The primary function of the chimney is to direct the smoke away from the stove and out of the house. However, this is not the only function of the stove, as it must also:
- Ensure that enough air is drawn into the stove for the fire to burn clean.
- Direct the smoke away to avoid smoke and smell nuisance in the neighbourhood.
The chimney's significance for the stove
For a stove to work perfectly, there must be a sufficient draft in the chimney. The chimney is the engine of the stove that controls the combustion, and without a well-functioning chimney with sufficient draft, the stove will not burn properly. This may lead to problems with ignition, smoke in the living room, bad combustion and smoke nuisance in the neighbourhood.
The heat in the smoke creates the chimney draft. The draft occurs due to the temperature difference between the air/smoke inside the chimney and the temperature outside. This creates an upward air-flow which is intensified, the higher the chimney is and the better the chimney is isolated. Therefore, steel chimneys are often better as brick chimneys, as they are faster and easier to heat.
The ideal chimney draft is between 12-15 Pascal, Pa, but varies throughout the year (higher in the winter, lower in the summer).
The cooperation between the stove and the chimney thus depends on a sufficient chimney draft. This is an important factor, when assessing the placement and height of the chimney. Surrounding trees, buildings and the location of the house (high/low) place demands on the height of the chimney. Furthermore, the wind outside plays a role for the height of the chimney as it can enhance or reduce the draft depending on local conditions.
Another and new important factor to consider is the condition of the house
Today, houses are isolated more often compared to 10 years ago. Low-energy houses with Genvex systems and large cooker hoods are being build, causing a "fight" for air. This often results in the stove losing its “breath”. In this case, we recommend an Aduro stove with air-connection (external air supply).
A good chimney is thus a prerequisite for the stove to function satisfactorily.
The chimney’s placement on the roof is critical to the chimney draft. Below is an illustration of the three zones your chimney can be placed in, along with their effect on the chimney’s draft.
In zone 1 you can be relatively sure, that the chimney’s draft is good. The draft will, however, be affected by tall trees and other buildings nearby the chimney.
In zone 2 you can experience bad chimney draft in periods. The draught will be particularly affected by the wind direction, while big trees and other buildings nearby the chimney can also have an effect on the draft.
Aduro recommends: If you experience difficulties in zone 2, it can be remedied by extending the chimney so it is placed in zone 1, or you can mount an Aduro DraftOptimizer.
In zone 3 you will experience, that the chimney’s draft is often poor.
Aduro recommends: To avoid difficulties with a chimney placed in zone 3, you should mount an Aduro DraftOptimizer upon installation.
The chimney draft and its significance
The chimney draft plays a crucial role for the stove to function.
There can be several reasons for a bad chimney draft. Below you find a list of the most common:
- The chimney is too low compared to the house (see illustration under "Placement")
- The house is sheltered by tall trees or houses
- The chimney is leaking and draws in false air (for example due to missing joints or bad isolation)
- There is a vacuum in the house, resulting in the stove not receiving enough air (due to cooker hood/ventilation system)
- The house is too isolated which means that the stove cannot receive enough air for the combustion.
A poor chimney draft can in some circumstances be improved by having a higher flue gas temperature, thus making the ignition very important. By using dry wood, you will experience a better combustion and a higher temperature in the stove, which will increase the draft in the chimney. Conversely, moist firewood can aggravate the chimney draft, as it lowers the temperature in the stove, giving less buoyancy in the chimney. We recommend using split birch or beech with a moisture content of max. 18%.
If the effect is not achieved by proper ignition, you can create a better chimney draft by:
- Extending the height of the chimney
- Installing a smoke exhauster – e.g. Aduro DraftOptimizer
- Securing the chimney for leaks
If necessary, talk to your local chimney sweep and let him evaluate the chimney.