Is there any difference between a stove tested for intermittent burning and one tested for continuous burning – do I need to choose one or the other?
In the past when a lot of stoves were used as the primary heat source and a lot of coal was burned, there was an advantage if a stove manufacturer could claim that their stove was suitable for long 'continuous' burn times, often referred to as 'suitable for overnight burning'. In order to do this a stove had to be well made, with no air leakage around the door or combustion controls, so that it had the capability of closing off virtually all of the air supply to the fire chamber, thus starving the fuel of combustion air and slowing the burn rate down to 'slumber mode'. Reputable manufacturers backed up their claims with a time-consuming, and consequently expensive, lab test which was in addition to the minimum refuel period required by the CE EN13240 or EN13229 tests of 45 minutes for wood and 1 hour for mineral fuel (non boiler stoves).
However, times have changed and although we all love our stoves not many of us actually use them as our primary heat source any more and this, combined with today's well-insulated homes – where a little heat goes a long way, has reduced the need for overnight burning. For manufacturers getting a higher efficiency and reducing emissions levels has become more important when undertaking their CE tests. More people too recognise the environmental damage of slumber burning which produces an awful lot of nuisance smoke, creates more soot and creosote and therefore increases the potential for flue system blockages. Slowing the burn rate down too much reduces the fire chamber temperature and also limits the effectiveness of the stove's airwash system so that you end up with an unpleasant grimy 'nicotine-stained' window.
A modern stove which is rated as 'suitable for continuous burning' is therefore now a very rare thing. Since most stoves are generally better made, more controllable and more efficient, extending the burn times can usually be easily accomplished by tweaking the air controls and monitoring the flue gas temperature (with a flue pipe thermometer) so that it doesn't fall below 120ºC, below which creosote forms and nuisance smoke is created. How long you can extend the burn time is then determined by the size of the fuel load, the quality of the fuel (eg oak logs are slower burning) and the characteristics of the flue draught. So in that respect most stoves are 'suitable (and safe) for overnight burning' to some extent. It is worth noting here that clean burning Defra Approved stoves have their air controls configured so that it is virtually impossible to turn them down to the point where they create smoke and creosote so it will be harder to extend the burn times as much as a similar quality non-Defra stove.