Chimneys and draught conditions – Trouble shooting

 

The main function of a chimney is to create draught for combustion and to transport the flue gases out of the building.
A good draught is vital for a good combustion. We consider a normally good draught to be between 10-20 Pa (1-2 mm VC). The chimney creates the draught, not the appliance.  

 

Essential for the draught is the construction of the chimney
A tall chimney gives more draught. If the draught is insufficient it can be a solution to build a taller chimney. The chimney diameter should never be less than the diameter on the appliance flue outlet. A circular chimney liner normally gives a better draught than a square. Use of flue pipe elbows reduces the draught.

If elbows are used it is better to install with 2 X 45o.

 

Combustion air is essential for the draught

An open fireplace requires approx 300m3 air each hr, while a “closed” fireplace requires approx 30m3 per hr. A kitchen duct /ventilator sucks much more air than a chimney. This will create a negative draught. Negative chimney draught causes smoke in the room.
Outside air leading directly to the appliance avoids insufficient combustion air.

 

 

Influence of the wind
Draught disturbance can be caused by tall trees, cliffs or tall buildings. The problem can normally be solved by making the chimney taller. In extreme situations an exhaust fan must be installed. A draught regulator stabilizes the chimney draught.  

 

Draught is simply hot air rising
High temperature creates strong chimney draught. A good result is achieved when the height and diameter of the chimney fits the appliance. Too strong a draught can cause the heat to be sucked too fast into the chimney. Too strong a draught can be regulated with a damper, draught regulators (flue pipes or chimneys), or restrictors.
Multiple uses of flue pipe elbows reduce the draught.      

 

 

Trouble shooting (wood burning fireplaces)

 

Sizing is important
An oversized stove can leave you with potential creosote and smoke problems while an undersized stove can lead to constant over firing - resulting in warped and cracked parts.

 

What is the intended use of the stove?

  • Heat source
  • Aesthetics (a piece of furniture)
  • Primary heat or back up

 

Too large a stove is not going to do you any good
You will constantly burn the stove in an air starved mode which generates a lot of smoke, soot, tar, and ash. The glass will be constantly dark when you burn the stove in an air-starved mode.

 

Too small a stove is not going to do the you any good either
You will always try to get more heat out of the stove than it can produce, leading to constant over firing. The consequences could be cracked and warped parts which is not covered under the Jøtul warranty (burn plates, baffle plates etc).

 

If it is difficult to start the fire the reasons could be:

 

  • Not enough air: Open the air valves. Sometimes the door must be opened (approx. 1 cm gap). Make sure that the damper is in open position. You can also open the door to an external ash pan.
  • Bad kindling: Use small pieces of split kindling together with crumbled newspaper, and add larger pieces. Remember: The smaller the better, the dryer the better.
  • Down draught/cold chimney: Heat up the chimney by twisting some newspaper into a torch and hold it up into the stove until the draught is reversed.
 

Smoke in the room can be caused by:

 

  • Wood quality: Wood with a lot of moisture can cause more smoke than the chimney can dispose of.
  • Air systems like air condition, bathroom or kitchen fans might take their need of air from the chimney (negative draught). In these cases you must bring outside air into the fireplace. 
  • Operating errors: Always open the damper and primary air control before you reload the stove - open the door slowly.
  • Flue pipes: Remember that elbows and horizontal flue pipes make restrictions on the draught.
  • A chimney that is too short could give insufficient draught for the fireplace. A chimney that is too cold can cause none - or negative draught. Flue liner must be correctly connected to the fireplace and the chimney - and have the right dimension.
  • Blocked chimney could be caused by a birds nest, soot, or tar.
 

 

   

Short burn time can be caused by:

  • Wood load: Large pieces will burn longer than small pieces. Use hard wood that has been cut, split and stored at a dry place for approx. 12 months.
  • Too strong draught in the chimney which gives insufficient combustion.
  • Operating errors: Primary air control should be slowly adjusted toward the closed position, and then finally closed after the fire is established.

Do not burn with the ash pan door open at a longer period of time - it will cause over firing of the fireplace.  

 

Back puffing (ignition of gases) can be caused by:

  • Closing the air control when reloading the burn chamber. Sometimes you must wait approx. 15 min. before you re-close the air-control.
  • Secondary air supply might be blocked by soot and tar.
  • Moist wood requires enormous amount of energy to dry out before it will burn. These flue gases can ignite and cause an explosion in the burn chamber.
  • Insufficient draught.

 

Hard to control (overfiring)

  • Poor gasket seals.
  • The fuel: Never burn old pallets, chipped wood, trash or waxed products.
  • Excessive draught in the chimney: Use a damper or a draught controller to reduce the draught.  

 

Low heat

If you find that the heat output is too low the most common reasons are:

  • Poor wood quality
  • Chimney draught
  • Operating errors
  • House construction (insulation, height to ceiling and such)
  • The stove is too small

When the wood burns slowly and at a low temperature, it produces tar and other organic vapours which - combine with moisture - form creosote. This creosote will stick to the chimney walls.

 

 
Creosote is the fuel for chimney fires
Excessive creosote can be caused by:

  • Poor wood quality and size
  • Chimney draught
  • Operating errors
  • Stove size (too low operating temperature)

 

Remember to sweep the chimney before the start of the firing season.