What was wrong with the old fire.
Some fires need more of a pull to make them work better
Precast flue chimneys with ridge vents are always the worst for fitting gas fires on.
Did the installer check the old flue before fitting the fire?
Can you ventilate the room more so the chimney can create more of a pull?
Can you fit a different ridge vent to increase the height of the flue and therefore create more pull?
The installer should have looked at all these options with you if he was any good
If the fire has a problem with the pull then lighting at full flame for 10 will heat the chimney - do you still have the problem then?