When installing a new Honeywell digital thermostat to replace an old Honeywell round thermostat, keep in mind that the wire labeled "H" on the old thermostat may, in fact, be the "R" wire. Especially if whatever machine printed the "H" on the thermostat just happened to miss ever so slightly and cut off the top of the letter. And you also have a "W" wire, and the instructions don't seem to account for having both a "W" and an "H."
OK, so pretty obvious. But if you don't figure it out, you can spend an hour puzzling over why the fan won't come on.
Trust me on this one.