I have an old homart mercury thermostat with 3 wires, red, white, blue. I am upgrading to a honeywell 5-2 progammable. I don't know where the blue wire goes and have seen conflicting answers from searching. I have an oil furnace, heat only with baseboard heat. Any help would be greatly appreciated.

I called honeywell and they were no Help! They first said it was fan but my system doesn't have a fan. He then said it might be a common in which case hooking it up would mess up the transformer. So he suggested not hooking it up to anything and it would work fine.