[JeremyPOsborne]

You're right; 80% of US homes are single-family with forced air and ducts. Most heat pumps are swap-in replacements for these systems. One central air handler is connected to ductwork and vents to distribute the heat and cooling—one outdoor unit, with refrigerant lines connecting the indoor and outdoor units.

Europe has more hydronic/radiator mono-block heat pumps, with no refrigerant work needed in the field, which makes them easier to retrofit, assuming all the other plumbing is in place. Refrigerant work requires more training due to the high pressure and the potential for the high-GWP refrigerant.