If the flame effect fire is on...
- 65-85% of the fire kW input goes up the chimney
- In so doing the air feeding combustion must be replaced
- In heating the chimney it will tend to draw more air also
- So drawing cold air into the house & into that room

If the flame effect fire is off...
- Block the chimney with a tiny vent set on a panel
- Ensure you have a rain-cap on the chimney pot

You need to limit rain ingress - 250cm annually.
LCD TV w/ flame effect picture might pay for itself in a year :-)