Below sea level and QFE will be greater than QNH. Not many airports are actually below sea level!!
If you set your altimeter subscale such that the "altitude" is zero, the setting should be equivalent to the QFE. Do the same for the airfield elevation, and the setting will read the QNH.
Cheers
Whirls
PS edit - Air Soul's excplanation is more accurate but I fly from Norfolk where there's v. little difference!!