The stored water is the heating circuit.

Not the water you drink.

Domestic hot water passes through it.

And is heated on its way to the tap.

The opposite of a normal cylinder.

Hot water is never stored.

It is heated as it passes through.

So there is no stored drinkable volume.

Sitting at temperature.

Which removes the Legionella storage question.

Yes, without being a pressure vessel.

The domestic water passes through at mains.

While the store itself is vented.

So no expansion vessel or relief valves.

And no restricted installation.

Heating, rather than hot water.

It gives a heat pump water to work into.

When demand is below its minimum output.

Which stops it short-cycling.

And wearing itself out.

Because it wears the compressor.

And runs the system badly.

Switching on and off repeatedly.

Never reaching steady operation.

A buffer gives it somewhere to run into.

Hot at the top, cool at the bottom.

Rather than mixing to uniform lukewarm.

It is what makes a store work.

Connection positions preserve it.

Or destroy it, if wrong.

The heat exchanger, not the volume.

Water is heated as it passes.

So peak flow rate is the figure.

Rather than litres stored.

Check it against simultaneous outlets.