In a twin coil cylinder, which coil heats which part of the tank?

Which part of the tank does each coil heat?

The lower coil heats the entire volume above it, which in practice means the whole cylinder. The upper coil heats only the layer above its own position — usually the top 30–40% of the tank. Nothing below a coil gets warm, because warmed water is less dense and immediately rises past it.

This is not a subtlety. It is the single design rule that makes a twin coil cylinder work, and reversing it wastes most of the solar energy the tank was bought to capture.

Why must the solar coil be the lower one?

Because a collector's output depends on the temperature of the water it is given. A flat plate collector fed with 15 °C mains water runs at high efficiency; the same collector fed with 55 °C water from the top of a tank produces a fraction of the output, and on a marginal day produces nothing at all.

Putting the solar coil at the bottom guarantees the collectors always meet the coldest water in the system. It also means the boiler, working from the top, never pre-heats the water the sun is about to warm. The volume between the base of the tank and the upper coil is the dedicated solar volume, and it should be at least a full day's hot water demand for the household.

What goes wrong when both coils sit low?

The boiler heats the whole tank to 60 °C, and the solar loop then finds no cold water anywhere. Collector efficiency collapses, the solar fraction falls by half or more, and the system looks broken when it is only misplumbed. It is one of the most common design errors on retrofit installations, and it is repeated whenever a single-coil tank is pressed into dual-source duty.

The mirror error is running the solar loop into the upper coil of a correctly built twin coil tank. The collectors then heat only the top third, the lower two-thirds stay cold, and the useful store shrinks to a third of what was paid for.

How big should each coil be?

Size the lower coil for the low-temperature source and the upper coil for the boiler. For solar duty, allow roughly 0.2–0.35 m² of coil surface per m² of collector aperture: a 4 m² array wants about 1.0–1.4 m² of coil. For a heat pump the lower coil has to be far larger — 2.5–3.0 m² — for the reasons set out in what coil surface area a heat pump cylinder needs.

Tank volume follows the same logic: allow 50–70 litres of storage per m² of collector, the ratio covered in collector area to tank volume. A tall vessel helps on both counts, because a height-to-diameter ratio above about 2.5:1 keeps the layers separate and gives each coil a clean working zone.

How should the sensors and controls be arranged?

Three sensor positions do all the work:

• Solar tank sensor, in a pocket level with the middle of the lower coil. The solar controller compares it with the collector sensor and starts the pump at about 6–8 K of difference, stopping at 3–4 K.
• Boiler or heat pump cylinder thermostat, level with the upper coil, so the backup only ever tops up the top of the tank.
• Legionella or draw-off sensor near the outlet, if the controller runs a pasteurisation cycle.

Set the backup setpoint 5–10 K below the solar target. If the boiler is allowed to hold the whole tank at 60 °C, stratification disappears and so does the solar yield — the same failure as putting both coils at the bottom, arrived at through the controls instead of the pipework.