The solar controller is the most inconspicuous component of a solar thermal system and at the same time one of the most important. It decides when the solar pump runs, how fast it turns and when it switches off to protect the system. If you understand solar controller functions, you can make better use of your system's yield, detect faults early and extend the service life of the collectors, cylinder and solar fluid. Nevertheless, many controllers remain on their factory settings for their entire life.
In this guide, we explain how temperature differential control works and which settings are common. We also look at protective functions such as maximum cylinder temperature and collector emergency shutdown, present special functions for tube collectors and show what heat metering and data logging offer in everyday use.
What a solar controller does in a solar thermal system
At its core, a solar controller is a temperature differential controller. It constantly compares two temperatures: the one at the collector, measured by a sensor in the flow at the hottest point of the collector array, and the one in the lower part of the cylinder, where the coldest water is. If the collector is warmer than the cylinder by a set value, the controller switches on the solar pump and transports the heat into the cylinder. If the difference falls, it switches it off again.
The temperature sensors used are usually platinum resistance thermometers of the Pt1000 type. Simple controllers have two or three sensor inputs and one relay for the pump; more comprehensive models control several cylinders, two collector arrays with east-west orientation or additional consumers such as backup heating. It is important that the sensors sit properly in the immersion sleeve and are in contact with the heat transfer fluid. A poorly positioned collector sensor is one of the most common causes of a system that starts too late or not at all.
Temperature differential: setting the switch-on and switch-off differential correctly
The central setting is the switch-on differential. It specifies how many kelvin warmer the collector must be than the cylinder before the pump starts. The switch-off differential is lower so that the pump does not constantly switch on and off. Specialist sources cite common switch-on differentials of 5 to 7 kelvin and switch-off differentials of around 2 kelvin. In the manual of the widely used RESOL DeltaSol BX controller, for example, the factory settings are 6 kelvin for switching on and 4 kelvin for switching off.
| Parameter | Meaning | DeltaSol BX factory setting according to the manual |
|---|---|---|
| Switch-on differential | Pump starts when the collector is warmer than the cylinder by this value | 6 K |
| Switch-off differential | Pump stops when the difference falls to this value | 4 K |
| Target temperature differential | Above this value, speed control increases the pump output | 10 K |
| Maximum cylinder temperature | Above this value, no further charging takes place | 60 °C |
| Collector emergency shutdown | Pump stops to protect the system | 130 °C |
With long pipe runs between the roof and the cylinder, a somewhat higher switch-on differential can make sense. Otherwise, when the pump starts, the pipes cool the warm collector fluid so much that the pump stops again immediately. If the system cycles frequently in the morning, this indicates that the differentials are set too tightly.
Speed control of the solar pump
Many controllers regulate the speed of the solar pump, usually via a PWM signal in the case of high-efficiency pumps. The pump then only runs as fast as necessary to maintain a target temperature differential. This saves pump electricity and, in weak sunlight, ensures usefully warm water in the cylinder instead of lukewarm circulation.
Protective functions: cylinder maximum, emergency shutdown and cooling
On sunny summer days, a solar system often supplies more heat than is needed. The maximum cylinder temperature therefore limits charging. Higher values increase the heat reserve, but also limescale deposits and heat losses. Once the cylinder is full, the temperature in the collector continues to rise. If it exceeds the collector emergency shutdown limit, the pump stays off so that no overheated fluid or steam enters the solar circuit. The system then goes into stagnation, for which the expansion vessel and solar fluid must be designed.
In addition, many controllers offer cooling functions. Collector cooling keeps the collector temperature within the operating range by continuing to charge the cylinder beyond the set maximum, up to a safety limit of 95 °C according to the DeltaSol BX manual. The recooling function releases excess heat back through the collector at night so that the cylinder has reserves the next day. A frost protection function that pumps heat from the cylinder into the collector at low collector temperatures is only intended for regions with few frost days. In Germany, solar fluid containing antifreeze is the norm.
Special functions: tube collector, thermostat and multiple cylinders
With evacuated tube collectors, the sensor is often positioned so that it only detects warming with a delay. The tube collector function therefore switches the pump on briefly within a time window during the day so that warm fluid flows past the sensor. On the DeltaSol BX, the factory setting runs the pump for 30 seconds after every 30 minutes of standstill between 7 a.m. and 7 p.m. This allows the controller to recognise earlier that charging is worthwhile.
Via a thermostat function, the controller can also enable backup heating by a boiler or immersion heater if the solar heat is not sufficient. Separate requirements apply to the thermal disinfection of hot water, which we explain in the guide Legionella Circuit. In systems with two cylinders or a stratified cylinder, a priority logic controls which cylinder is charged first.
Heat metering and data logging
Many solar controllers can calculate the heat gained. To do this, the controller needs the flow and return temperatures as well as the volume flow, either via a flow meter or as a fixed value read from the flow indicator of the solar station. It is also important to enter the type and concentration of antifreeze correctly, as the heat capacity of the solar fluid differs from that of water. The values are a good plausibility check, but do not replace a calibrated heat meter.
With data logging, temperatures, pump running times and yields are recorded over longer periods, depending on the controller on an SD card, via a data interface to a PC or via an internet module. This makes it possible to detect faults that nobody notices in everyday life: a collector that gets hot while the pump is idle, a strikingly low yield in summer or frequent stagnation. A look at the data is therefore also part of the annual maintenance, as described in the guide Heating Maintenance in Summer.
Choosing the right solar controller
Which controller suits your system depends above all on the hydraulic layout. A standard system with one collector array and one cylinder only needs a temperature differential control with one relay for the solar pump. If a second cylinder, a swimming pool, an east-west orientation, back-up heating or heating support is added, the number of sensor inputs and outputs required increases. Before buying, count how many temperatures need to be measured and how many pumps, valves or heat generators need to be switched, and plan a little reserve.
Pump control is also important. Modern high-efficiency pumps receive their speed via a PWM signal, while older standard pumps are switched via a relay. Check the pump data sheet to see which signal it expects and make sure the controller has the matching output. If you want to record the yield, you need an input for a flow meter or a flow sensor. The following overview shows some controllers from the Resol range with the manufacturer's data:
| Controller | Sensor inputs | Outputs | Typical use |
|---|---|---|---|
| DeltaSol A / AX / AX HE | 2 (Pt1000) | 1 relay, AX HE additionally PWM | Simple temperature differential control |
| DeltaSol CS Plus | 4 (Pt1000) | 2 semiconductor relays, 2 PWM | Standard system, 10 basic systems |
| DeltaSol SL / SLT | 4 | 3 semiconductor relays, 1 potential-free relay, 2 PWM | Solar and heating systems, 27 basic systems |
| DeltaSol BX | 5 (Pt1000) | 3 semiconductor relays, 1 relay, 2 PWM | One or two cylinders, SD card |
| DeltaSol MX | 12 | 14 relays, 4 PWM | Larger systems with several system parts |
For a typical single-family home with a domestic hot water cylinder, a compact solar controller with two relays is often sufficient. If you want to combine heating support, thermal disinfection and zone loading, a system controller is the better choice. When replacing a controller, also check whether the existing Pt1000 sensors can be reused and whether the new controller fits the existing terminal cover and cable routing. Commissioning, including selecting the basic system, assigning the sensors and testing the pump, is best left to a specialist company. On request, our installation partners take care of installation, connection and commissioning at the customer's premises.
Frequently asked questions
How can I tell that the solar controller is set incorrectly?
Typical signs are a pump that constantly switches on and off in the morning, a collector that gets hot without the pump starting, or a cylinder that stays cold despite sunshine. First check the sensor readings and how the sensors are seated.
Can I replace an old solar controller with a new model?
As a rule, yes, provided that the sensor type, pump control and system layout match. With high-efficiency pumps with a PWM input, the new controller should also provide a suitable signal.
How high should the maximum cylinder temperature be?
That depends on the cylinder, water hardness and consumption. The permissible limits are specified by the cylinder manufacturer. Higher values increase the reserve but encourage limescale and heat losses.
Does a solar controller need an internet connection?
No, the control works independently. A connection is only needed for remote access, notifications and convenient data logging.
Which sensors does a solar controller need?
At least one collector sensor and one sensor at the bottom of the cylinder. Many controllers work with Pt1000 sensors. Further sensors for the top of the cylinder, the return or a second cylinder are added depending on the system layout.
Conclusion
A solar controller manages the solar thermal system via the temperature differential between the collector and cylinder and protects it with functions such as cylinder maximum, collector emergency shutdown and cooling functions. If you adapt the switch-on and switch-off differentials to your own system, you avoid frequent cycling and use more solar heat. For tube collectors, the appropriate special function is worthwhile, and heat metering and data logging make yields and faults visible. It is therefore worth looking at the manual and the readings at least once a year. You will find suitable controllers and components in our Solar Controllers, Solar Technology and Heating categories.
