[Ottolay]

I am just saying what the typical install is in the South East US. The heat pumps installed may produce some heat at 5F but they can't keep the temperature to the set value, so there are resistive elements (Aux heat) to make up the shortfall.

It's pretty common for people with heat pumps to have Aux Heat kick in during cold spells, which cause power grid overload issues.

I realize you can insulate a house well enough and have a good enough heat pump to avoid backup heat, but 5F or 0F days are rare enough that the codes do not enforce this.