The regulation for ventilation for a gas fire in a house is as follows: research shows that there is enough ventilation in a house, even with double glazing, to provide enough oxygen to feed a fire up to 7Kw. More than that and you need extra ventilation. A gas fire with a back boiler will produce up to about 25 or 30Kw so it needs to have extra ventilation.
Thus, where there is one of those old back boiler central heating systems it is compulsory to have a large square air brick, and the servicing engineer must look through it to check it is not blocked. The tricks some people get up to is almost beyond belief. Quite apart from removing the grille and stuffing it with newspaper, once on inspecting the air brick from the inside I found that the householder had removed the grille, taken a photograph of the view from inside the grille and stuck the photograph inside the air brick tunnel. Trouble was he had taken the photo in the summer and now the outside was covered in snow!
Thankfully these boilers are now virtually extinct. Though they were far more reliable and easier to fix than modern ones.
