| Boiling Point No. 05 - September 1983 |
The installation of chimney stoves to remove smoke from houses has been one of the major objectives of many stove programmes.
Chimney stoves have advantages and disadvantages over non-chimney stoves. The main advantage is the reduction of pollutants in the kitchen. Chimneys can assist to ignite and burn low quality and wet fuels. In Nepal, chimney stoves are used successfully to burn maize and rice stalks.
Chimney stoves usually cost more than non-chimney stoves for the initial purchase, and there are greater recurrent costs. They also take longer to install. Low mass mud and ceramic stoves, with chimneys, provide less room heat since the flue gas cannot easily warm the room. It is more difficult to make use of the flue gas for drying the roof, crops, or fuelwood, unless a short chimney is used (ie an indoor chimney), or the pots are removed and the smoke allowed to escape through the pot-hole(s).
Unless chimneys are cleaned regularly the performance of the stove will decrease as the chimney blocks up and there is the possibility that tar formed in the chimney will ignite. This applies especially when agricultural residues are burnt.
Chimneys must be installed correctly otherwise they will not draw properly. Incorrectly installed chimneys can result in roofs (especially thatch) catching fire.
Chimney caps must also be carefully designed, and the chimney placed on the leeward side of the house if wind is not to blow down the chimney (ie backdraughts are to be avoided).
Children may easily block chimneys with dirt or stones if the exit is not protected, or placed out of reach.
It requires more effort to train people to install chimney stoves, and more intensive monitoring to ensure the quality of installation is maintained.
Hints on Designing and Installing Chimneys
1. Never make a chimney with a diameter of less than 10 cm and a height of 1 metre (for further details see Technical Notes No 2 'The Optimisation of Chimney Stoves', issued with BP No 4).
2. Make sure that a chimney is 1.5 metre below a ceiling or overhang (if placed indoors) or 0.5 metres above a roof (Figure 1). Never install a chimney under a thatch roof (Figure 2) as it could catch fire.
3. When putting a metal chimney through a flammable roof, install a sheet metal thimble, (Figure 3).
4. Always make provision for easy maintenance of the chimney. This can be achieved by
- leaving a 10cm well at the bottom of the chimney (Figure 4) so soot does not block the exit passage from the stove
- providing access at the base to remove the soot
- sloping the stove exit passage down so that soot can be easily pushed into the well
- regularly tapping the chimney to knock loose soot into the well
- if the chimney is taken out through the wall, providing a cap (Figure 5) at the bend to make cleaning easier.
5. Install a chimney cap as illustrated in Figure 6 if local conditions (eg prevailing winds) will cause backdraught problems.