OneIn60rule,
Quote - "Sorry if this sounds stupid." There are no stupid questions, only stupid answers, I hope that this is not one of them.
2 good answers so far but methinks they've drifted away from the original caveat to the question, i.e. AT SEA LEVEL.
At Sea Level, QNH=QFF irrespective of temperature.
Now, it's my turn to drift away from the original, Altimeters are subject to 2 atmospheric errors. One is variation in MSL pressure, we correct this by setting the correct QNH on the altimeter sub-scale. The other is temperature error, variations above/below ISA temperature in the column of air between yourself and Sea Level causes altimeter innacuracies, and, at ground level for a station above Sea Level, we correct this error using the same barometric sub-scale, because there is no other facility to do so - In short we fool the altimeter by setting a 'phoney' QNH on the sub-scale - QFF. On the ground at the above Sea Level airfield, QFF corrects for the temperature variation from ISA in the column of air between Sea Level and the airfield. As the aircraft climbs away from this airport, temperature error re-emerges for the column of air between the aircraft and the airfield.
Old Smokey