If anyone is worried about combustion byproducts in their living space, I highly recommend looking into upgrading your furnace and hot water heater to direct vent models. Both appliances have sealed combustion chambers, and there are two pipes coming down from the roof that supply air in, and two pipes that vent exhaust gasses out.
I did this because I live in a converted industrial loft, so my furnace and hot water heater are essentially in the same space as my bed, and having open flames in the same space seemed risky.
What? I thought ventilation of water heaters was a legal requirement. I am sure it is in the UK. It is illegal not to do this. Because of CO poisioning.
Same with furnaces, heaters, etc.
I like my current setup - the entire boiler is outside and away from any windows.
I did this because I live in a converted industrial loft, so my furnace and hot water heater are essentially in the same space as my bed, and having open flames in the same space seemed risky.