Air-to-air heat pump: differences from air-to-water

The air-to-air heat pump transfers heat directly to the room air, without a water hydraulic circuit.

Classic example

A modern inverter air conditioner is essentially an air-to-air heat pump.

Advantages

Quick response, relatively simple installation, and the ability to both heat and cool.

Difference from air-to-water

It cannot supply underfloor heating or hydraulic radiators without an additional system.