Vents must be fitted in areas that carbon based fuels are burnt, (unless the appliance is room sealed)
When your fire burns it creates , carbon dioxide,C02 the 02 is important as the fire has sufficient levels of oxygen. In the depths of winter when you have a big fire on, it will consume massive amounts of oxygen. When the fire doesn't have required oxygen it creates Carbon Monoxide CO.
As your home is old, it probably has alot nooks and crannies that allow air in and out, so ventilation was pretty natural. When you renovate, you will seal the house more so than before.
I would strongly advice you put a vent in your room, pick the spot carefully and you can forget about it.
The window opened for 10 minutes sounds like it was made up and has no factual basis as far as I can see.