I have a victorian fireplace with a modern gas fire installed. The fire place has a hatch at the top to open/close the chimney, which the fitters have connected to a wire to force it to be fully open constantly. However, it loses a lot of heat through this. I believe it is partly ventilated from underneath. How much should this kind of hatch be open? can I close it completely? Can I close it and have a CO monitor? Surely as long as this is OK there is nothing to worry about?
Your flue should always be open when the fireplace is operating. It is a fire and smoke risk to close the flue while the fire is lit.
When not burning, the flue should be closed to prevent heat loss.