HETAS (although these standards are fudged because they don’t mention weight) and it is something of a mystery why Trading Standards have not picked up on this
One company which is good with information is the Champfleurie estate and here their price information is used. Their prices are fairly typical of the major suppliers. Champfleurie also make the very valid point about our unnecessary obsession with hardwood rather than softwood.
The bottom line is that you need a certain weight of timber at a certain moisture content in order to produce a certain amount of heat. For instance ‘500 kg at less than 20% moisture’. If you do have to buy it by volume then ask whether the logs are loose in the container or close packed; it makes a considerable difference, in the order of 33%. Hardwood and softwood both give the same amount of heat per kg. of weight.
A large builder’s bulk bag is normally 1m x 1m x 1m; that is one cubic metre. Not to be confused with the slightly smaller builder’s bag which is 85 cm x 85 cm x 85 cm. (the volume of this slightly smaller bag is only 65% of the large bag – a lot of log suppliers use this smaller bag without making it clear).
A cubic metre bag may hold a tonne of sand but it does not hold a tonne of logs.
In fact the sides of the bag tend to bulge slightly and you get about an extra 20% of logs in – about 1.2 cu. m. Such a bag holds between 200 – 240 kg of softwood logs. Excluding delivery, a bag like this can be purchased for about £72 (excluding the 5% VATValue added tax can, to a large extent, be reclaimed on self build. see the page on VAT). This makes the cost of such logs between about £300/tonne and £360/tonne. The calorific value is about 4200 kWh/tonne (providing the logs are dried to 20% moisture content).
This means you are paying between 7p/kWh and 8.6p/kWh. This compares with about 15p/kWh for electricity and 5p/kWh for natural gas. (Of course the efficiency of your stove also needs factoring in, as does the efficiency of a boiler)
It is generally considered important to burn timber at 25% or less because the other factor which needs considering is tar production. If damp wood is burnt slowly and particularly if the flue pipe is not adequately insulated then wood tar and soot will condense out on the inside of the flue. Two things might then happen. You might get a blocked flue which can be dangerous because of smoke and carbon monoxide coming into the house, or the tar in the flue might set on fire. Fluespipe to conduct gas, typically ventilation air or boiler exhaust. see more on Flues for log stoves are normally double skinned with insulation between and made of materials able to withstand such a fire e.g. the Selkirk twin wall insulated flue.
- There is no convenient established method for delivery, handling and storage (although a few suppliers such as the Champfleurie estate and Lindrick Logs have addressed most of the issues)
- logs really need to be burnt in batches for clean burning. There can be problems with smoke emissions as has been experienced recently in Germany and the US where their widespread popularity has led to public debates on the subject. Legally in the UK the question of smoke emissions depends on whether you are in a smokeless zone. The legislation around the Clean Air Act does allow for dark smoke to be emitted providing ‘that the alleged emission was solely due to the lighting up of a furnace which was cold and that all practicable steps had been taken to prevent or minimise the emission of dark smoke’. The Log Pile Website has good information on emissions and also on the safety aspects of burning wood.
- log burning works best with a water heat store because of the irregularity of the burning.
- If you are harvesting your own logs it is worth giving consideration to the storage of the timber. Ideally it should be allowed to dry under a cover or roof with good air circulation for several months, preferably over a summer. Kindling will dry in a week or two if there is a storage place near the stove.
- All combustion appliances require an air supply and wood burning stoves are no exception. As houses become more airtight, the correct design of air supply becomes more of an issue. Many stoves are now designed so that a combustion air supply pipe goes directly to the stove rather than the stove getting its air from the room. This also cuts down on cold drafts within the room.
These types of log burning stoves are becoming increasingly available in the UK. They work by burning the logs in an atmosphere low on oxygen in the first chamber which produces wood gas. In the second chamber the correct amount of air is introduced (controlled by a lambda sensor) to burn this gas extremely cleanly. This means they can cope (to some extent) with fuel of varying dryness . As is usually the case with burning timber it works best with batch firings simply because some smoke is inevitably produced during light up. This in turn means that it is important to have a correctly sized heat store in order to absorb all the energy from the burn. Such stoves are also now available as visual features including glazed doors, with a few companies such as SHS even supplying models with double doors so you can see the combustion occurring in both chambers.
cooking on stoves
As with water heating (above), cooking on wood burning stoves is almost always out of balance with room heating and water heating. In summer it is in direct conflict, even with models containing baffles and adjustable grates. The reason that so many people drool on about cast iron ranges such as AGA and ESSE is that they remember them like some shrine to heat in an otherwise cold and damp house – the only place that was reliably warm as the frost crept across the stone floor. Possibly mother baking cakes helped too. Nothing wrong with that except these antiquarian behemoths are usually inefficient, extremely slow to react and in terms of cooking in and on them, – well – juggling plates springs to mind. All sensible cooks have a couple of gas rings or induction hobs and a microwave close to hand, especially for cooking in summer when the stove doesn’t need to be on. Nostalgia isn’t what it used to be
Should you decide to go for a combined water heater and ‘visual’ room stove then bear in mind that normal old fashioned water jackets on stoves do not work well with wood burners. They were designed for solid fuels such as coal and coke. Wood burns at a lower temperature when in contact with a relatively cool water jacket. This causes poor combustion and tarry condensates on the cold surfaces including any glass windows in the stove. For added efficiency in these types of installations you may want to use a valve such as the Laddomat which controls the flow of water between the boiler and the storage tank in such a way that the boiler reaches full operating temperature quickly. It also extracts any remaining heat out of the boiler when it is going out.
combustion air supply for stoves in habitable rooms
Combustion stoves all need a supply of air to burn the fuel and it varies depending on the stove output and fuel. The manufacturers state how much air is needed in the form of so many square millimetres of opening area. It is a requirement of the Building regulationsThese are the mass of regulations that cover safety, health, welfare, convenience, energy efficiency etc. in the way buildings are constructed. Not to be confused with Planning consent (which is more to do with whether you can put up the building in the first place) that this is provided because not doing so could cause the stove to produce deadly carbon monoxide if it is starved of air. The question becomes how the air gets to the stove. With a well sealed draught proof house it would be self defeating to use the old fashioned method of putting vents in an outside wall. Anyway people object to the draught and block up the vents, which is dangerous. The best way is to use a stove which has a built in connection so that you can run a ventilation duct directly from the stove to an external wall.
The Biomass Energy Centre have recently placed a great deal of useful info about wood fuels on their web site
The Log Pile Website – sources of log and pellet fuel and lots more info.
Also the Forestry Commission has become more active in promoting wood burning and is beginning to publish information on the net. There is a graph which shows heat output in kWh per tonne of logs and compares green logs (newly felled) with seasoned ones with the percentage moisture contents.
In Wales there is Woodfuel Wales
HETAS is the official body recognised by Government to approve solid fuel domestic heating appliances, fuels and services including the registration of competent installers.
The Building Regulations (part JThe Approved documents, (England) part J, deals with Combustion appliances and fuel storage systems) covers combustion appliances and fuel storage systems
A nifty little gadget is the Ecofan which sits on top of a stove and uses the stove’s heat to power itself in order to circulate the warm air coming off the stove.
Space heating using electrical resistance heaters (electric radiators and fan heaters), along with water heating using immersion heaters is not the way to go because they inefficiently depend on large amounts of fossil fuel. There are a few exceptions to this, concerning heat pumps, POUPoint of Use (for instance an electric water heater right next to a tap) water heating and night time use of electricity. See below.
Kissing good bye to fossil fuel
In the long term, from an environmental point of view, heating of both space and water is quite tricky to get to grips with and depends partly on your point of view of the likely or desirable future energy generation mix in the UK. Most of our electricity is generated from fossil fuel and electricity will always be in high demand, especially if electric cars start to gain in popularity. Electricity is so versatile in what it can do that it is crazy to burn it directly for heating. It is also about three times the price of other fuels.
If you believe that we are already much too far down the road of the burning of fossil fuels and that we need to decarbonise very rapidly then the main practicable alternatives to hand are wind, sun, tidal/wave, hydro and biomass along with better nuclear. This is mainly down to government policy or enlightened business investment policy.
The problem with electricity
Although, on the face of it, the idea of simply signing up with a supplier of renewable electricity for heating seems like a good one there is a major flaw in this approach. (We are talking here about electrical resistance heating rather than electrically powered heat pumps). This is because the long term demand for electricity is very high and it is generally very inefficient to generate it in the UK. It is better to save what electricity there is for uses which can only be fulfilled by electricity (such as lighting, telecoms etc). The argument is powerfully put by David Olivier and Cath Hassell here
There are at least three main situations when space or water heating by mains electricity may be a greener option:
- POU electric water heaters (especially for low volume use such as washing of hands, and when the tap is on a dead leg a long distance from the main water heater).
- Heat pumps used to provide space heating and DHWDomestic Hot Water. see the section on domestic hot water heating when they are powered by electricity from PVPhoto Voltaic. referring to the generation of electricity from sunlight panels on the roof of the house. See the page on Heat Pumps.
- Off peak use The Economy 7 and (to a lesser extent) Economy 10 tariffs utilise base load electricity which the generators cannot reduce at night. However this is an extremely complex issue. The base load might be better utilised for manufacturing, battery charging (of say – electric cars) or even motorway lighting.
Boilers are available for burning straw for domestic heating and constitute an almost carbon neutral source of heat as the carbon is recycled annually. Various sizes of bales can be used and are usually batch loaded by tractor. There is nearly always a hot water storage accumulatorusually a large water tank used to store surplus heat (from say a wood fired boiler or thermal solar collector). see the page on Heat Stores employed to store the heat. Some of them can be used to burn other biomass fuels such as wood waste. The cheaper end of these boilers tend to be quite inefficient and polluting but the better ones have secondary combustion systems which burn off the smoke.
It is important to get the design of the accumulator and associated pipework correct, and also the header tank which usually needs to be able to cope with a large expansion of the water in the system.
There are three possible drawbacks
- Being light compared with their volume, considerable dry storage area is required nearby for the bales
- The boilers themselves are large: a domestic boiler of 40 kW output will be about a metre high by a metre wide by nearly 2 metres long
- Batch feeding may be required two or three times a day so this can be difficult to organise. Probably the best situation for straw bale heating is a large farm building where staff and machinery are available to feed the boiler.
There is an interesting Navitron forum with some discussion of the practical aspects of burning straw and biomass
Of the fossil fuels, natural gas is considerably better than either oil or coal. This is because natural gas, CH4, is partly hydrogen which burns to form water rather than carbon dioxide. It produces about 70% of the CO2 compared with oil. However it is still a major polluter and if supply networks are not well maintained and become leaky then methane is a potent greenhouse gas.
On the positive side
- is its ability to burn very cleanly (in terms of particulates), producing little sulphur dioxide and NOx. Also very little ash.
- It can be burnt very efficiently.
- Gas is very storable compared with electricity so it is more efficient at coping with short term variations in demand.
- There is also a considerable future potential for feeding renewable methane produced from land fill sites into the gas supply network and this could supply nearly half the domestic heating needs of the UK if a report from National Grid is to be believed. See this BBC article. (There is of course also the Ambridge scenario)
- There is also potential for the direct gasification of materials such as waste timber and household waste, a technique which has been almost ignored in the UK. See the report by Juniper Consulting
- Furthermore there is the possibility that hydrogen, maybe powered from renewables such as solar or wind, could be fed into a converted national gas supply network in the future.
For the self builder who is on the gas supply network there are a few points to consider.
- Gas requires no on-site storage so this saves on building costs compared with say burning wood.
- Gas boilers are relatively cheap to purchase and easy to install
- Maintenance contracts are reasonably economical
- Combination boilers maintain water pressure through the hot side of the system. This may be useful for showers on top floors (and removes the need for header tank space).
- The Passivhaus design approach has not favoured gas heating. Rather it incorporates electric heating directly into the air supply.
- Co-generation, or CHP Combined heat and power - where the heat which is produced when electricity is generated is used within a heating system rather than wasted. This can happen at different levels - within a single house, a housing development, a town etc. , is a possibility with gas and although the internal and external combustion engine approach to this has been slow in taking off, the emerging fuel cell technology for achieving CHP is currently about to hit the market.
see Gas Safe registered engineers
Oil and LPG
It is difficult to make any sort of case for using oil or LPG (Liquefied petroleum gas) as a heating fuel. If you are not on mains gas then probably the best green option is to go for wood burning (logs, pellets or chip) or possibly straw bale if you have sufficient storage space and access to the fuel. Particularly with wood pellet as a fuel you have almost as much convenience as with oil or gas.
Of course the main thing is to insulate extremely well. If you can achieve a standard such as Passivhaus then there will be no need for a central heating system
The relative costs and attributes of domestic fuels can be found on Nottingham Energy Partnership’s ENERGY COST COMPARISON web site. It includes their CO2 emissions taking into account the efficiencies of the fuel’s boiler.
The Building Regulations and fuel.
The type of fuel you use can have a considerable bearing on the design and construction of a house. There are several factors at work:
- Does the fuel need to be stored? (eg biofuel such as logs, wood chip, wood pellets, straw bale). How much space and where? Gas and Oil have special storage requirements
- How is the heat distributed in the house? eg a log stove distributes its heat by air convection unless a boiler and pump etc. is involved
- Biofuel usually requires a source of combustion air which may impact on the air tightness of the house
- Chimneys and flues need careful design: Gas balanced flues usually need to be on an external wall (or quite close to one) and chimney flues need to rise fairly vertically from the fireplace, stove or boiler right up through the roof, taking a suitable route which is safe (cannot set fire to the building structure) and emerge through the roof at a place that cannot cause fumes or smoke to be a problem.
The Building Regulations cover a lot of this ground:
Combustion appliances and fuel storage has a detailed section in the Approved Documents Approved documents are detailed publications which come under the English Building Regulations. They are based on tried and tested methods of building and if you follow them you are assured of complying with the Regs. The equivalents for Scotland are the Technical HandbookUnder the Scottish Building Regulations, the Technical Handbook gives construction principles, which, if you follow them guarantee compliance with the Regulations, for Wales: the Approved documents (Wales), and for N.I. the Technical BookletsUnder the Northern Ireland Building Regulations, the Technical Booklets give construction principles, which, if you follow them guarantee compliance with the Regulations (part J) and this makes reference to several aspects of the design and construction of houses including:
- the construction of chimneys and flueblock chimneys along with wall thicknesses
- hearths, gathers, and bases for back boilers for gas fires
- fireplaces including large and unusual ones
- flues and their sizing including flues for gas appliances
- flue heights and how flue outlets relate to roof design and adjacent buildings
- ventilation and air supply for appliances (this has a large bearing on ventilation and air tightness generally).
- fire resistance[for the purposes of part B of the Approved Documents] - The ability of a component or construction of a building to satisfy, for a stated period of time, some or all of the appropriate criteria specified in the relevant part of BSBritish Standard 476. of construction close to an appliance
- the testing and repair of old chimneys
- storage of gas bottles and oil tanks
The Regulations also include a ‘fuel factor‘ which is used in calculating the TERTarget emission rate. The minimum energy performance for a new dwelling as defined in the Building Regulations. see Target CO2Carbon dioxide is a gas which is given off when carbon based materials such as fossil fuels (coal, oil, and natural gas) are burned. It is called a greenhouse gas because it works like the glazing of a greenhouse and causes global warming Emission Rate) and this is influenced by the type of fuel you use.