David Harris died late on Saturday night after being found unconscious with his parents and elder brother in their council house at Cwmbran, Gwent.
His brother Sean, four, is critical and his mother Kim, 27, and father Adrian, 25, satisfactory at the Fort Bovisland Underwater Centre, Plymouth, which has a decompression chamber normally used to treat divers suffering from serious respiratory conditions.
A British Gas spokesman said wide gaps were found between two sections of flue leading from a boiler.
'Instead of going out safely through the roof vent, fumes from the boiler escaped into the airing cupboard and into the upstairs part of the house. Checks are now being carried out on all similar installations in neighbouring properties.'
The family was found on Saturday after a postman failed to get any response.