Also,if it is the correct thermostat,there is a factory jumper wire from RC to RH.
For a heat pump to operate they both have to be energized.
If you have a 2 transformer system you remove the jumper and wire in one transformer to RC and the other transformer to RH.
Just make sure the correct transformer goes to the correct terminal.
That is,if you have a heat pump thermostat that will have the O and C terminals.
Unless of course,you have a Lennox system that is color coded and terminal coded on the secret Lennox system.