The burning of wood

A friend Iliff has commissioned a study on the burning of wood. The Folly of Firewood He has realised that due to following government guide lines in the past his woodland does not have the capacity any longer to meet his wood burning needs in a way friendly to the environmentally. Clearly from the research done the answer would be not to burn wood however to reduce how much is burnt would go a long way in helping the woodland recover.

When we burn wood as a batch there are two distinct stages. At first we drive out all the chemicals in the wood, and at this stage temperature and burn rate is critical at least if we are to ensure these chemicals are not released into the atmosphere or deposited in the flue system. As the wood is reduced to charcoal we then can adjust the output without worrying about the gases released. To burn wood in an ecologically friendly way therefore means we have a very limited control in the burners output. Each burner will be designed for a set bulk of material and a set output, and although most wood has the same calorific value by weight, the burner takes a load by volume, and there is a wide variation in calorific value by volume.

This leaves the designer with a problem as the burner is likely to be used with a verity of woods. I am sure there would be a way to alter the firebox to accommodate various wood types, but in the main it is set to just one type. This fact gives rise to users of wood burners giving conflicting advice on what type of wood to burn, with some saying soft wood is best, others saying hard wood is best, and yet others saying how it needs a mixture. All are likely correct for their wood burner.

carbon cycle There are two very different designs of wood burner. Some are designed of occasional use to give ambience, never designed to run in an ecologically friendly way, often classed as multi-fuel as there is no real attempt to ensure particular emissions, or tar like deposits in flues are reduced to a minimum. In some cases because of the use, there is really no way to stop these problems, for example when used for cooking. Clearly the food temperature must come first, although careful use of dampers and other controls can redirect the flue gases so the oven temperature can be varied independent of the fires burn rate, this is clearly limited. Traditionally charcoal or coke would be used in these, where the burn rate does not effect emissions output. Don’t be kidded that wood burning helps the planet. To left shows the cycle and although the CO² may be recycled there are other considerations.

Plumbing The ecologically friendly burner however is a very different beast, in the main set up for one type of wood, and run at a set output. Clearly we don’t want a set output, so as well as the burner we also need a means to store the heat produced. There are some very clever units like the rocket stove which heat rocks, which will then liberate heat very like the old economy 7 electric storage heaters, and by opening and closing vents the heat output is controlled. However in the main water is used, this is now also used with electric off peak heating, the idea being very well insulated water tanks, which can store the energy until required with very little loss. This means one can run a fire in the burner in the evening then store that heat for the next day.

Condensing Stove However the wood burner has one problem not found with electric. It can’t be switched off, it takes time for the fire to burn out. The normal method is to have one radiator which is always on, able to get rid of any excess heat. With a well insulated heat store simply placing ones hand on the store, will not indicate how much heat is stored, so unlike the electric version some means of viewing the energy level is required.

With gas central heating it was found relying on the expertise of the plumber to install motorised valves, cisterns, and control gear resulted in some systems being rather poor, where his skill was clearly lacking. The introduction of the combi boiler where all the valves and controls were included in the boiler removed the problem of the lack of skill of the installer. With the wood burner we however rely heavily on the skill of the installer for them to work in an ecologically friendly way. There are specials where it is all built in the example to the right is a prototype condensing stove super efficient but sadly only a prototype.

Skip The energy saving trust estimates a bill of at least £11,500 to install a wood burner in an ecologically friendly way. They also estimate a saving of £600 a year compared with electric heating however since electric heating is the most expensive anyway, the trust says electric is three times more expensive then gas. To use a wood burner where gas is not available could over the years save a lot of money, but where gas is available it is unlikely to save one any money, unless one is able to get the wood free. In real terms even if you owned woodland the wood is not free, as one has to balance it against loss of revenue if it was sold. Plus of course the cost to cut, move, and store the wood.

Add to this the cost in time to light and clean the wood burner, and one realises the only way people make a wood burner pay is by hiding the costs, or theft. It is not unknown for the wood burner owner to load his car boot with wood stolen from some woodland, he regards this as free wood, even when he uses extra petrol or diesel to collect his bootie. Wood theft is on the increase, and clearly there will be a point where police start to crack down on this theft. There are times where free wood is available, I know of many who have been allowed to take home pallets from work or similar items. However these often contain preservatives, which when burnt are hazardous to health, so when deciding if fitting of a wood burner makes sense then one should consider the retail cost of wood.

The big problem is the DIY sheds. These shops sell allsorts from electrical consumer units, to wood burning stoves, which the normal DIY man can not legally fit at a cost less than a professional would charge. There is however nothing to stop one selling the items. It is not just wood stoves, but a whole host of items including complete gas central heating boilers, which are sold in these DIY sheds. As an electrician I have waked around these to grab something I have missed from whole sale outlet, and been amazed at the wrong information given on signs around the shop.

Vulcan-thermometer Clearly some people have inherited a wood burner, and to rip out and replace with gas is expensive, so next question is how to run a wood burner as efficient as one can, without too much expense.

First has to be flue temperature. In the main the fuel should be at 150 degs C, this is hot enough to ensure no water condenses on the flue, but cool enough to ensure heat is not lost to atmosphere at too great of a rate. Second air supply for the combustion air needs to be as direct to burner as it can. Ducts supplying air very near to the fire will reduce drafts though rest of the home.

Because the fire burn rate is set to keep flue at 150 degs C, to extract more or less heat from the fire one needs to turn fans off and on, to vary the output. There are special sterling engine driven fans, but clearly these are expensive.

I have considered positive pressure fans bringing air in from outside rather than relying on the fire sucking the air, but this has not been tested.

It seems odd now but I now realise why we got rid of our open fires. In the main it was the draft they caused. Yes dust, carpet damage, fuel storage, and non automatic operation also prompted the change, but draft was the major one. Yet that was the one thing that with correct vents could have been avoided.

mechanical-heat-recovery-system Today we have many extractors in the house, mainly in the bathroom, kitchen and not forgetting the tumble drier each causing a negative pressure, and possible danger of drawing flue gasses into the house. The heat recovery unit would reduce the problems as it blows air in as wall as sucking air out, but not only are the expensive, but also they are large units and finding somewhere to mount them is not always easy.

Just as extracting air is replaced but air sucked in through every hole it can find, blowing air into the house would result in air escaping through every hole it can find, but if that air was blown in past the wood burner then it would be warm air, not cold air, so the house would feel warm throughout.

stove-ventilation I looked at how much air a wood burner requires, but every wood burner uses excess air to ensure no flue gasses enter the house, so very hard to calculate how much air must enter the house. Minimum flue size for defra exempt is 125 mm an extractor fan uses a 100mm hole diameter, so would likely would supply more air than required. (95 m³/hr at 5 m³/kg that’s 19 kg of fuel per hour) however that’s for wood, and multi-fuel burners could use Anthracite at 9.5 m³/kg so 10 kg of fuel per hour, and likely therefore much more gas is expelled through the flue. Also Anthracite needs 40% excess air so down to burning around 6kg of fuel to use the air forced in with a standard extractor fan working in reverse. (I note the Exodraft chimney fan rated at, up to 370 m³/hr, and at £855.60 rather expensive, and as with all electric items used with wood stoves, what happens with a power failure? Can’t have auto shut down with wood stoves.)

Storing wood to dry is also clearly a problem. The picture shows just part of this guys store, where he feels it takes three years to dry to a stage where it burns without problem.

Wood_store Unfortunately there are far too many variables, and testing in one home does not mean it will work in another home. One problem with positive pressure is the fire may burn too rapid, most extractors don’t have a speed control so only way is to limit air into the device, or allow air from inside the house to enter the ducting. Noise is another problem and one does not want a fan running 24/7.

Automatic operation is clearly preferred and likely easiest way is with a standard room thermostat situated close to the fire so once the fire goes out the fan will auto stop. Having a by-pass switch would however likely make lighting the fire easier.

stove-ventilation A 100mm pipe however is not small, and hiding it from view would not be easy in many cases.

Having been involved with installing a wood burner on a narrow boat with restricted flue height and cold interior which hinders lighting I am aware of the problems. In this case there was also a problem with the side boiler, as again not enough height for thermo-siphon. Since the boat had a bank of batteries (400Ah) using 12 vdc pumps was an option, with no chance of a power failure. However unless some battery back-up is included, relying on electric to ensure safe running of a wood burner is not really an option.

I am old enough to remember open fires and stoves, and also remember why we had quarry tiled floors, having watched my mother rake out the burning coals, when the water was turned off unexpected. Running off water then the stove got too hot to stop it boiling, and seeing the water cascading from the eves when it did.

We of course had many dangerous items in the home. I am sure today if some one invented the gas stove it would not be allowed for home use. It is only because of its historic use it is allowed, and the fact we have been brought up with the dangers from childhood and so treat them with the care required.

wallnoefer The problem with the wood burner is we got rid of them all, and so are unaware of the dangers, so bringing them back into the home is far more dangerous than if we had been brought up with their use.

Do I have a wood stove now? No way in the same way as I would not use gas for cooking they are far too dangerous. It is different question where there is no option. I use gas for cooking in the caravan for example.

There are some very clever stoves which burn very efficient the one shown has flames which go down (5) as well as up into the area (4) once the control (2) is moved heating the water (3) so the flue temperature (1) is held reasonably constant, however when either of the glass fronted doors (7 and 8) are opened (2) has to be manually opened, and in spite of all the clever controls it needs manually operation, and this is a big problem that the burners are so labour intensive. Using the fire in the evening while watching TV and heating up a water heat store as shown further up the page these systems can work well as an integrated part of a complex system, but the cost would take a very long time to recover.

wallnoefer One of the big questions I keep asking with many of these high efficient systems is what happens with a power cut or even if a circulation pump fails. With the diagram you see how (6) is a pump to circulate the water. Although with my central heating in 25 years I am only on my second pump so they are quite reliable, there still needs to be some safety device. Opening (2) would reduce heat to water jacket but likely it would require raking out the fire to stop it boiling the water. The prototype shown near top of the page has the same problem. The auxiliary radiator on the plumbing diagram again required in case the heat store can’t take all the heat produced to my mind just too much to go wrong.

The system to the right uses rocks or concrete to store the heat. It is a true condensing burner so is super efficient. It has no moving parts so nothing to fail with power cut. It seems to have everything going for it. However it would seem these are home made units, so getting insurance for a home with one of these fitted would be likely a problem. And clearly a large unit, and would have to be integral part of the house. The Utube videos of the device, show above where it says "exhaust" is made into a seat, so you sit on the hot seat. The combustion chamber is an old oil drum, with removable top for cleaning, but how often the drum would need renewing, and what would need checking on a regular basis is not referred to, as all home made. However it does show it can be done, the big question has to be why is this not sold as a commercial system? Is there something which has not been declared? For example how is the fire started?

So to recap, flue temperature gauge, fan, ducted combustion air, will help, but to run efficiently means one has to spend a lot of money on heat stores and even then there are some real safety issues. Clearly Lancaster boilers have been used for years to heat the like of schools and colleges, but these need regular servicing, and some one in attendance. When I was young women did not work once married, so there was some one at home to keep a watchful eye. Today however women work, so often a home is left unattended. So to use wood requires a heat store, and loads of money to install the system.

As to leaving wood in the woodland to rot clearly this is going to assist in giving a habitat for fungus, birds, mammals, and other woodland species and for a SSSI woodland this is not only important but legally required to avoid up to a £20,000 fine which would result if found guilty of without reasonable excuse, intentionally or recklessly destroy or damage any of the flora, fauna or physiographical features by reason of which the land is of special interest, or to intentionally or recklessly disturb or take away any of those features, or to intentionally or recklessly introduce non-native flora and fauna. This is all under section 28 of the Wildlife and Countryside Act 1961 (As amended) it was under the Countryside Council for Wales but now has been renamed on the 1st April 2013 to Natural Resources Wales (Welsh: Cyfoeth Naturiol Cymru).

However for other woodlands a balance between revenue and environment must be struck other wise these woodlands will cease to exist. There are other forms of revenue as well as selling the wood from wild life viewing to mountain bike or horse trekking. But unless public funds are allotted to keep or create woodland there has to be some revenue gain to the owner.

The major problem is other than fire wood to fell a tree and extract plus deliver to a saw mill costs more for a single tree than one would get for the wood only using semi voluntary labour can single trees be felled and produce money. The rise in sales of wood burners has produced a market for wood to burn and so in turn a reason to keep or plant woodlands.