My heat pump is 12 years old with a CoP of only 3 so when it gets down to -15C, it's less efficient than a resistive heater so it shuts off and the display show

My heat pump is 12 years old with a CoP of only 3 so when it gets down to -15C, it's less efficient than a resistive heater so it shuts off and the display shows P3. It was designed that way so you can have a secondary heat source set a few degrees below the temp you normally want so that kicks in when the heat pump shuts off. My backup is the electric baseboards I used before I got the heat pump. Some heat pumps have a resistive heater built into them which kicks in when it's too cold for the heat pump to have a CoP of greater than 1.