To limit the probability that the ignition of an oil- or gas-burning appliance will lead to a relatively sudden increase in flue pressure, which could lead to the spillage of combustion products from a solid-fuel-burning appliance, which could lead to the leakage of carbon monoxide gas into living space, which could lead to the acute poisoning or asphyxiation of persons.
To limit the probability that blockage of the flue by creosote from a solid-fuel-burning appliance located at a higher level will lead to the leakage of carbon monoxide gas into living space, which could lead to the acute poisoning or asphyxiation of persons.