We had a "real effect" fire that was used very rarely, but turned it on one cold night when the central heating went down.
Within minutes I was feeling giddy and sick but had the presence to consider CO so turned the fire off and opened the outside door. Was fine in 20 mins.
Had the flue checked and the gas man shone a torch up the flue and could see the light through the wallpaper, the flue and plaster had cracked but the paper stretched.
The fire has gone and the flue blocked up, but still gives me the shivers, had I not been aware of CO I and the kids – asleep upstairs – would have been goners
Trevor