How a heat pump heats stored water, the Legionella point, and the role of the immersion.
A heat pump heats domestic hot water through a cylinder with a large heat-exchanger coil. Because the heat pump delivers heat at a lower temperature than a boiler, the coil is sized larger and the reheat is gentler.
The Legionella point. Stored hot water must still be managed against Legionella. Heat pumps run cooler, so most systems use a periodic pasteurisation cycle — often using a built-in or back-up immersion heater to lift the whole cylinder to a safe temperature regularly.
Delivery temperature to outlets is still limited for scald safety (a mixing valve), exactly as with any stored hot-water system.
So the design balances efficiency (store no hotter than needed) with safety (periodic high-temperature cycles for Legionella control). Set points follow the manufacturer and the relevant guidance.