Chimneys
2 pages link to Chimneys
Table of contents
How and why they work.
Fireplaces work because of the way they are constructed, a properly built fire place and chimney shouldnt smoke.Height shouldnt be a problem with regards to the fireplace opening size.
The hot gases naturally wont to rise, but if they encounter any restrictions in the fireplace or flue they will back out into the room taking the path of least resistance.
The rollback directs heat out into the room and higher up forms the smoke shelf and throat, its primarily the smoke shelf and throat that will determine if the fireplace works or not.
The smoke shelf is always built higher than the fireplace opening by at least 100mm, the front of roll back, and the back of the fireplace opening form the throat, minimum opening size is 100mm.
The throat forms a venturi
(cache) that increases the velocity of the hot gases.The flue takes away the hot gases but must be properly formed and gathered (tapered) to aid in a smooth flow.
Building
As with all projects it was laid out dry to make sure everything was going to fit before laying.
Building the fire box and roll back, the roll back directs heat out into the room and also forms the smoke shelf higher up.
The rollback can start rolling forwards from the first course or higher up as desired.
It pays to mark out the rollback on some plywood before you start so you know how much you have to roll each course out as the work progresses.
If the rollback comes out too quickly the bricks wont want to stay where laid and you will have to prop them for each course.
The roll back and back wall, the back wall forms the base of the flue.
The fireplace opening is always lower than the smoke shelf by a minimum of 100mm, in this case it is about 180mm.
If the smoke shelf is too low in relationship to the opening the fire wont draw properly and will smoke into the room.
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How to fix a smoker
Chimneys smoke because the hot gasses are restricted on the way out of the top of flue and find the path of least resistance to escape, the fireplace opening.Down drafts.
Down drafts are caused by higher pressure air at the top of the flue opening.
Usually caused by the flue being built too low in relation to the roof ridge, or by trees growing higher in the area near the flue.
Wind blowing across the top of the trees or the roof ridge line will cause high pressure air at the flue opening and cause down drafts.
The flue opening needs to be at least 300mm higher than the roof ridge.
Or 300mm higher than a level line drawn 3.5M from the roof.
The pic below gives you the idea , not to scale obviously.
Flue too small.
A small flue will restrict the gases from escaping.
The rule of thumb is the flue area needs to be at least 10% that of the fire place opening.
Once built it is very hard to readjust a flue to make it bigger.
A bigger flue is more desirable that a smaller one.
Restrictions in the flue.
Could be caused by loose bricks that have become detached from the inside of the flue, or some other obstruction.
Remove the obstruction.
Soot.
Remove the soot.
Burn dry wood to eliminate the soot build up.
Other Reasons for chimneys to smoke
Rollback not built high enough.
Rollback too low in relationship to the fire opening, the top of the rollback needs to be at least 100 higher then the fire opening.
Put 1 more course on the roll back, just dry laid will give you an idea if it is going to fix the problem or not.
Smoke shelf not formed properly.
The smoke shelf needs to be at least 100mm wide to form any real venturi effect and needs to have a sharp edge where it finishes inside the flue.
If the venturi
(cache) rounds off rather than having a sharp edge it may not work properly. glossaryThroat too narrow.
Fireplace throats need to be at least 100mm wide to work or they will restrict the hot gases.
Wider throats are preferred on larger chimneys.
The throat in the picture of the fireplace below is 230mm wide.
Wider throats are are also preferred where the fireplace opening is higher than normal.
2 pages link to Chimneys