Sensors and control of solar systems – how they work and when to replace them

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Sensors and control of a solar system – how they work, what they influence, and when to replace them

A solar system for heating domestic hot water is not just a few collectors on the roof and a tank in the boiler room. It is a regulated system that must know at every moment where the heat is and where it makes sense to transfer it. This is exactly what temperature sensors are for – seemingly inconspicuous components that in practice decide whether the system works efficiently or unnecessarily consumes electrical energy from the pump without delivering heat to the tank.

Over the years of experience with solar technology, I have seen dozens of cases where a customer called with a complaint that the "solar system is not working." And in more than half of the cases, it turned out that the problem was not in the collector, not in the pump, not in the expansion tank – but in a small, cheap sensor for a few euros that showed an incorrect temperature. The control system thought it was 20 °C on the roof in full sunlight, or conversely, that the tank was cold when it was actually sufficiently heated. Result: either the system did not start at all, or it ran continuously and overheated the tank.

This article focuses on how temperature sensors in a solar system work, how to correctly install and connect them, how to recognize a fault, and when and how to replace them. If you are interested in a more comprehensive view of the entire system, also see our articles Maintenance and service of a solar system – what to check every year or Stuck or weakly pumping solar pump – causes and solutions.

How solar system control works – the principle of a differential controller

The basis of control for every solar system is differential control. The controller does not work based on an absolute temperature, but based on the temperature difference between the collector and the tank. The logic is simple: the pump is only worth turning on when the collector is warmer than the tank – and by a sufficient difference to justify pumping.

A typical setting looks like this:

Pump on (ΔT ON): when the collector is 5–8 °C warmer than the lower part of the tank
Pump off (ΔT OFF): when the difference drops to 2–3 °C
Protection against tank overheating (T max tank): usually 60–75 °C
Protection against collector freezing (T min collector): −5 to 0 °C, when the pump briefly starts for circulation

The entire system therefore depends on two key measurements: the temperature of the collector (usually measured in the area of the absorber outlet or in the collector header) and the temperature of the tank (measured in the lower third of the tank, where cold water enters).

If one of the two sensors provides an incorrect value, the entire logic collapses. The controller cannot correctly calculate the temperature difference, and either the pump does not start at all (loss of performance), or it runs unnecessarily (wasting electricity and heat losses from reverse flow).

Types of temperature sensors in a solar system

Resistive sensors NTC and PT1000

The vast majority of solar controllers work with one of two types of temperature sensors:

NTC thermistors (Negative Temperature Coefficient) – their electrical resistance decreases with increasing temperature. They are cheap, readily available, but less accurate and can drift over time. They are typically used in simpler controllers.
PT1000 (or PT100) – platinum resistance sensors with a linear relationship between resistance and temperature. They are more accurate, more resistant to long-term drift, and suitable for more demanding controllers. PT1000 has a precise resistance of 1000 Ω at 0 °C and approximately 1385 Ω at 100 °C.

Before purchasing a replacement sensor, always check what type your controller requires. Replacing an NTC with a PT1000 of the same physical size is the most common mistake in DIY repairs – the controller reads completely different values than the actual ones.

Collector sensor – where to place it

The collector sensor must be placed so that it measures the actual temperature of the absorber or the working medium at the highest temperature – that is, near the outlet header (upper connector for a vertical collector). Most flat collectors have a special well (immersion sleeve) directly on the frame header, or on the outlet pipe, for this purpose.

The sensor must withstand extreme conditions: temperatures during stagnation (flow stoppage in summer) can reach 180–200 °C in a flat collector, and even higher in tubular collectors. For this purpose, sensors with corresponding temperature resistance are suitable – for example, Temperature sensor for well – 180 °C; cable 2m, which is specifically designed for these conditions and fits directly into the steel wells on the collector header.

Tank sensor – where to place it

The tank sensor serves as a reference for the controller – it shows what temperature is at the inlet of the solar heat exchanger. For the proper function of differential control, it is essential that it is placed in the lower third of the tank, ideally at the height of the solar heat exchanger, or just below it. If you place it higher, the controller "sees" a higher temperature than the actual temperature at the inlet of the heat exchanger – and the system starts unnecessarily late.

The sensor is mounted in the tank through an immersion sleeve (well), which prevents direct contact of the sensor part with water and allows replacement without draining the tank. For this purpose, for example, Sensor for well – PVC cable 4m; 95 °C is suitable, with sufficient cable length for convenient routing to the controller.

How the sensor communicates with the controller – cables, length, interference

The connection between the sensor and the controller is done via a two-core cable. It is important to realize that the signal from the sensor is analog and low-voltage – any electromagnetic interference or high contact resistance (oxidized contact, loose clamping) can distort the measured values.

Practical rules for sensor cabling:

Run the sensor cable separately from power cables (230 V grid, pump). If they must run parallel, maintain at least 10 cm distance or use a shielded cable.
The maximum cable length depends on the cross-sectional area of the conductor. For PT1000 sensors, a length of up to approximately 50 m is acceptable with a cross-section of 0.5 mm² without significant error influence. For NTC sensors, a maximum of 30 m is recommended with the same cross-section.
Extending the sensor cable must be done properly – welded or crimped joints, not twisted. An oxidized joint adds resistance and the controller reads an incorrect temperature.
Solar pipe on the roof is exposed to significant temperature differences (−20 °C in winter, +80 °C in summer) – if the sensor cable runs along the pipe (which is common), it must be resistant to these temperatures. In installations where the sensor cable runs together with the medium in a protected pipe, a practical solution is a double stainless steel connection including the cable – for example Double stainless steel pipe with rubber insulation and cable – 2x pr. 16; 10m, where the sensor cable is integrated directly into the thermal insulation of the pipe.

Solar system controllers – from basic to advanced

Simple differential control

The most basic solar system controller operates exclusively on the principle of differential control – it compares T1 (collector) and T2 (storage tank) and switches the pump on/off based on the set ΔT. Such controllers are inexpensive, reliable, and fully sufficient for a simple single-loop system.

Controller with multiple functions and outputs

More modern controllers usually offer:

Multiple sensor inputs (T1 collector, T2 storage tank, T3 backup tank or pool)
Flow regulation via a PWM signal to the circulation pump
Anti-legionella function (heating the storage tank to 60 °C once a week)
Cooling function for the storage tank at night in case of overheating (night cooling)
Power and total energy measurement (with flow meter)
Bus communication for integration into a smart home

An example of a compact solution, where the controller is integrated directly into the pump group, is Solar pump unit IVAR.SOLAR K with controller IVAR.SOLAR IMTDC – in one compact block are the pump, controller, flow meter, thermometer, safety valve, and ball valves. The IMTDC controller allows setting all key parameters and displays current temperatures on an LCD screen.

Diagnosis – how to find out that the sensor is faulty

A faulty sensor usually shows one of several typical symptoms. An experienced service technician can recognize them quite quickly, but even an average user can help themselves with simple checks.

Symptom 1: The pump does not start even on a sunny day

This is the most likely sign of a faulty collector sensor. If the controller displays a collector temperature of 25–30 °C even under strong sunlight in summer (in reality it should be 70–100 °C), it means the sensor is not reading correctly – either the circuit is broken (resistance is infinite) or shorted (resistance is zero). The controller evaluates that ΔT is too small and does not start the pump.

Symptom 2: The pump runs continuously, the storage tank does not overheat

If the storage tank sensor constantly shows a low temperature (e.g. 20 °C) even though the tank is actually at 65 °C, the controller thinks the tank is always cold and the pump will run continuously. The tank does not heat further because the collector can only transfer heat as long as it is warmer than the medium in the tank.

Symptom 3: Extreme temperature jumps on the display

If the controller displays, for example, −40 °C or +250 °C, or the value constantly jumps by tens of degrees, it is a typical sign of a broken or shorted cable, or a poor contact in the terminal block.

How to test the sensor with a multimeter

The procedure is simple: disconnect the sensor cable from the controller and measure the resistance at the sensor terminals at room temperature (approx. 20–25 °C):

PT1000: at 20 °C the resistance should be approximately 1078 Ω, at 25 °C approximately 1097 Ω. If the multimeter shows "OL" (open circuit) or 0 Ω (short circuit), the sensor is damaged.
NTC 10kΩ (common type): at 25 °C the resistance should be approximately 10 000 Ω. At 60 °C it drops to approximately 2 500–3 000 Ω. Values vary depending on the specific type of NTC, compare with the table in the controller manual.

If you want to verify the entire circuit including the cable, keep the sensor connected and measure at the controller terminals – if the resistance is different than directly at the sensor, the problem is in the cable or connections.

When to Replace a Sensor – and When It's Worth Replacing the Controller Too

The lifespan of a temperature sensor in a solar system depends on several factors: construction quality, ambient temperature, number of temperature cycles, and whether it was ever exposed to stagnation (extreme temperatures with no flow in summer). The typical lifespan of a quality sensor is 10–20 years. However, there are situations when replacement is necessary sooner:

The sensor in the collector has survived several summer stagnations without protection (cracked housing, deformed cable)
Rubber or PVC cable insulation is corroded, brittle, cracked
Measured values systematically deviate from the actual temperature by more than 3–5 °C (verifiable with an external thermometer)
The sensor has been soaked in water or condensation (corrosion on contacts)
The controller reports a sensor error (E1, E2, Err sensor, etc.)

When replacing a sensor, it is essential to buy the same type (PT1000 for PT1000, NTC 10k for NTC 10k) with the same characteristics. If you are unsure what type of sensor was originally installed, you can verify it in the controller manual or by measuring the resistance of a working sensor at two different temperatures (room temperature vs. hot water) – for example, by immersing it in 60 °C water.

When to Consider Replacing the Entire Controller

Solar system controllers are electromechanical devices with a long lifespan – they typically function for 15–25 years. Replacement of the controller is justified in these cases:

The display does not work, buttons do not respond
The controller does not start the pump despite correct temperatures and functional sensors
The controller's relay is welded (the pump runs even when the sensors are disconnected)
You want to add features (more zones, energy measurement, remote monitoring)
Replacement sensors for the old controller are no longer available on the market

Practical Field Experience – Typical Cases

Case 1: Solar "Worked" Only in the Morning

The customer had a complaint that the solar system heated the tank in the morning, but stopped after lunch. The pump did not start after lunch despite the continued sunshine. It turned out that the tank sensor was placed about 60 cm from the bottom – that is, in the upper zone of the tank. In the morning, when the tank was cold, the controller correctly detected the low temperature and the system worked. After partial heating, stratification in the tank developed – the upper layer was at 65 °C, the lower one still at 35 °C. However, the sensor measured the upper 65 °C, the controller evaluated the tank as "full" and turned off the pump. Solution: relocation of the sensor to the lower third of the tank, the system then worked properly all day.

Case 2: The Pump Runs at Night

The customer noticed that the pump was running constantly at night. The tank temperature was low (28 °C), the collector according to the controller showed 45 °C. Naturally, the collector could not have 45 °C at night. Problem: short circuit in the collector sensor cable – moisture got into the connection in the central terminal block and shorted the sensor resistance. The controller evaluated the lower resistance as a higher temperature. The pump ran unnecessarily all night and drained the heat from the tank back into the environment through the unheated collector. The customer had an unnecessarily cold tank and increased electricity consumption.

Case 3: The System Works, but Performance is Lower

During an annual inspection, we measured that the system delivered about 25 % less energy than in previous years. Collectors were clean, the pump worked, the flow was correct. The collector sensor measured only 72 °C at a temperature of 80 °C – drift caused by aging of the NTC thermistor. Result: the controller turned off the pump earlier than it should have (ΔT dropped below the threshold earlier), because it saw a lower temperature on the collector. Replacing the sensor with a new one restored the original performance.

Sensor Replacement – Step-by-Step Procedure

Replacing a collector or tank sensor is a relatively simple operation that even a skilled DIY enthusiast can handle. It is important to follow a few rules:

Turn off the system from the electrical grid before any work on the controller or wiring.
Do not pull out the collector sensor on a hot summer day when the system is stationary – the collector temperature can be 150–180 °C and the pipe will be extremely hot. Work in the morning or on cloudy days.
Remove the old cable from the installation carefully – mark where it was fastened, so you can route the new cable the same way.
Insert the new sensor into the pipe all the way to the bottom – only a sensor in full contact with the pipe measures the correct temperature. Tighten the pipe (if threaded) or fix it according to the construction.
Secure the cable to the pipe or structure at intervals of max. 30–50 cm, so it does not hang freely and is not damaged by vibrations or wind.
Connect the cable to the controller terminal block according to the diagram in the manual. Most controllers are not polarity-sensitive for sensors (both wires are equivalent), but verify this in the documentation.
Immediately after turning the system on, check the displayed temperatures – they must be physically meaningful for the current conditions.

If you are replacing the tank sensor and the tank is not equipped with a drain pipe (dry pipe), you must partially drain the tank (at least below the pipe level) before removing the sensor. Modern tanks for solar systems always have dry pipes.

Integration of Sensors in Advanced Systems

In larger installations – for example, a combined system of solar + heat pump + boiler, or a system with two tanks and a pool – the number of sensors increases. A controller can manage 3–6 sensors at once:

T1 – collector (output)
T2 – primary tank (lower zone)
T3 – secondary tank or pool
T4 – tank inlet (for performance measurement)
T5 – outside temperature (for night cooling function)
Flow sensor (for heat output calculation)

For such systems, it is important that all sensors are of the same type (or that the controller supports configuration of different types on different inputs). At the same time, with multiple sensors, diagnostics becomes more complex – a problem with one sensor can affect the behavior of the entire system in unpredictable ways, so regular checking of sensor values is part of the annual service inspection. More about the correct annual inspection can be found in the article Maintenance and Service of a Solar System – What to Check Every Year.

If you are planning to install an entire system from scratch and you are wondering how to choose the right components including sensors, also see our articles Dimensioning Solar Pipe and Pump Unit – How to Do It and Installation of a Solar System Step by Step – From Collector to Tank. For choosing the right collector, the article Flat vs. Tubular Solar Collector – Comparison of Types for Slovak Conditions is also useful – for example, the Flat Frame Solar Collector IVAR.SOLAR 210 M5 is suitable to be paired with a controller that has a pipe directly on the header.

Calibration and Verification of Sensor Accuracy

For users who want to be sure about the accuracy of their system, there is a simple verification method:

Prepare a container with water and a verified accurate thermometer (a kitchen digital thermometer with ±0.5 °C accuracy is sufficient).
Immerse the sensor and the reference thermometer in the same place in the container.
Heat the water gradually to 30, 50 and 70 °C and at each temperature compare the controller display with the actual value.
If the values differ by less than ±2 °C, the sensor is still in good condition. A difference of 3–5 °C indicates sensor aging, more than 5 °C is a reason for replacement.

Some advanced controllers allow manual correction (offset) of the sensor – that is, setting a system-wide correction by a constant value (e.g. +3 °C). This is a temporary solution, not a substitute for physical sensor replacement, because the offset is constant and does not correct the non-linear drift of the sensor across the entire temperature range.

Most Frequently Asked Questions (FAQ)

What happens if I connect a PT1000 sensor to an input of a controller designed for NTC?

The controller will read completely incorrect values. For example, at room temperature of 25 °C, the PT1000 has a resistance of ~1097 Ω. A controller with NTC characteristics (NTC 10k) would interpret this resistance as a temperature around −30 °C to −40 °C, because NTC has a similar resistance at that temperature. The system would never start the pump. Always use the type of sensor the controller requires – this is stated in the technical documentation.

Can I extend the sensor cable if it is too short?

Yes, but you must follow some rules: use a cable of the same or larger cross-section (min. 0.5 mm²), the connection must be solid and waterproof (soldered or crimped, not twisted), the total length should not exceed 30–50 m (depending on the sensor and controller type). Route the cable separately from power lines. A longer cable adds resistance, which may slightly distort the measurement – for PT1000, with 0.5 mm² and 30 m cable, it is approximately 1.4 Ω per meter (two-wire), so about 84 Ω total cable resistance for 30 m, which corresponds to an error of about 2 °C. For more accurate measurements, use a larger cross-section.

The controller displays an error message E1 (or Err Sensor) – what does it mean?

Error code of the sensor (E1, E2, Err, Sensor Error, etc.) usually means that the controller detects a sensor resistance outside the range – either too high (open circuit) or too low (short circuit). Procedure: disconnect the cable from the controller and measure the resistance at the sensor terminals with a multimeter. If the resistance is OK, the problem is in the cable or the connections. If it is infinite or zero, the sensor is damaged and must be replaced.

Will the collector sensor withstand stagnation? When should it be replaced preventively?

A high-quality sensor designed for solar applications (such as Temperature Sensor for Well – 180 °C; Cable 2m) is dimensioned for repeated stagnation up to 180 °C. However, after 10–15 years of operation, during which the system has experienced dozens of stagnation cycles, we recommend replacing the collector sensor preventively – even if it still works. The reason is the aging of the cable insulation and gradual drift of the NTC thermistor. A tank sensor (with lower max. temperature) can last up to 20 years without problems.

Is it better to have a sensor with a shorter or longer cable?

From a technical point of view, a shorter cable is better (less resistance, less interference). The cable should be just long enough to be comfortably routed without unnecessary winding. Do not coil the excess cable into a spool – it creates inductance, which can cause measurement noise in some types of controllers. If you have an excess of 1–2 m, simply route it in a loop along the pipe. If the excess is large, cut the cable and terminate it with a quality terminal block.

Can temperature sensors of a solar system communicate wirelessly?

In standard solar systems for family homes, sensors are exclusively wired – and for a good reason. Wireless sensors require batteries or external power, are prone to interference, and on roofs with metal structures, the signal can be problematic. For home solar systems, we always recommend wired PT1000 or NTC sensors. Wireless solutions are more of a matter for industrial monitoring systems.

Conclusion – A Small Component, A Big Impact

Temperature sensors are the cheapest yet decisive components of a solar system. Without accurate measurement of the collector and tank temperatures, the controller cannot perform its job – and the entire investment in collectors, pump unit, tank, and installation works inefficiently or not at all. Regular checking of the sensor readings (at least once a year) and timely replacement at the first signs of failure is the cheapest way to keep your solar system at full performance throughout its lifetime.

If you are unsure what type of sensor your controller requires, or need advice when choosing a replacement part, check out the available sensors and accessories in our solar systems category.

Do you have a question about this topic?

Having trouble deciding or dealing with a specific situation in your home? Write to us – we are happy to help.