Navitron supplies a range of ultra-high efficiency woodstoves with or without dampers.
The use of flue dampers is a bit of a black art, so this page is designed to provide a little advice and some tips for the use of flue dampers. A damper isn't necessary on a woodstove, but does allow an extra level of control. Dampers are often misused, and frequently lead to significant loss of efficiency.
What is a flue damper?
Woodstove Dampers are butterfly valves usually situated in the throat of the flue exiting the woodstove. They are designed to restrict the flow of the combustion gases which in turn reduces the intensity of the fire (which has a knock-on effect of reducing the amount of air entering the fire), and causes the fire to burn slower. Slowere burning fires have more time to release heat into the room, however, incorrect adjustment of the flue damper (closed too far) can easily result in insufficient air entering the fire, and a reduction in fire temperature, either of which may result in a smokey, inefficient fire.
How not to use a flue damper!
Some stove users fill their stoves with wood and shut the damper fully, immediately prior to retiring for the night, in order to keep the woodstove burning all night. This allows the stove to be rekindled in the morning simply by adding more fuel and openning up the air vents. However, this practice should be avoided at all costs - although the burn rate is reduced to such a level that the fire is still burning the following day, it reduces the fire to a smokey destructive distillation process. Instead of burning the wood to produce mainly carbon dioxide, water and heat, the process is reduced to conversion of the wood into heavy, tarry chemicals which are subsequently deposited on the insides of the flue or chimney. This is a very inefficient process - releasing only a small fraction of the available heat, and causes severe tarring of the flue/chimney which greatly increases the risks of chimney fire, and makes sweeping the chimney necessary more often.
How a flue damper should be used.
The flue damper should be fully open when starting a fire or reloading a fire. If the flue damper is partially open or closed, the chimney will not draw properly, which makes starting a fire more difficult, and is likely to cause smoke to billow into the room.
When fresh wood has been added, or when the fire is going strongly, the flue damper should normally be fully open. Closing the flue damper will reduce the speed that the flue gases can escape, and will result in insufficient air, and a smokey inefficient fire.
The process of wood combustion takes place in two stages. Firstly the volatiles are driven off, which tend to burn relatively quickly. Once these have been driven off, a mass of charcoal is left, which burns more slowly. Depending on the type of wood, the ratio of volatiles to charcoal will vary. The flue damper may be partially closed after the volatiles have been burnt off.
The optimum position of the damper will vary according to the state of the fire. The best way to determine the optimum position is to purchase and fit a magnetic flue thermometer, which can be fitted to the fluepipe leaving the woodstove. The optimum position of the damper is a compromise between the optimum combustion efficiency and the optimum thermal efficiency (ie the amount of heat released to the house)
How do you find the best damper adjustment?
The best damper adjustment for a wood burning stove is one that delivers the best balance between combustion efficiency (percentage of energy in the wood that gets converted to heat) and thermal efficiency (percentage of heat that stays in the house and does not go up the chimney).
Ideally, the flue temperature should remain approximately in the range 300-450°C. If the flue temperature goes above this, then you will be wasting heat up the flue, and if the temperature drops beow this, you are likely to cause accelerated deposition of tar and soot in the chimney. In order to keep the chimney/flue in good condition, it is necessary to accept 20% heatlosses up the chimney.
Other things to consider that may reduce wood stove efficiency:
Old woodstoves are less efficient than modern designs. Some antique woodstoves are extremely inefficient and emit higher levels of emissions - replace with an ultra-efficient Navitron stove!
Damaged or missing firebricks inside the stove will allow the stove to cool excessively. You need to have a hot firebox in order to achieve complete combustion. Replace missing or damaged firebricks.
Replace damaged or missing firebricks. The firebricks insulate the firebox, which needs to be maintained at a high temperature in order to fully-combust the fuel. If the firebox is less than adequately insulated, the combustion temperaure decreases leading to an increase in smoke, and lower efficiency.