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How to reduce smoke emissions from domestic wood-burning appliances

Posted by Sandra Hayes on 13 July 2017 at 2:30 pm

Burning wood in a domestic stove or boiler can generally be regarded as a more sustainable approach to heating than traditional methods such as coal or gas heating. This is because the carbon dioxide (CO²) emissions produced as a result of burning wood fuel remain part of the carbon cycle, being taken up by new plants elsewhere via photosynthesis.

In common with conventional combustion systems, burning wood in a wood burning stove or boiler can emit a number of pollutants including nitrogen dioxide (NO2), particulate matter (PM), carbon monoxide (CO) and volatile organic compounds (VOCs) and sulphur dioxide (SO2).  The mix and amounts of pollution produced will depend on the size and design of the stove or boiler, the quality of the fuel used and the presence of any emissions abatement (cleaning) equipment (such as a secondary flow or a catalytic convertor -  in very large wood fuel boilers).  Generally a well maintained stove or boiler will produce more pollution than a similar gas system, but less than an equivalent coal or oil fired stove or boiler.

It is very important when using a wood burning stove or boiler to ensure that combustion is complete as incomplete combustion increases the level of pollutants and smoke production.  This can happen when wet wood is burned or if the fire is damped down to keep it going overnight.  Burning wood contaminated with paint or other finishes will also cause pollution and is likely to damage the stove or boiler.  Appliances also need to be maintained in compliance with the manufacturer’s instructions in order to keep potential pollutants to a minimum.

Stoves and boilers with a secondary air system will generally burn cleaner due to the increased levels of oxygen they provide.  Fuels such as wood pellets will generally burn the cleanest as they are small (meaning they have a greater surface area for their size) with a consistent, low moisture content. 

Wood can also be manufactured into compressed briquettes (also referred to as heat logs), which are usually between 60 mm and 150 mm in length and can be used as an alternative to logs in a log burning stove. Briquettes can offer a cleaner alternative to logs due to their higher energy density, low moisture content and steady combustion, although it is always worth checking whether they contain any additives before you buy.

Smaller chips and pellets can offer an even cleaner burn, as the large surface area to volume ratio means combustion is achieved in a very efficient manner. This makes a pellet-fuelled boiler or stove a suitable choice for those living within smoke-controlled areas (where only exempt appliances can be used) and other areas were background levels of pollutants are already an issue, usually in towns or cities.

Using an energy efficient stove or boiler is also a way of reducing potential pollution levels. As of 2022, any new stoves produced must meet lower energy-usage requirements set out by the European Union.  Further information on this initiative can be found at: http://www.which.co.uk/news/2017/03/new-wood-burning-stoves-to-produce-fewer-emissions/ and http://www.hetas.co.uk/ecodesign-ready-scheme/

Further useful information on this topic can be found at http://www.environmental-protection.org.uk/policy-areas/air-quality/air-pollution-law-and-policy/using-wood-and-coal-for-home-heating/

Image credit: Steve Douglas.
 

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1 comments - read them below or add one

Eco Andrew

Eco AndrewComment left on: 4 August 2017 at 1:55 am

My WOODBURNER STOVE TIPS

 

1.  Use well-seasoned (dry) wood with moisture content 20% or below.  Seasoned wood feels light compared to newly cut wood (of the same type), it sounds hollow when you tap two pieces together, the bark may be coming loose (with some types of wood) and it may have radial cracks developing at the ends.  Generally wood needs to be seasoned (covered from the rain, raised off the ground, well ventilated - not wrapped up - and preferably in sunshine rather than shade) for two summers.  Ideally check with a moisture gauge.  Hard wood is preferable - soft wood burns more quickly so the fire needs re-filling frequently (but I wouldn’t throw it away!).

 

2.  Use plenty of paper to get your kindling sticks going.  Crumple the paper LIGHTLY, to get a fast, hot burn.  Don’t fold, twist or roll the paper because this excludes air and will result in a slow, smoky burn, which won’t get your kindling burning.  I almost fill the firebox with paper - we generally have a surplus of paper from non-glossy junk mail and office/school waste so I don’t need to be sparing.  However be careful not to block your flue with paper!

Add your kindling sticks on top and stand some on end at the front of the fire. 

A large amount of paper not only lights the kindling quickly but also rapidly warms the flue, creating an up-draught which powers the fire.

If you have greasy/oily pans after cooking wipe them with kitchen towel and save them for the fire - they work as excellent fire-lighters.  Better for your drains too!

 

3.  Light the paper at the base and push the door to straight away, without fastening it closed, to create a turbulent flow of air into the fire which helps the fire spread to all the paper quickly.

 

4. Once the paper is burning strongly (usually after just a few seconds) fasten the door closed, ensuring the top and bottom vents are fully open first (the fire will die down almost instantly if you have left the vents closed).

 

5.  In a short time (a minute or two) the kindling will be burning strongly. This is the time to add a few small logs (preferably pre-warmed under the fire).  Don’t add too many at one time, or you may quench the fire.  Continue adding a few (warmed) logs at intervals, using larger logs as the fire becomes stronger.  While doing this minimise the time the door is open.  Keeping the door open allows too much cool air into the fire and will quickly dampen down the fire.  Have logs in your hand, ready to toss in before you open the door then close the door immediately once the logs are in.   

Wood-burners gain their efficiency from regulating the air-flow, as opposed to open fires which allow uncontrolled, excess air flow, meaning heat is wasted going up the flue.  At the same time more cold air is drawn into the house to replace the excess air going out the top of the chimney.

 

6.  Try to position the logs with about 1-2 cm between them to allow a good air-flow, while being close enough so that they help each other burn.  

 

7.  The vents in most wood-burners are positioned to encourage an air-flow from top front, down the inside surface of the door (helping keep the glass clean), then along the floor of the fire, front to back, then up the back wall of the fire, then along the top of the firebox towards the front of the fire and up the flue.  This air flow helps the fire to burn efficiently by making the air take a long path through the fire so giving time for more of the oxygen to be utilised.  Avoid positioning your wood width-ways across this flow, especially if you’re using LARGE logs, because this impedes the flow. 

 

8.  This is rarely a problem when using a lot of small logs, (as opposed to a few large logs) because there are plenty of air spaces, in which case you don’t need to be so careful about how you position your logs.  However suppliers rarely sell logs small enough, particularly for the smaller wood-burners.  My stove is small (Morso Squirrel 5kw) so I always cut my logs short (about 10cm).

 

9.  If you’re burning square-section/ flat-faced (waste) wood, rather than logs, it is especially important to leave gaps.  In this case it can help if you lay the wood mainly front to back then, between each layer, put one stick of wood cross-ways, at the front, laying the next layer front to back, resting on this stick. 

 

10.  Once the fire is well-established (probably in about 10 minutes) and giving out substantial heat (you feel the warmth on your face from arms length) start to close the bottom vent a little at a time.  If you do this too quickly you may start to quench the fire.  In time you will be able to fully close the bottom vent.  This slows the burn, your wood last longer, while not wasting too much heat up the flue or drawing too much cold (replacement) air into your house, so you get the best heat output from your wood.  You may be surprised to find this actually increases the heat from your fire despite the reduced air flow and a less fierce burn.

 

11.  Start to close your top vent too if your fire continues to burn well, further improving the efficiency of your fire.  If you find that this starts to cause your fire to die down then re-open the vent sufficient to sustain the burn.  While you still have flames from your wood (as opposed to just hot glowing ‘coals’) leave the top vent partly open to avoid sooting up the glass.

As you gain experience with your own fire you will learn to judge the best times to turn down the vents.  

 

12. Signs of an efficient burn are:

- there is no hissing noise when you open the door of the stove (this is the noise of moisture being boiled off showing that your wood is not sufficiently seasoned)

- blue tinges to the flames (like a gas ring/ bunsen burner) rather than purely yellow/orange flames

- flames seem to ‘float’ gently in the air, as opposed to a fierce burn

- sooty blackness on the fire bricks on the walls of your fire burns off to give a clean, light-brown colour

- the air-flow into the fire is not noisy (air vents only open small amount)

- go outside and look - no smoke from your chimney!

 

13. Continue to add logs at intervals while you still have a substantial amount of hot glowing ‘coals’ , and while the fire can be easily revived.

 

14.  When you won’t need the fire to keep burning much longer (you’re going to bed / going out) - let the fire burn down till you just have glowing ‘coals’ (no flames) then shut down the vents almost completely (if you have a DEFRA-approved stove the vents can be fully closed because there will be some small vents that are constantly open).  Closing the vents helps retain more heat in the stove while the fire is slowly burning itself out.  To get the most out of your wood gather the glowing coals together from time-to-time.

 

15.  Don’t worry about cleaning out ash too thoroughly - the ash provides some insulation at the base of the fire and helps establish a strong burn.  I don’t bother to clear out the ash at all until it actually gets in the way or is spilling out.  Rescue the bits of charcoal from your ash - it’s great kindling.  If your fire is burning efficiently you will rarely need to clean your door glass with anything other than a quick rub with the paper that you light your kindling with.  If you have stubborn deposits a cream bathroom cleaner (e.g. Ecover) works well, but NOT while the glass is hot - risk of fracturing the glass.

 

16.  Review your user-guide instructions from time-to-time especially with regard to maintenance (cleaning, inspecting for gaps/cracks, renewing seals, having the flue swept annually).

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