black soot on new gas fire..

[ray316]

l have just had a new gas fire fitted, the chimney was swept and smoke test ok.. but after only 2 weeks the fibre boards at the back right at the top where the fumes/heat escape have a black soot like mark on one side that seems to be getting bigger every day we use the fire..
What could cause this, surely this should'nt happen should it.

 

[gigz]

When you first turn the fire on do you put it on high for roughly 15 minutes ?

 

[ray316]

When you first turn the fire on do you put it on high for roughly 15 minutes ?[/quoteMost of the time l leave it on high longer than 15 minutes.

 

[gas112]

a coal in just the wrong place even a few mill can make the difference