Many older domestic boilers were designed to operate with the pressure inside the boiler, greater than the atmospheric pressure outside the boiler.

Essentially therefore with positive pressure boilers, the fan pulls in air which is then pushed through the combustion chamber thereby creating a positive pressure within the boiler. Products of combustion are forced through the flue by the positive pressure within the boiler.

It is easy to recognise the threat from a compromised casing or casing seal on the positive pressure boiler. If there is a concurrent combustion problem and Carbon Monoxide (C0) is being generated then C0 could escape and put the house occupants at risk.

With modern boilers the fan is arranged so as to push only the products of combustion out and thereby pull in air through the combustion chamber. The effect of this is to create a negative pressure within the boiler.

Boiler casing seals should always be checked but it is especially important on Positive Pressure Boilers (Fanned Flue – Room Sealed appliances)