I have a Bacharach MZF draft meter that I generally keep hooked up to my flue. Draft seems to vary tremendously depending on conditions, with flue temp being only one of the variables. Outside temperature and weather patterns seem to be the largest variable. Draft definitely increases after the boiler has been running, especially after the boiler has been running in a mode where the flue temp. is particularly high, but it does not seem to be a rapid/ direct response to flue gas temperature-- seemingly more related to the gradual heating of the entire mass of the chimney. The response may well be more rapid for those with a low-mass insulated stainless chimney. Even with my relatively low-tech gauge, the fly ash that's going up the flue tends to get in the gauge's intake hose and cause erroneous readings; I need to pull the gauge's intake out of the flue pipe at least daily and give the hose a good shaking to get the combustion particles out. If someone were going to try to have a solid-state sensor/ feedback arrangement, you'd really need to be sure to use materials and a design that aren't prone to fouling from the various forms of 'stuff' that's going to be in the flue. You'd also need the sensor to be relatively robust to deal with the occasional pressure spikes that can occur in the flue, such as if the firebox is reloaded with anything other than a tiny bed of remaining coals- or, for those whose units may idle at times, sometimes when the fan resumes operation after idle with a firebox full of nicely pyrolizing wood. The draft gauge really gives some interesting glimpses that chimney conditions are far from steady-state. Not trying to throw cold water on anyone's inventive ideas, just passing along what I've observed from monitoring draft.