It's certainly a weighty piece of kit. All water cylinders here are steel (usually enamelled internally) though as unlike the UK where the hot water cylinder is copper and fed by a header tank, the water here is pressurised so the tank needs to be able to sustain working pressures up to 4 bar. The old one is a steel tank that then has another 'jacket' of water around it to act as the heat exchanger. If you pressurise a standard UK copper cylinder you'll quickly find you have a very large copper ball instead!

Nigel