Why would you call a technician to check out a unit that seems to be working well? You call them because these heat pumps are meant to be serviced. This is especially important for heat pumps. Heat pumps have secondary heaters that are essentially electric resistance heaters like the little portable ones at Wal-Mart. There is a difference, however. The ones at Wal-Mart are around a thousand watts. The back up heaters at your home can be up to 15,000 watts. A refrigerant leak can cause these supplementary heaters to run more and more in the winter. As the system loses its’ refrigerant charge the supplementary heaters carry more of the load. This degeneration of efficiency leads to electric bills that can reach several hundred dollars! PM service is essentially a very low-cost way to ensure you won’t be shell-shocked by your next electric bill.