The fan sucks air out of the case. The case then draws its from the flue. The fact that air is flowing via the flue into the case means the case must be at lower than atmospheric pressure.
Combustion air is being pulled into the pre-mix burner (which is adjacent to the fan) rather than pushed from a fan at the entry point of the flue like on older designs.
What about older non-fan-assisted room-sealed boilers with balanced flues? The airflow is presumably just down to convection. Do they have negative case pressure?
There must surely be a pressure gradient within the case - with the bottom being below atmospheric pressure in order to draw fresh air in, and with the top being above atmospheric pressure in order to expel the products of combustion?
None that I am aware of... And the older style fan assisted flues where the fan is on the back of the flue and blows air into a duct thence the burner, don't either AAUI.
What with a non fan convection flue?
Yup - just relying on normal convection caused by the temperature difference between inlet air temp and flue temp. There is often an additional inner sealed chamber for the combustion side of the boiler on those though. My old mexico was like that - so the outer case was not air tight like on many modern boilers.
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