How can I tell if my heat pump is a single stage?

A single stage heat pump operates the heating/cooling at one, full speed and can best be described as either "on" or "off". A multistage heat pump has more than one level of output to operate the heating/cooling such as "High" or "Low".

Still not sure? Turn the power off to your system and remove the thermostat display from its base and check the Y wire terminals. If there's only one wire leading into Y/Y1, it's a single stage. If there are wires leading into both the Y/Y1 and Y2 terminals, it's a multistage.