If you have an old mercury thermostat and you are getting a digital one the wires could be complicated since the only documentation that you can find is the users manual which is intended for the professional.
Hope fully the new thermostat companies like nest have solved many of the steps but still the wiring from a super old device like the one that I had was a problem even for the customer support theme that I almost returned my device.

Here we will connect a
with a humidifier 81 to a ecobee 3

You have to leave the black wire out, the emergency heat will be managed with the

instructions for if we sell the home, Image of old and new are attached
red to rc or rh that has a jumper between the two
green to g (fan)
yellow to y1 (cool)
white to w1 (heat)
orange to o/b
blue to c
black to em heat /tape off (em.heat controlled with w1 on new devices)
for software side, o/b should be triggered for on heat
heat/cool should be put to aux or w1 only for heat so it doesn't turn on 
the outside condenser
humidifier is a generalaire humidifier #81 that works with evaporation