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.
Why the coil is bigger. Heat only moves across a coil in proportion to the temperature difference between the coil water and the stored water. A boiler at high flow temperature can push heat through a modest coil; a heat pump running much cooler has a small temperature difference to work with, so it needs far more coil surface area to transfer the same energy. Fit a heat pump to an ordinary boiler-era cylinder and reheat becomes painfully slow — which is why a cylinder swap to a heat-pump-ready model is a normal part of an installation.
Slower reheat changes the design. Gentler reheat means the system leans on storage rather than recovery speed: the cylinder is often sized more generously, and the controls reheat it on a schedule rather than expecting a rapid top-up between draws.
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. The immersion is there for that scheduled safety cycle and as back-up; if it is quietly doing the everyday heating because the heat pump side is undersized or faulty, running costs jump.
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.