Some time ago I looked into the idea of wood burning.

This can really be expanded to include all solid fuel. The solid fuel can be divided into two rough categories. Processed fuel where nasty bits are removed, like coke and charcoal, and unprocessed fuel where one has to burn it under controlled conditions. There are a few specials like the pellet burner but in the main burners designed to burn raw unprocessed material like coal and wood have to be designed to run within a very narrow band of heat output. This is because they need a double burn and a hot burn to ensure tar and creosote is not deposited in the fuel and there are no particular emissions. Both wood and coal will turn into charcoal and coke so at the tail end of the burn the fire can be allowed to die down but within minuets of lighting there is an opium temperature which will ensure too much heat is not sent out of the flue and there is a complete burn.

The different woods and coals have different density so although all wood gives off nearly the same heat per unit mass it’s not the same per unit volume. But the fire box has a volume it can take not a weight. This gives raise to what would seem conflicting reports were some say mix hard and soft and others say soft is better and others hard is better. The problem is each stove is designed for a set wood clearly if one is selling a stove where soft wood is the normal wood burnt then it’s designed to burn that wood and visa versa.

Heat Store

The burner is designed in general so the heart of the fire is around the 600 degrees C and the flue temperature is around 150 degrees C. And batch burnt. This means the fire is lit it is quickly brought to running temperature and runs until enough heat has been produced then allowed to go out. By circulating air and/or water the amount of heat produced can vary keeping the flame and flue temperatures constant but not by enough to satisfy the varying demands of most properties so some method of storing heat energy is required if the burner is to run efficient and emissions free.

Using large lumps of rock and concrete can allow heat to be stored in the same basic way as the old electric storage heaters worked. Having vents which can be opened and closed to vary the rate of heat release can spread the energy of a burn over an extended time. However to control these methods of heat storage is far from easy as found with the electric storage radiators and one as with the electric storage radiators has to look at multi burns per day. And to install that much rock or concrete it really needs to be part of the house design. Not really something, which can be added latter.

This brings us to the water systems. It could be any liquid really and adding anti-freeze is common. The idea is a large well insulated tank, full of liquid to store the heat, with a series of coils to put heat in or out of the liquid. A series of heat exchangers which can combine solar, solid fuel, and electric inputs to run a series of radiators and heat the domestic hot water as an output.

This means the wood burner can be run in the evening and the excess heat stored for gradual release through the night and next day. This is really the only practical way to run a wood burner as an efficient environmentally friendly heat source.

Even then one is still pumping out flue gasses at 150 degrees C where really we should be looking at below 80 degrees C like the condensate gas boilers. Plus to grow enough wood to satisfy the British need for heat would need over 5 times the area of the UK to grow it. So to use land to grow wood means that some one some where will have to go without food as a result. So to burn waste may have some good points but to grow to burn is clearly wrong. Using the waste straw from crops in pellet burners is a good way of using waste. But all the paints and preservatives used on wood mean to burn waste wood the temperatures need to be well above the 600 degrees C used for virgin wood.

So although on a small scale wood burning may work it is rather limited, and really electric heating, and central power stations burning waste, harnessing wave and wind power, and hydro power is the only true environmentally friendly way to heat the home.

Although I realise that burning wood is in the main not an environmentally sound idea never the less warming ones hands on a real fire has some built in feel good factor and as long as one does not try to kid ones self into believing one is doing the right thing I see no reason not to use the wood burner. However wood is expensive and most will want to run the system as efficiently as they can. So the first move in right direction must be the flue temperature gauge. All stoves are different and it is no good blindly copying what others have done. It has to be all worked out for your burner and the first job is to work out the flue temperature required.

If the flue is too cool then likely the fire is also too cool so you will get creosote building up on the flue, also moisture will likely condense in the flue causing acid to form attacking the flue it’s self. If flue is too hot then loads of heat is being wasted.

Guage

The fire should draw air from outside. This does too major things. One it stops drafts and two it means that extractors like tumble driers, kitchen hoods, and bathroom moisture will not draw combustion produces into the room. Where old stoves without external piped combustion air are being used then using a positive fan assisted feed to the area around the fire can both assist in stopping the fire causing drafts or at least ensuring the draft is a warm one from the fire not a cold one to the fire.

Automation is always a good point. Fans and pumps which auto switch on are always better than having to manually start and stop. Water cooling is a problem as any failure can cause boiling. Warning lamps or buzzers doubling up on pumps and sensors and where possible thermo siphon to ensure flow can’t fail. Heat sinks as shown with the towel rail in the diagram where heat can be radiated when the store is full are important. The big problem with the water heat store is they are so well insulated it is hard to work out how much capacity is left. Pockets with temperature senders are required to give some indication of if near empty or near full of energy.

It was noted it is much harder to light a fire when the cooling water is circulating but any system to stop it for lighting must fail safe. The whole idea of fail safe is a problem with solid fuel. It is very easy to auto close a gas valve or oil supply extinguishing the flame when there is a fault. With solid fuel this is not so easy. CO2 sensors, and smoke alarms are a must. Unlike with yesteryear where we put most of the heat up the flue today we are sailing close to the wind and some thing as simple as putting on a tumble drier can draw deadly fumes into the home.

As with all open flame devices there is always a danger. Dust is explosive in certain concentrations. In the kitchen the stove should always have an extractor hood to remove any dust like flower before it can get to the explosive concentration.

Personally I use an induction hob and electric oven with a whole host of safety features built in. One problem is we get use to the safety features and tend to forget these are not built into solid fuel burners. One would not dream of going out leaving the baby at home to fend for its self. The same applies to the wood burner. The old idea of banking down a fire does not work with modern appliances. It produces creosote and particular emissions and running the fire so cool likely the flue will be under temperature so CO2 can bleed into the room. You just can’t go to bed until the fire is out. Simple but often not what we want to do. Using non open flues that’s where it draws combustion air from outside and processed fuel like coke or charcoal then may be one could get away with running over night.

Condensing stove Condensing stove

There are some really special stoves made. When I first saw the stove to the left I though at last a condensing stove that burns wood. However it seems it's only a prototype and at the time of writing just a pipe dream. Looking at the diagram as with many of the super wood stoves one realises it relies on electric power. Unlike the gas and oil wood burners will not just go out when power is removed. But without electric likely draft would be lost so it would only smoulder but I think I would want to see some form of uninterruptable power supply to ensure it could at least burn the fuel already loaded. There is a system called the Rocket Stove which is in the main a home made system which also is condensing. The idea is the fire goes into a barrel where the flue gases are cooled to below 100 degrees C and are lead outside with a down ward facing pipe so condensate can simply run out. Unlike the other system it does not require power. However as with any home made system there is a problem with insurance.

exodraft-chimney-fan

I would love to say there is a good way to burn wood. In fact the whole idea was to show how good burning wood was being a renewable fuel source. However we are already seeing how people tricked into thinking wood burning would be cheap have found the burner they have is no where near the 75% efficiency expected but more like 25% where it is run hotter than the design temperature. The result is they try to find cheap or free wood where ever they can. Wood left on the woodland floor is basically stolen to feed these inefficient units. I don't blame them for the theft if I was tricked into fitting a part system which is inefficient I guess I would also steal fuel. I blame the DIY sheds and the government who allow the sale of stoves which under British law can't be legally used but can be sold.

wallnoefer

While I was looking for facts on the wood burners I came across many interesting ideas and facts. Many raised even more questions rather than giving an answer. For example exodraft chimney fan designed to ensure combustion products did not get into the room, water pumps on the Hughes Condensing stove and Wallnoefer stove where the flame goes down as well as up. All require electric power but there was nothing shown as to what would happen with a power cut. Clearly one could use 24 volt DC pumps with battery back-up and even automated change over should a pump fail but it is some local plumber DIY guy installing the systems and unless there are clear instructions on how back up is required likely it will not be fitted. Even if it is fitted regulator tests would still be required.

biofuel

The idea of producing charcoal therefore removing all the nasty stuff from the wood seemed good and with a proper refractory vessel all these nasty products are collected and put to good use. However much of the charcoal is not made in that way but uses a rather Heath Robinson affair allowing all the nasty stuff to go into the atmosphere. There does seem to be a problem for the small woodland owner. To cut and transport a single tree to a saw mill will likely cost more then he gets for the tree. So there is a big financial incentive to sell wood to burn. In some ways creating an outlet for the small woodland owner's wood is very important if we are to retain our small woodlands. However to transport wood to the refractory vessel has the same problem as to saw mill hiring a wagon to transport wood is not cheap. What he wants is wood at a size he can transport in a pick-up. However I watched a program called the wild wood and there is clearly a problem in cutting down single trees where they don't always land as expected. This means for fire wood production smaller trees are better as easy to handle but these don't help the wild life in the woodland. We need at least some mature trees.

charcoal

I looked at a modified carbon cycle showing how it was not as simple as tree absorbs CO2 then stove releases CO2 but how greenhouse gases are released with incomplete combustion also there are particulate emissions, loss of habitat for woodland creatures as the wood is harvested before it is mature and no rotting wood is left behind plus soil erosion where too many trees are cut down in one area. It is all well and good saying you should not fell groups of trees but in real terms getting a single tree out of the woodland is not easy. I have looked at new groups of trees and even when planted right next to an existing woodland after 12 years the undergrowth had not become natural the big question is how many trees can be felled without causing a problem with regeneration.

cabon cycle plus

Looking at the guy who alerted me of this problem, he had a very small woodland, and I started to ask was this really representative, but when I looked at others who were saying how they were testing, the area used was sanitised with nice boarders around each patch, and was not even enough area to park 50 cars, so was far smaller clearly the results were next to useless.

nec-air2

I came to the conclusion that Eco-friendly was just a way of getting more money out of the unsuspecting public with no thought, or testing, to see if really helping the environment, which has made me very cynical of anything with an eco label. Records and history is the way forward. We have to look at 100's of years not 12 and man just does not live long enough. So we have to turn to the historian and the archaeologist to tell us what happened in the past. Even in my living memory I remember smog so unless we burn wood very different to the way were burn coal we will have a problem. I also remember my mother raking out the stove in the kitchen when the water was turned off and taking the burning coals into the garden. The kitchen floor was quarry tiles and only the lose mat could burn. Carrying live coals over a fitted carpet would be a completely different story.

Socrates said "Anything in moderation nothing in excess" although Oscar Wilde said "Moderation is a fatal thing. Nothing succeeds like excess." However to me for a few people to have wood burners is not a problem. It is when there is a promotion and many people use the wood burner that the problems start. The temple of Apollo at Delphi bore the inscription Meden Agan and I do think we need to look to history and adopt anything in moderation nothing in excess.