Would it be wood? (to keep you warm)

Following on from my last blog entry, with gas bills that are beginning to make us all wince, many people are endeavouring to reduce their heating bills by burning cheap coal and logs.

Installing a multi-fuel stove was a fashionable addition to a home a few years ago, but that romantic real living flame is now being worked hard for every therm of heat it can produce.

Multi-fuel stoves can burn many things from peat to compressed paper briquettes, but for most people the fuel of choice will be either logs, coal or both.

Firstly, its a shame not many stoves are plumbed-in to also provide hot water via a “back boiler” – but that doesn’t prevent them from being good sources of cheap heat…especially if the logs come free from sources such as fallen trees.

But are wood burning and multi-fuel stoves efficient and are they green?

Well, for wood burners, if one makes the assumption that a new tree is planted for everyone chopped down, then they are indeed carbon neutral as the carbon released into the atmosphere will be absorbed by the growing new tree.

So where natural gas kicks out 0.19 kg of carbon dioxide for every KWhour of energy produced and electricity a whopping 0.42 kg, then wood’s puny 0.03 kg looks rather good.
This figure assumes that the wood is correctly dried (seasoned) and then some pretty high efficiencies can be achieved of around 80% which is almost as good as a modern condensing boiler working under optimum load.

Wood, when freshly felled, can have a moisture content as high as 60%.   This figure wants to be 15-20% when burnt and that can take 1-2 years of seasoning.
So freshly collected timber – and unfortunately some bought logs – are still full of moisture which will burn cooler and less efficiently than they would if dry – meaning that not only will you need to burn more timber to get the heat you need, but the flame is dirty, producing tar (sometimes called creosote) to line your chimney.

Lots of heat is used up turning that high water content into steam, which is then lost up the chimney.  The lower flame temperature also means that the smoke will cool quickly and the wood resin will condense on the inside of the chimney as creosote and then gradually soak into the masonry.

This will start in the loft and slowly work it’s way down the chimney.     It can both leach through to stain the internal decorations of the house leaving a real eye-sore to deal with, but can also build up to become a fire hazard within the chimney.    One is a problem, the other a threat.    Both are best avoided!

Choosing the sensible option of lining the chimney cannot eliminate the latter risk.   A creosote fuelled chimney fire can produce temperatures of 1200 degrees C – hot enough for materials to self-combust without direct contact with a flame.
Be prepared for a chimney fire by reading this.

How do the logs burn?

Wood burns in 3 stages, but they all happen together when the fire is established:  the first is boiling off that water content.

The second is the combustion of the wood resins that first turn to gas.
The only efficient way to burn these is with a secondary hot air flow…this is often called an air-wash system – keeping the viewing glass clean is simply a by-product.  If the resin gases aren’t burnt before they escape up the chimney then around 30% of the energy of the log is lost!    So make sure your stove has such a system in it’s design.

The final stage is the charcoal, which burns hottest and most efficiently leaving such little ash.  (And also the perfect temperature for baking those spuds in tin foil.)

The Price of  Coal

Bought so cheaply in bags from corner shops, coal’s real costs to the health of our bodies and the environment are far higher.   It is full of sulphur, contributing to damaging acid rain, as well as other pollutants that cause respiratory problems.   And its carbon footprint, although better than electric, is still poor at 0.27 kg of carbon dioxide.

Although coal has double the calorific value of wood, contains little water and burns hotter than wood, it does this with poorer efficiencies in a multi-fuel stove.  Power stations grind up the coal to dust which is then injected into the combustion chambers – not a convenient option in your front room.

That poor efficiency produces the smoke seen escaping from an increasing number of chimney pots these days, but it also produces the silent killer of carbon monoxide and the extra heat of coal will not prevent the smoke from cooling substantially before it escapes from those chimney pots.

Any masonry chimney is unlikely to have a 100% airtight structure and any adjacent chimney, whether in your own home – cosmetically sealed bedroom fireplace? – or a neighbours, that doesn’t have a fire burning below, will let any migrating smoke sink as it cools.

Carbon monoxide can then build to dangerous levels in poorly ventilated house rooms endangering occupants.   See this NHS website for symptoms.

There is only one answer: have the chimney lined with a proprietary liner (flexible metal tube) to safely transport the smoke to outside.  But I would still strongly suggest you place a carbon monoxide detector (available from good DIY chain stores) within the rooms – especially bedrooms – adjacent to your chimney to alert you to a possible defective installation and serious health risk.

Should it be wood?

So it would seem that wood is the friendlier option from the Earth’s point of view as well as our lung’s.   Unfortunately, as ever, there are always complications to urban myths!

Wood, although producing so little ash (up to 20 times less than coal) does give off particles of microscopic harmful dust – probable human carcinogens –  into the atmosphere called wood smoke particulate matter (WSPM).  These are just the right size to lodge within the deepest recesses of our lungs.

In Germany, where amazingly the emissions from wood burning stoves eclipse vehicle emissions, a law came into effect last March that required any stove over 4kW (a normal domestic stove) to be inspected for compliance with maximum permissible WSPM emissions.

The UK still only has a small percentage of wood burning stoves when compared to Germany or Scandinavia, but this is definitely something to keep watching the government for.

Never together…

Regardless of which solid fuel you choose, never burn coal and wood at the same time.

Not only does a multi-fuel stove need setting up differently to maximise the efficiency of combustion for each fuel – coal needs an airflow from below, whereas wood burns from above – but the water produced by the burning wood will combine nicely with the sulphur given off by the coal to produce sulphuric acid in your chimney that will rot most things especially anything metallic like your liner.

Breathe easily and toast the marshmallows..?

Once all that nasty smoke and its pollutants are safely outside, you can breathe easily within your toasty home, can’t you?    Well, perhaps not as many previously industrial areas imposed the Clean Air Act in 1956.
The Clean Air Act states that, in Smoke Control Areas,

“Only approved smokeless fuel may be burned”

Wood is not an approved smokeless fuel and the only way that wood can be burned legally in a smokeless zone is if the woodburning stove used is approved by Defra.

To blatantly ignore this, might be to invite the local Environmental Health Department to become involved with a maximum £1000 fine for each offence.    A typical list of Defra approved stoves can be found here.

By far the best fuels to burn are the compressed paper or wood waste briquettes or pellets.  Less water content (as low as 3%) means they burn hotter and longer producing less pollutants and don’t need at least a year’s seasoning!   You can even produce your own paper logs from old newspapers.   Be careful with using wood chippings though as they often have high moisture contents.

If you have not yet got a multi-fuel or wood burning stove and are keen to have one, the best place to start researching is the government recognised official body HETAS.

This recent cold snap is a reminder of the Big Chill in the run up to and during Christmas. Remember it’s still only February, so there’s plenty of winter left to chill us all again yet!

How ever you heat your home, stay warm and remember you can keep up with all the new blog posts from YBC by subscribing to an email alert.