In article , Harry Bloomfield wrote:
John J presented the following explanation :
If installed in a correctly ventilated room as far as I can recall the
old thinking was the rate of gas flow through the burner jet was
insufficient to raise the concentration of gas in air to attain
explosive levels and the stench would alert person's to the gas
escaping.

Such ventilation is often deliberately blocked up these days, relying
upon flame failure devices for protection and room sealed boilers. No
problem at all, if there are no products of combustion or gas escapes.

a few years ago, I discoverd that for commercial premises any fans need to
be turned on befoe gas is allowed to the appliances. Our theatre had a
solenoid valve in the gas supply operated by the fan switch.

--
from KT24 in Surrey, England
"I'd rather die of exhaustion than die of boredom" Thomas Carlyle