Generally speaking, this is when a stove, which was working perfectly fine when you last used it, then inexplicably doesn't work. It will be difficult or impossible to light, possibly causing smoke to enter the room. There could actually be a number of causes (see below). However, at certain times of the year when there are dramatic swings in temperature and especially because the problem has occurred suddenly, it is more than likely to be caused by 'Spring and Autumn Syndrome' and is not likely to be a problem with the stove.
The stove and its flue uses the temperature differences between the room and the air outside. Under normal circumstances the air inside a home is usually warmer and lighter (because as it warms it expands and becomes less dense) than the colder outside air and this creates a pressure difference which induces a natural updraught inside the flue – where warmer air has to rise up through the flue because it is under greater pressure. This natural draught will exist whether or not the stove is being used and the greater the temperature difference between inside and outside, then the stronger the updraught or 'draw'. This process will be accelerated when hot combustion gases are introduced into the flue thus enabling them to be successfully exhausted into the atmosphere.
Problems occur with updraught when the temperatures are reversed. For example, those days when it is suddenly warmer outside than inside – usually during unseasonably hot spells in spring or autumn. This is when lighting your stove becomes difficult because suddenly there's no natural updraught to kick-start the process. The air inside is colder and heavier than the air outside and this creates a kind of heavy cold 'plug' in the flue which wants to fall back into the room thus creating downdraught. You will probably not know that this is happening until you try to light your stove. Once you are aware of the conditions which cause spring and autumn syndrome however it is actually easy to deal with.
Prepare your fire, but this time use much more quick-burning kindling than normal and one or two additional firelighters. The Scandinavian 'top fire' method is always to be preferred in these situations. This is where the bigger logs are placed on the firegrate, with kindling on top of the logs and then firelighters on top of the kindling. Ensure the stove's air controls are fully open. The additional firelighters and larger amount of kindling will produce a fast burst of flame and therefore generate heat quickly to help increase the updraught. Do not add any big logs until you are satisfied that the stove is operating normally. At the start you may see some very minor smoke leakage but this should quickly disappear as the heat inside the flue increases. It is worth noting that all of this can take longer with a boiler stove due to the cold water inside the boiler generally keeping the fire chamber and flue gases at a lower temperature for longer.
Where a stove has a Direct External Air Supply (DEAS) the conditions for creating spring and autumn syndrome are eliminated because the system is 'sealed' and therefore the room temperature (and pressure) should not affect the performance of the flue because none of the colder air from the room is being used inside the stove.
Please note: Other factors which can increase the likelihood of poor updraught include insufficient height of the flue system or chimney, the number of bends and length of any straights, whether or not it is insulated (or on an external wall), possible blockage (perhaps in the stove flueway itself) and an insufficient air supply to the room. These important topics are discussed in other Stove Yard FAQs.
IMPORTANT: Never use your stove if smoke continues to enter the room. Please seek professional advice and do not use the stove again until the problem has been identified and rectified. See our FAQ on stove emergencies.