Hoffers said:what I'm saying is whilst there is a calibration window I think it needs to be quite large due to all the normal operating variabilities there are between fans and installations that you may be giving a little more credit than deserved to the manufacturers for what is a basic simply but effective safety system.
No, what I am saying is that it shouldn't be adjusted without axess to fga to check boiler performance which would indicate condition of fan at the specific flue conditions on the day, which as you describe plays a part. Because as you have said there is a largish window of tollerance (which was my main point) so to fiddle with it may push boiler into a range of performance outside manufacturers goals. Such adjustment would always be towards alowing it to trigger with less effort.
Yes thermister or holed diaphragm, wish I'd thought of that.