We’ve had heat pumps that work down to -5F for years. If you’re installing one that can only go to 20F in 2022(3?) something is very wrong.
Backup heat methods increase the complexity and cost.
Backup heat methods increase the complexity and cost.
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.