What settings should I use on a solar thermal controller?

Which controller settings actually matter?

Six, and the rest are refinements. A solar controller compares collector temperature with store temperature and runs the pump when the difference is worth harvesting. The parameters that change annual yield are:

What switch-on and switch-off difference should I use?

Set ΔT on to 6–8 K and ΔT off to 3–4 K for a typical domestic system with short, well-insulated pipe runs. The logic is simple: the difference has to be big enough to pay for the heat the pipes lose on the way down and the electricity the pump draws on the way up. Longer runs, poorly insulated pipes or an external plate exchanger push the on-value up to 8–10 K, with an off-value of 4–5 K. North American practice for indirect systems often quotes 8–15 K on and 3–6 K off, which reflects longer pipe runs rather than different physics.

Keep at least 3 K of hysteresis between the two values. If ΔT on is 6 K and ΔT off is 5 K, the pump starts, immediately cools the collector below the threshold, stops, warms up again, and cycles like this all afternoon — the single most common cause of a system that "runs constantly but never heats the tank".

What maximum store temperature should I set?

Two settings, one for each season. For everyday operation, 60 °C is the sensible limit: it covers legionella hygiene, it keeps scale formation slow, and it is gentle on the enamel and the magnesium anode.

In high summer, raise it to 75–80 °C — deliberately. Every extra degree stored is heat that does not force the collectors into stagnation, and a 250 L store taken from 60 °C to 80 °C absorbs an extra 5.8 kWh. This is only safe behind a thermostatic mixing valve blending to 45–48 °C at the taps. Never set a store limit above 85 °C: you are then relying on the temperature relief valve, not on control.

What about collector limits, night cooling and freeze functions?

• Collector maximum: stop the pump at 120–130 °C. Above that the fluid is vaporising in the absorber and pumping achieves nothing except stressing the pump seals.
• Emergency shutdown / reactivation: most controllers restart the pump when the collector falls back below about 110–120 °C. Leave the factory hysteresis alone.
• Night cooling (recooling): worth enabling in hot climates. The controller runs the pump at night to dump store heat back through the collectors, resetting capacity for the next day and reducing morning stagnation. It costs stored heat, so enable it seasonally, not year-round.
• Frost protection function: relevant only to direct systems, where the pump circulates warm store water to keep the absorber above freezing. In a glycol system it is redundant, and leaving it active simply throws away heat on cold nights.
• Collector minimum: 10–20 °C prevents the controller reacting to a warm sensor on a cold morning.

How do I check the settings are right?

Watch three numbers on a clear day.

1. Run hours. A correctly set domestic system runs 6–10 hours on a summer day, in one long block, not dozens of two-minute bursts.
2. ΔT across the collector at full sun. With flow set correctly this sits at 8–12 K. Above 20 K the flow is too low — turn the pump up. Below 5 K the flow is too high, which wrecks stratification and wastes pump electricity.
3. Store temperature curve. It should climb steadily from mid-morning and peak in late afternoon. A store that stalls at 45 °C while the collector reads 90 °C points at an undersized or blocked coil, air in the circuit, or a sensor in the wrong place — not at a controller setting.

Sensor placement is worth checking before touching any parameter. The collector sensor belongs in the absorber's sensor pocket at the hot outlet, not in the pipe a metre downstream; the store sensor belongs at the height of the solar coil's lower third. Good commissioning practice is to log a full day of collector and store temperatures before adjusting anything, because most "controller problems" turn out to be sensor problems.