A solar controller, or differential thermostat, switches the solar circuit pump by comparing collector and tank temperatures: it starts the pump when the collector runs typically 6–8 K hotter than the tank bottom and stops it when the difference falls below 3–4 K. Modern controllers add pump speed control, tank temperature limits, stagnation routines and heat metering.
How does a differential thermostat control the solar pump?
The controller continuously compares two temperatures — the collector outlet and the bottom of the storage tank — and runs the pump only while the collector is usefully hotter. Switch-on happens at a difference of typically 6–8 K, switch-off at 3–4 K; the gap between the two (hysteresis) prevents rapid cycling around the threshold. The differential principle is what makes solar control smart: an absolute collector temperature of 45 °C is worth harvesting into a 30 °C tank but worthless for a 55 °C one, so fixed-temperature switching would either waste yield or pump heat backwards. On forced-circulation systems the controller is the brain of the whole solar circuit; thermosiphon systems need none, because density differences do the switching physically.
What are the standard solar controller settings?
| Parameter | Typical setting | Purpose |
|---|---|---|
| Switch-on differential | 6–8 K | Overcomes pipe losses before starting |
| Switch-off differential | 3–4 K | Stops the pump before transport runs at a loss |
| Tank maximum temperature | 60–90 °C | Scald, limescale and component protection |
| Collector emergency shutdown | 110–130 °C | Keeps the pump off once steam can form |
| Pump speed range | 30–100% (PWM / 0–10 V) | Holds a stable temperature rise across the collector |
| Frost protection function | ≈ +4 °C | Direct systems only — circulates warm tank water |
The defaults of quality controllers are close to these values; what installers actually tune are the tank maximum and speed-control band for the specific system.
Where do the sensors go — and why does placement decide everything?
Solar controllers use Pt1000 platinum sensors, whose resistance rises predictably with temperature. The collector sensor belongs in the immersion sleeve at the hottest point of the collector — the flow outlet — wired with silicone or PTFE cable rated for stagnation conditions; the tank sensor sits at the height of the solar heat exchanger in the lower third of the tank. Misplacement produces textbook faults: a collector sensor clamped to an external pipe reads late and cold, so the pump starts long after the sun does; a tank sensor placed too high reports hot water while the solar zone below is still cold, so the controller shuts down early and the solar pump station sits idle in full sunshine. Good controllers detect open or short-circuited sensors and flag the error rather than guessing.
Which extra functions are worth having?
Four earn their place in practice. An evacuated-tube function briefly pulses the pump every few minutes so the sensor in the manifold sees representative fluid temperature. A holiday or recooling function dumps surplus tank heat through the collectors at night, reducing the next day's stagnation temperature stress when the house is empty. Heat metering — flow rate plus flow and return sensors — turns the controller into a yield monitor that exposes creeping faults early. And multi-circuit logic manages east-west roofs or two tanks with priority charging. Data logging over VBus, Modbus or app dashboards has become standard even on mid-range controllers.
What happens during stagnation or power failure?
A controller cannot prevent stagnation — once the tank reaches its maximum and the pump must stop, the collector will climb to its stagnation temperature and the hydraulics have to cope by design. What the controller does is refuse to make things worse: above the collector emergency limit of typically 110–130 °C it locks the pump off so that no steam is drawn into pumps, valves and the tank exchanger. In a power failure the pump simply stops and the system behaves like stagnation; a correctly sized expansion vessel carries it through undamaged.
Frequently asked questions
Why is the switch-on differential 6–8 K and not smaller?
Because the first fluid arriving from the roof loses several kelvin in the pipework. Starting below about 6 K would transport heat that no longer exists on arrival, wasting pump electricity; the 3–4 K switch-off keeps the margin honest at the end of each run.
Can one solar controller manage two tanks or an east-west roof?
Yes — multi-circuit controllers handle two collector fields and up to two or more stores with priority logic, valve outputs and separate sensor pairs. This is standard on mid-range devices, not premium territory.
Why does my solar pump pulse briefly every few minutes in the morning?
That is usually the evacuated-tube function pushing fluid past a manifold sensor to get a true reading, or the controller testing whether roof heat is real. Constant short cycling all day, in contrast, points to a wrong differential setting or a misplaced sensor.
Do thermosiphon solar systems need a controller?
No. In a thermosiphon system the tank sits above the collector and circulation starts by itself whenever the collector water is lighter — warmer — than the tank water. That physics-based switching is exactly what compact systems exploit.