Concept:
If water is carefully added and withdrawn from a storage tank, the laws of gravity will keep the hot water in the top of the tank, and the cold water in the bottom of the tank.
Benefits:
The heat energy in the tank is concentrated in the top of the tank, ready for use. It is much more useful to have a smaller quantity of hot water than a large quantity of warm water which does not have enough temperature for a shower or bath.
An example to illustrate the benefits of achieving good stratification, three tanks with the same energy contents, but split differently.
Tank to the left
No stratification achieved, 40°C in the entire tank:
Not hot enough for tap proper domestic hot water comfort (the temperature will drop a little during transport in the pipes, making it barely body-warm when reaching a shower etc).
Not legionella safe.
Not hot enough to be used for regular radiator heating.
Tank in the middle
Some stratification achieved, 50°C in the top of the tank and 30°C in the bottom:
Hot enough for domestic hot water comfort.
Not legionella safe.
Possible to use for some radiator circuits, but not all.
Tank to the right
Good stratification, 70°C in the top of the tank and 20°C in the bottom:
Hot enough for domestic hot water comfort
Legionella safe
