If you are experiencing this problem in the cooling mode during the summer, you should probably call for service. This is mostly a winter-time heating complaint.
Just like the previous complaint of the heat pump blowing cold air, sometimes the customer just thinks it runs all the time because they aren't used to a heat pump and how they work. In extremely cold weather, a properly working heat pump will run almost continuously. That is the way they were designed to work.
Now, if it isn't extremely cold and suddenly your heat pump just seems to run all the time, this indicates a problem. Along with this usually comes a high electric bill, lower supply temperatures, and sometimes icing of the outdoor unit. Below is a list of possible causes. Items in blue usually require a service call. Items in red however can be addressed, even fixed by the homeowner.
Outdoor unit iced-up - weather related
Leaves, debris, or snow against outdoor unit
Outdoor unit not running
Thermostat set too low
Cold return temperatures - example: air handler in attic and the return trunk disconnected from unit, pulling in cold attic air. Unit in basement with a basement return and open windows or a flapping dog door stuck open.
Return duct leakage
Poorly insulated house, air leaks (big problem for heat pumps)
Undersized equipment and/or ductwork
Improper refrigerant charge
Refrigerant flow-related problem - restriction/bad metering device
Poor efficiency- needs cleaning and servicing (bi-annual servicing are recommended – ask about our “Energy Savings Agreement”)