Wood stoves and fireplaces are tested with different methods and with different results in mind. The Norwegian clean burn approval requires low particle emissions on both low and high burn rates; research shows that most people are burning wood on a low burn rate.
Efficiency testing focuses only on the maximum level of efficiency, known as nominal output, and has no references to how the product is used in day-to-day life. This means that a wood stove with 80% efficiency has to be burnt at the “peak testing level” all the time to make sure it gives 80% efficiency.
A wood stove that is clean burning according to Norwegian standard is clean burning on all burn rates. When technical data says that a clean burn stove has e.g. an efficiency of 76% it means that this is the level that is kept within the heat output range of e.g. 3-9 kW. The peak level of efficiency may well be 90% e.g. at 9 kW, but since most people burn on low burn rates most of the time the real efficiency will not be 90% when the stove is used in everyday situations.
We can therefore conclude that a wood stove that has been given a Norwegian clean burn approval is likely to be more efficient in everyday use than a stove that has only been tested for maximum efficiency, even if the efficiency numbers says otherwise.
However, some products get a high score on efficiency also if they are built to be clean burn on all burn rates.
The illustration shows the efficiency level for Jøtul F 162.
The illustration shows the comparatives between non-clean-burn and clean burn stoves as required in the Norwegian environmental test NS 3058.