Unfortunately there are no strict definitions of the term 'Clean Burn' when it is applied to multi fuel stoves and wood burners, and in reality it should be seen as a relative term. No doubt you'll come across Clean Burn or Clean Burning elsewhere on the internet and it is worth pointing out that it is sometimes deliberately used to mislead customers regarding the benefits and features of inferior quality, usually imported, lower priced stoves. All imported stoves, and from July 2013 all domestically produced stoves, need to be CE tested for efficiency and emissions, but this is no guarantee that a stove is clean burning – just that it has passed some very minimum EU emissions standards.
At The Stove Yard we would regard any stove which has passed the strict DEFRA Smoke Exempt Appliance (or DEFRA Approved) emissions tests as worthy of being called 'clean burning' because the stove has been set up differently to reduce the amount of smoke it produces, since smoke is essentially unburned 'dirty' particulates. When a stove is in slumber mode it is in fact burning inefficiently – of course, it will still produce a certain level of heat and also slow down the fuel consumption, but it will also produce much more smoke than is acceptable under the Clean Air Act. A DEFRA Smoke Exempt Appliance therefore limits the ability to 'slumber' the stove so that it will always burn cleanly, provided it is operated in the correct way.
In addition, the more efficient a stove is then the cleaner burning it is. The extra efficiency comes from the stove combusting more of the wood's gases and unwanted particulates, thus wringing out more heat from the wood and more percentage points of efficiency. This greater efficiency leaves less pollutants to escape via the flue gases. A stove with nearly 85% efficiency (eg Precision I) is going to be more cleaner burning than one with a typical efficiency of around the mid 70%. Unfortunately a stove with a higher output does not necessarily mean that it is more efficient and therefore because of this, a clean burn stove – it could simply just have a bigger fire chamber with more fuel in it when it was tested.
At one time a Secondary Air System (or Airwash) was a good indication of a cleaner burning wood burner as it produced a more efficient combustion of wood. However, even though every stove that we sell at The Stove Yard now has this feature we still wouldn't regard them all to be clean burn stoves, just more cleaner burning than what was available ten years ago. Today, for us any stove which has an efficient Pre-heated Tertiary Air System (eg Morsø Badger or Charnwood Island II) and an above average efficiency, has the credentials to be rightly described as a clean burn stove – even though many manufacturer's themselves don't necessarily use the term – Dan Skan, probably being the best example.
Pre-heated Tertiary Air is another level of air introduced at the top of the fire chamber which helps combust any of the remaining particulates, which would otherwise have escaped un-combusted into the flue system, thus making the stove truly clean burning. When working properly the effect of the tertiary air can be seen as a row of gas jets at the top of the fire chamber back, with each jet producing heat which would otherwise be lost. This accounts for the top-end efficiencies of stoves which feature this Pre-heated Tertiary Air system and which is becoming a more common feature, even on modestly priced stoves like the Defra Approved Precision I