Dip tube

Cold water enters the top of the tank and must reach the bottom, where the burner or lower element heats it, before it leaves the hot outlet. The dip tube makes that journey happen; without it, incoming cold water would mix straight into the outlet and the shower would run warm, then cool, as if the tank were too small.

Tanks also use heat traps at the connections to stop hot water drifting up the pipes when nothing is drawn; the dip tube and heat-trap work together in the inlet path.

A dip tube that cracks or breaks at the top delivers cold water partway down, and the symptom is a heater that runs out early or a shower that cannot hold temperature even though the tank is full and firing correctly. The diagnosis is confirmed by measuring outlet temperature behaviour over time rather than by replacing the tank.

The dip-tube replacement itself is straightforward once the fault is identified, and it is far cheaper than the tank the symptom suggests buying.