THE FLUE SYSTEM…
…THE ENGINE OF THE STOVE
FLUE DRAUGHT AND COMBUSTION
There are three important considerations necessary to maximise the performance of your stove:
- Perfect post-combustion with pre-heated air – automatic tertiary air.
- The correct fuel – good burning, well-seasoned wood with less than 18 % moisture content.
- A flue system with good up-draught – the ‘engine’ of the stove.
A stove does not produce its own up-draught. This is created through differences in air pressure between the inside of the house and the pressure of the outside air caused by the difference between the two temperatures causing the normally warmer internal air to rise through the flue system. A chimney with the wrong dimensions (ie a much greater cross-section than the stove flue outlet) will hinder the development of good up-draught. Weather and atmospheric conditions will also affect the draught performance. All DAN SKAN stoves are manufactured using the latest laser technology to create a uniform quality criteria to ensure that they all deliver the same special high performance combustion, but like a car this performance will very much depend upon a good engine and good fuel.
SIMPLE PERFORMANCE TESTS
If you want to test the performance of the stove yourself, try the following simple tests:
Safety Match Test for Up-draught
When the stove is cold, open the stove door – just a small gap, and then hold a burning match just in front of the gap (about 50 mm / 2"). If the flame is drawn into the fire chamber, the flue system has up-draught. However, if the flame is not drawn in, or is pushed outwards into the room then the flue could have no up-draught at all or possibly a downdraught problem. If the match is drawn in so strongly that it goes out, then you probably have very good up-draught.
Soot Test for Fuel Quality
Should you see persistent carbon deposits on the fire chamber fittings or darkening of the glass then we recommend that you undertake the following simple visual checks. Remove the Skamol panel or ‘lid’ at the inside top of the fire chamber or alternatively remove the inspection hatch on the flue pipe. Check for the build-up of flakey soot / carbon which may also have an unpleasant acrid smell would be a clear sign that poor quality or damp fuel has been burned. These deposits could eventually ‘fur’ up the flue system, reduce the up-draught, or potentially cause a chimney fire. A fine covering of dry ash powder which can be easily rubbed away with your finger is perfectly normal.
CORRECT DIMENSIONS OF THE CHIMNEY / FLUE SYSTEM
The prerequisite for a proper functioning stove is the suitability of the chimney or flue system (minimum cross-section, effective stack heightand positioning etc.) which should conform to all local and national Building Regulations – Document J for England & Wales (Scotland andNorthern Ireland have their own versions) and Approved Document J:Heat Producing Appliances for the Republic of Ireland.
Chimneys with a cross-sectional internal diameter of more than 200 mm (8" and / or less than 5 m (15") stack height measured from the top of the fire box could cause draught problems. In our experience the simplest and most cost-effective way to reduce the cross-sectional diameter of an existing chimney is to fit an approved solid fuel flexible chimney liner of approximately 150 mm (6") diameter.
Your installer or DAN SKAN partner will be able to advise you on this as well as various methods to compensate for insufficient stack height and poor up-draught. A chimney or flue system which matches the flue outlet diameter of the stove combined with a flue height of more than 6 m (20") is advantageous and strongly recommended. The chimney or fluesystem internal pressure should ideally reach at least 12 Pa.
EXPOSED EXTERNAL CHIMNEYS
Exposed external chimneys, especially steel flue systems, can sometimes create draught problems when lighting the stove. For example, if the air temperature outside is ice-cold then the air inside the chimney stack will probably be at the same temperature. This could form an ice-cold ‘block’ which produces a cold down-draught inside the room. However, there is a simple remedy: inside the room where the stove stands open an outside door or window against the wind to create a lower pressure. Then open the stove door for a few minutes so that the higher room pressure can ‘escape’ through the stove pipe and flue system … your chimney draught is now is working in the right direction!
HEATING IN THE TRANSITION BETWEEN SUMMER AND WINTER
In the Autumn air temperatures can vary extremely quickly and this could adversely affect your flue draught and the performance of your stove, making it difficult to light the fire and making it very slow to burn as normal. For example, a sudden rise in temperature will disturb the pressure balance that normally makes the flue work so that the flue gases find it difficult to rise. Strong winds can have a similar effect and is most noticeable when opening the stove door to add fuel.
If in doubt always try the safety match test discussed above. Using this method you can easily see whether you have sufficient up-draught or if there is likely to be a down-draught problem. In such instances we always recommend lighting with the ‘top fire’ method described on the inside back cover of this brochure and with a larger than normal ‘pre-fire’ to help get the draught moving in the right direction.
It is important that before you light the larger pre-fire you open a window or door (in the opposite direction of the wind) so that a lower pressure can be quickly achieved in the room where the stove is located than that in the flue system. Once your fire begins to burn normally then the door or window should be closed.
MAINTAINING A CLEAN BURN
To maintain a high performing clean burning stove you must ensure that the flue gases always move through the chimney or flue system as quickly as possible limiting anything which might slow them down and therefore cool the gas and smoke to the point where they can create creosote and soot deposits which will further enhance the slowing and cooling process. We recommend that the flueways are cleaned by an experienced chimney sweep at least once a year and preferably before the beginning of the heating season. The performance of the air wash system and its ability to maintain a clean glass, both up and down, will also depend on the speedy removal of the flue gases through a flue system which has a good up-draught. Smoke escaping from the stove into the room during reloading could indicate that you have a temporary problem with the weather conditions. Persistent smoke escape could indicate a problem with the flue system, possibly sooting up, and this should be investigated as a matter of urgency by the installer or chimney sweep. If you plan to use an existing chimney which has a much greater cross-sectional area than recommended by DAN SKAN (see page 85) then it is probably advisable to choose a stove with double doors, where only one door needs to be opened for refuelling, thus limiting the disturbance to the fire chamber pressure and up-draught.
A good chimney draught depends on good flames. Without heat producing flame the up-draught is limited and so too is the air wash performance.
Remember that poor burning woods (eg Laburnum, Leylandii etc) and damp or unseasoned wood with a moisture content of more than 18% will produce a lot of smoke and soot as well as a very poor flame. Slow burning hardwoods, such as Beech and Oak, work better when started with some highly flammable wood such as Spruce or Pine to encourage the flames.
Beech wood contains a lot of tar and is better mixed to help reduce the sticky black spots it may leave on the glass, which even our superior air wash system will find difficult to clear.
If the fire has burned down so that only a few embers are left, when any new fuel is added, always open the glass door slightly to encourage a strong up-draught which will perform like a blacksmith‘s bellows and quickly help the new logs to light. This should only take a minute or two and then the door should be closed to avoid dangerously over-firing the stove.
PROMOTING A GOOD UP-DRAUGHT
Always start your fire with a good starting or pre-fire – this will save time in the long run (see the inside back cover for our ‘Top fire’ instructions). Only when the whole system, the stove and the chimney, have good basic heat throughout, can the correct conditions for an efficient up-draught and clean burning be achieved. Only a warm system can supply sufficient up-draught.
When it is very cold outside then the bigger your pre-fire should be so that the rising hot gases from the pre-fire quickly warm the flue system to encourage a good up-draught – just like starting an engine! If your chimney or flue system performs poorly then you may be able to improve this by reducing the size of the baffle plate by trimming a little bit at a time from the front edge until you arrive at the best ‘setting’.
The Skamol baffle plate can be easily cut using a manual saw or electric jig-saw. You can even try operating the stove without the baffle plate. This will be safe because your DAN SKAN stove is manufactured to the highest standards so that the bodywork will easily withstand the heat and still remain perfectly stable.