A properly functioning chimney has enough draft to pull smoke out of your home and provide enough oxygen for hot, complete burning of wood to occur. It is important that your chimney is the right type and style for your firebox or stove. In addition, it should be installed properly in your home in compliance with building codes and manufacturer instructions. A well-designed system has the following performance characteristics:
» fires light easily and burn bright and hot
» draft builds quickly
» lighting fires does not cause smoke to fill room
» opening the door does not cause smoke to spill out
» when fire is not in use, air flows into the stove and cold air and bad smells do not come out.
Common repairs
The purpose of your chimney is two-fold. It must remove harmful 
byproducts created when fuel is burned, and it must carry excessive heat away from combustible items near the fire. Besides being cleaned and inspected each year, your chimney will also need occasional repairs to ensure that it is performing these functions properly. Repair work can be done by masons or fireplace professionals.
» Firebox re-pointing and rebuilding : the firebox is the brick area where you build your fire; loose
bricks and degraded mortar should be repaired.
» Mortar crown repair : the mortar crown is the concrete like surface at the top of your chimney that keeps water out of your flue; a cracked or broken mortar crown should be repaired immediately to avoid serious water damage.
» Damper repair : A damper closes the flue when you aren’t using the fireplace so heat doesn’t escape through the chimney.
» Flue repair : the flu is the internal part of the chimney that funnels 
exhaust from your fireplace to the outside; if it cracks, breaks, or is blocked by debris or creosote build up it is a safety hazard that needs to be repaired.
A note on carbon monoxide
A chimney functions to remove the byproducts of burning fuel. Carbon monoxide is one of these byproducts. It is a colorless, odorless gas that is a serious health hazard. It causes illness and death if it is inhaled in large amounts. For the safety of your family and guests is critical to that your chimney is maintained properly so that carbon monoxide and other harmful fuel byproducts do not enter your home. Annual inspections are important. Also, you should have carbon monoxide detectors installed on each floor of your home, as well as in sleeping areas. Carbon monoxide detectors can let you know if there are possible problems with the venting systems of your furnace, gas hot water heater, or gas stove and will keep you from being harmed from prolonged exposure to low levels of carbon monoxide.
Other safety tips
» Do not bank fires or close dampers to prolong burning.
» Burn properly aged wood, not green wood.
» Burn fires hot to minimize creosote buildup.
» Regularly check the outside parts of your chimney and any connection for any visible damage.
» Have your chimney annually inspected by a professional for any structural damage,
and cleaned as needed.