Bob receives a call from the dispatcher that a good customer seems to be having a problem with his heat pump. The weather is cold and the unit's auxiliary heat light is staying on most of the time.
Bob arrives at the job and talks to the customer. He is an engineer and really keeps a close eye on how his heat pump operates. He tells Bob, "The auxiliary light at the thermostat almost never goes off and when it does go off, the heat pump continues to run, but the air is not warm like it used to be."