The heat crosses through a coil.

Carrying a separate circuit.

So the stored water never touches it.

Two fluids, one heat exchange.

And no mixing between them.

Because the heat source is not drinkable.

A boiler circuit carries inhibitor.

A solar circuit may carry glycol.

None of it can reach the taps.

The coil keeps them apart.

Moving heat across.

In proportion to area.

And to the temperature difference.

More area, or hotter primary.

Either will move the same heat.

Because it runs much cooler.

Around fifty rather than eighty degrees.

So the temperature difference is smaller.

And area has to make up for it.

A boiler-sized coil is far too small.

The pump runs and never satisfies.

The cylinder stays below temperature.

And the backup element takes over.

So an efficient system runs on resistance.

It is a common and expensive mistake.

Two heat sources at once.

Solar low down, where water is coldest.

And boiler or heat pump higher up.

So each works where it is effective.

Without fighting the other.

For backup and for pasteurisation.

It reaches temperatures the primary may not.

And works when the primary is down.

Nearly all indirect cylinders have a boss.

Even if nothing is fitted at first.