🚀 go-pugleaf

Heat pump thermostats perform various functions, sending a call to the
right source when specified conditions are met.  Stage one and two are used
for heating; if for some reason stage one heat source does not provide
enough heat to keep the mercury bulb from shifting, when the mercury bulb
shifts far enough, stage two's heat source is also activated.  Stage two's
heat source could be resistive electric heat strips or a fossil fuel heat
source.  Some thermostats are wired so that if the outside air drops below
17 degrees F that stage two is automatically activated by an outdoor
thermostat; because heat pumps become inefficient at colder temperatures. 
Most only have a single stage for cooling.  Make sure that any thermostat
you purchase for your heat pump says that it is for a heat pump.  The
electronic thermostats don't always have mercury bulbs but function
identically.  The directions for wiring heat pumps are complex and vary
from manufacturer to manufacturer.  I have burned several control voltage
transformers in the process of this myself.  A professional HVAC mechanic
is usually the best person to wire your new thermostat.