~ Smoke is entering the room from the appliance ~
If burning wood, make sure it is well seasoned with moisture content well below 20%.
This usually requires the wood to be seasoned for 2-3 years. Trying to burn green wood is like trying to burn water!
Ensure you are not closing the air controls before the fire is established
Check for blockage in chimney (consult your chimney sweep if necessary)
If problems persist, arrange for your dealer or professional chimney specialist to report on the suitability of your chimney

~ Tar Formation ~
Use of unseasoned wood
Poor chimney draught
Operating the appliance while the fire is at very low temperature
Seals on the flue joints in convection chamber are not smoke tight
Walls of convection chamber are not clean

~ Cast iron grates distorting ~
Ash compartment over-full, causing the grates to be sandwiched in hot ash

~ Fire does not burn for long periods ~
Wood can contain a number of variable factors related to species, weight, age and moisture content.
It is difficult therefore to be precise with regards to the length of burn time
If it is thought that the fuel is burning too quickly:
The diameter of the logs are too small
If softwoods are being used, change to hardwoods
Excessive chimney draught. If so, fit a draught stabiliser
Worn out door and glass seals need replacing