Chimneys--An unexact science !

Did You Know that a adequate chimney is actually more important for successful wood burning than a good stove or fireplace? In other words it would be better to have a good chimney and a poor stove than the opposite.

As the title implies, chimneys are an unexact science. The intent of this section is to educate you so you can make the proper decisions concerning your wood and gas burning systems. Problems with chimneys -If a stove smokes into the room, an inadequate chimney is the most likely cause. See gas and gas questions page for vent free/ novent info. Creosote problems can often be lessened by good chimneys. Low heating efficiencies can also often be traced to poor chimney draft--In short, 90% of all wood burning problems (in our experience) can be traced to natural draft and chimneys. Forced dafts by means of mechanical extraction can be controled more effectively.

Fundamentals- The function of a chimney is two-fold, first to carry the undesirable products of combustion (smoke and fumes) out of the home: secondly to supply the draft (pressure) to feed air to the fire. The draft of a chimney comes from the tendency of hot air to rise. The draft is directly related to the difference between the chimney inner and outside temperature x the height. The flue termination play a critical role in up- and down draft.

This flow up the chimney is determined by many factors such as:
1. Chimney Height
2. Chimney Diameter
3. Chimney Location
4. Bends in the Chimney
5. Construction of Chimney (Chimney temperature)
6. Tightness of House

Reversing Chimneys - a common problem Wind Related Downdrafts

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By following some of the guidelines given in this section you can learn which type of chimney to build or how to upgrade your present chimney to a higher standard.

Chimney Height- The taller the better ! A taller chimney will have more draft than a shorter chimney (all other things being equal). Its relatively easy to decrease the draft in a chimney which is too strong, but improving the draft on a short chimney can be more difficult. Longer flues can be a little smaller diameter and shorter flue need to be bigger in diameter.In South African conditions a chimney must genarally not be shorter than 2,4m. It is measured from the top of the fireplace or braai opening to the top of the chimney ; not the cowl. On extremely cold days chimneys longer tan 6m can be a problem since it allows the smoke to cool excessively, thereby slowing it down and causing a block in the chimney. Insulated chimneys daft better and can overcome this blocking problem.

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Chimney Diameter- Chimney diameter also has an effect on the draft of your chimney.

In case of stoves and close combustion burners having a chimney that is too wide (as compared to the outlet on the stove) can be a problem since it allows the smoke to cool excessively, thereby slowing it down. If possible, the chimney diameter should be similar to the diameter of the flue outlet on the stove. In no case should it be more than 3 or 4 times the cross-sectional area of the stove outlet. If your chimney is oversized, you may consider relining it with a smaller diameter flue.

Vented gas firepaces need almost the same size chimney than a wood fireplace. Natural draft needs a minimum diameter and temperature to form a draft. Gas fireplaces do give of fumes. Please do not under design your chimney.

Chimney diameters for open fire braai’ is about 6- 11% bigger than fireplaces. Fireplaces must keep the heat inside and braai’s must take the heat and extra smoke generated by the fat dripping on the coals outside. Depending on the fireplace design a chimney can go as low as 13% of the fireplace opening.(Keeping in mind the chimney height, material, fire-box, smoke shelf and smoke chamber or gather.) Closed combustion stoves can go down to 8.5%. If you try to braai in a fireplace, hearth or braai and it work well when the fire go and get smoky when you open the coals to braai, the chances are good that the chimney is designed for a fireplace and not a braai.

Chimney Location- The early settlers knew some things we have forgot. They located the chimney inside the structure, where it would stay warm and draft better. This indoor location also meant that the warm chimney added some heat to the home. Take a hint from our forefathers...locate your chimney inside if possible ! Bends in the Chimney—Each turn of a chimney lowers the draft. A rule of thumb is that each 90 degree turn cuts 1.5m off of the effective height of a chimney. Therefore, if you have a 3.6m chimney with two turns, the effective chimney height is 0.6m ! --surely not enough height for a stove fireplace or braai to work properly. Make use of 45 degree bends where possible, they only reduce the effective chimney heigt by 0.65m per bend. hight.

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Construction of Chimney—A round chimney flue drafts better than a square or rectangular one. In addition, an insulated chimney (either a metal one or properly constructed masonry chimney) will get warm and stay warm easier, making it easier for the chimney to draft properly. Steel insulated chimneys warm faster than masonry chimneys and will create a faster draft. This is useful for South African conditions when we light fires on cold days and use fireplace heating for short periods, not giving masonry chimneys enough time to reach a 100% working temperature.

Tightness of Home—All real fire heating appliances need air to support the combustion, incuding gas fires. A house that is too tight may not allow enough air to feed the fire, and a smoking stove or fireplace can result. Many fireplaces and stoves allow for the connection of a pipe which draws the combustion air from outside the home. In addition, a tight house can aggravate the problem of Flow Reversal (down draft).
The same effect can cause the room to smell the next morning, You make a fire and the chimney draft create a low pressure(vacuum) in the house.Once the fire go out and the room cools down, the room can only fill the vacuum through the chimney, bringing with it the smell .

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Other Chimney Problems :
Flow Reversal or Down drafting chimney—This is not a common problem in South African conditions. Cold air falls and warm air rises, we all know this and take it for granted...well, your chimney knows it too ! When it’s not in use, the cold air from outdoors falls down the chimney and tries to enter the house. Clothes dryers and bathroom/kitchen fans agravate the problem further since they suck air from the home and push it outside. This air needs to be replaced from somewhere...and your chimney is just waiting to supply it. So, if you just open your stove or fireplace damper and light a fire, you may get a surprise.

Wind Related Downdraft—If your stove or fireplace spills smoke into the house only on certain days, then your problem may be wind related. In certain cases, installing a special type of draft increasing chimney cap can help with this problem. Increasing the height of the chimney can also be of benefit – steel chimney extenders such as those made by us can raise chimneys without the use of masonry. Chimney fans can also be installed.

Thanks, enjoy your effective chimney.