Installing a Temperature Sensor for a Solar Collector – Placement and Wiring

We are still preparing products.

But you can look at other categories.

Installing a Temperature Sensor for a Solar Collector – Placement and Wiring

A temperature sensor is a seemingly minor component of a solar system – a small probe, a few meters of cable, one connector. Yet this small device determines whether your solar controller can correctly evaluate energy, start the pump, and protect the system from overheating. An incorrectly placed or badly wired sensor can completely ruin the performance of a well-designed system. In practice, I've seen installations where the collector was producing plenty of energy, but the controller simply couldn't see it – because the sensor was in the wrong spot, or the polarity was swapped. This article will explain in detail where and how to mount the sensor, why every detail matters, and what happens when things go wrong.

Why the Temperature Sensor Is a Key Element of the Whole System

A solar controller doesn't operate blindly. It makes decisions based on the temperature difference between the collector (sensor T1 or S1) and the hot water tank (sensor T2 or S2). When the collector temperature is a set number of degrees higher than the tank temperature – typically 5 to 8 °C – the controller starts the circulation pump. When the difference drops below the set hysteresis (usually 2–4 °C), the pump stops again. This simple rule protects the tank from cooling down and the collector from so-called reverse heat losses.

Without accurate sensor readings, this whole logic falls apart. A sensor mounted in the wrong place can report a temperature 15–20 °C lower or higher than the actual value. The controller then either runs the pump unnecessarily (cooling down the tank), or fails to start it at all even when the collector is at 90 °C and the tank is barely 30 °C.

The diagram shows the basic arrangement: sensor T1 measures the temperature of the collector outlet, sensor T2 measures the temperature in the lower part of the tank. The controller compares these two values and starts the pump when the collector is warmer by the set difference.

Types of Temperature Sensors Used in Solar Systems

Before we get into placement, it's important to understand what you're holding in your hands. Solar systems commonly use three types of temperature sensors:

NTC Resistance Sensors

NTC (Negative Temperature Coefficient) sensors are by far the most widespread in solar systems. Their resistance decreases as temperature rises – a typical value is 10 kΩ at 25 °C. Most European controllers (Resol, Steca, Teplota, Wagner) work with NTC sensors with a Pt1000 or NTC10k characteristic. Before buying, always check which characteristic your controller's manufacturer specifies – not all NTC sensors are interchangeable.

Pt1000 Resistance Sensors

Platinum resistance sensors Pt1000 have a resistance of exactly 1,000 Ω at 0 °C, rising linearly with temperature. They are more accurate than NTC, but also more expensive. Most modern differential controllers work exclusively with Pt1000. The advantage is that longer cable runs have less impact on accuracy – wiring affects the measured resistance, and with the linear characteristic of Pt1000, correction is easier.

Thermoelectric Sensors (Thermocouples)

In typical residential solar systems, thermocouples are practically never used – they are common in industrial applications. If you come across a thermocouple, it's likely in an older installation or a special application.

Collector Sensor (T1) – Where Exactly to Place It

This is the most important part of the whole installation, and also the one where most mistakes are made. The T1 sensor must measure the temperature the fluid has actually reached after being heated in the collector – not the ambient temperature, the frame temperature, or the temperature of the incoming cold fluid.

Correct Location: Outlet Manifold or Return Chamber of the Absorber

Sensor T1 belongs in a special immersion pocket (sleeve) that is part of every solar flat-plate or tube collector. This pocket is located either directly in the outlet manifold (distribution pipe), or on the side of the collector near the outlet. It's a small blind tube – usually 6 or 8 mm in diameter, 80 to 100 mm deep – into which you simply insert the sensor.

A few rules that must be followed:

The sensor must be in contact with the fluid or directly with the metal near the fluid. An air gap between the sensor and the pocket dramatically worsens accuracy. That's why the sensor should always be bonded into the pocket with thermally conductive paste.
The sensor must be on the outlet side of the collector, not the inlet side. The inlet side (cold branch) can be 20–40 °C cooler than the outlet – the controller would be operating in an information vacuum.
With multiple collectors connected in series, the sensor belongs on the outlet of the last collector in the row. With parallel connection the situation is more complex – find out more in the article Series vs. Parallel Connection of Solar Collectors – Which to Choose.
The sensor must not be exposed to direct sunlight without protection. If the pocket isn't covered by the collector glass, the sun can heat the probe itself to 60–80 °C even when the fluid is only 30 °C – causing the controller to start the pump unnecessarily.

What If the Collector Doesn't Have an Immersion Pocket?

Older or lower-quality collectors sometimes lack a dedicated pocket. In that case, you have two options:

Attach the sensor directly to the collector's outlet pipe using thermally conductive paste and secure it with aluminum tape. The sensor must be insulated from outside air – covered with at least 10 cm of thermal insulation.
Install a flow fitting with an immersion pocket directly on the outlet connection – a more elegant solution, but it requires space and the correct thread size.

I clearly prefer an immersion pocket built directly into the collector – it's hydrodynamically cleaner, the sensor reacts faster, and the measurement is more accurate. That's why, when choosing a collector, I always check whether it has an integrated pocket.

Tank Sensor (T2) – Where to Place It in the Tank

The tank sensor T2 measures the water temperature in the tank and serves as the reference value for the controller. Hot water tanks have immersion sleeves or direct immersion ports for this purpose, most often with a G1/2" or G3/8" thread.

Placement Height in the Tank

This is something that surprises many people: the T2 sensor does not belong in the middle or top of the tank, but in the lower third – usually at a height of 1/5 to 1/3 of the tank's height. Why? A tank works on the principle of thermal stratification – the hottest water is at the top, the coldest at the bottom. The controller needs to know whether there's enough cold water at the bottom worth heating with the collector. If the sensor were at the top, where the water is always warm (heated by a boiler or electric heating element), the controller would think the tank is full of heat and wouldn't start the pump – even though there's 25 °C cold water at the bottom.

In practice, I go by this: a 200-liter tank, about 140 cm tall – the T2 sensor goes at a height of about 25–35 cm from the bottom. A 300-liter tank, about 175 cm tall – sensor at 30–45 cm from the bottom. Always also check the tank manufacturer's instructions – some tanks have a dedicated immersion pocket for the solar sensor labeled "Solar" or "T-solar".

Relation to the Solar Coil

The T2 sensor should be placed as close as possible to the outlet of the solar coil (heat exchanger), not to the electric heating coil. The solar coil is usually located in the lower part of the tank and heats the water from below. If your tank has electric backup heating – for example with an electric heating element 2 kW for OKC tanks – it is usually located in the upper part of the tank and works independently of the solar circuit. The T2 sensor should be below the level of this electric heating element, so that the electric backup heating doesn't affect the solar controller's decisions.

Sensor Cable – Length, Cross-Section and Routing

Sensors are typically supplied with a cable 2 m or 3 m long. In practice, this is almost never enough – the collector is on the roof, the controller in the boiler room. You need an extension, and important rules apply here.

Maximum Cable Length

For NTC10k sensors, the wire's resistance adds to the sensor's resistance and causes a systematic measurement error. A standard installation cable of 2×0.5 mm² has a resistance of about 36 Ω/100 m. For a 10 m cable run (20 m of wire there and back), that's 7.2 Ω – for an NTC10k sensor with a value of 10,000 Ω at 25 °C, this is a deviation of less than 0.1 °C, which is negligible. For a Pt1000 sensor with a value of 1097 Ω at 25 °C, however, this is a deviation of 0.6 °C, which is small but starts to become problematic on longer runs (30+ m), and some premium controllers allow calibration/wire correction.

For NTC sensors: maximum cable length approx. 50–80 m, cross-section min. 0.5 mm²
For Pt1000 sensors: maximum cable length approx. 30–50 m (longer only with correction or 4-wire connection)
Never route the sensor cable in the same bundle as 230V mains wiring – electromagnetic interference causes false fluctuations in the displayed temperature

Extending the Cable

Extending a sensor cable is simple, but it must be done correctly. Use the same type of wire, make joints in junction boxes (not loosely hanging terminal connectors), and use UV-resistant insulated cable when routing across the roof. Polarity doesn't matter for NTC and Pt1000 – they are resistive sensors without polarity. If the controller reports an incorrect value after extension, check the contacts and the resistance of the entire run with a multimeter.

Step-by-Step Installation Procedure

Sensor installation usually takes place alongside mounting the collector and piping. Here's the procedure I use for a typical job with two collectors on a pitched roof:

Detailed procedure:

Locate the T1 immersion pocket on the collector. In flat-plate collectors, it's usually on one of the side connections in a plastic or metal housing. In tube collectors, the pocket is usually in the top manifold. Check the collector's technical documentation.
Apply thermally conductive paste to the sensor body. A small amount – pea-sized – evenly over the probe's body. The paste eliminates air pockets in the pocket and improves thermal conductivity.
Insert the sensor into the pocket all the way. If the pocket has a screw-on retaining cap or a rubber sealing sleeve, tighten it so the sensor doesn't fall out, without tearing the cable's waterproof sleeve.
Route the sensor cable along with the piping to the boiler room. The cable should be protected from mechanical damage, UV radiation, and direct contact with high-temperature piping. Ideally, run it in a separate conduit next to the pipe insulation.
Connect the cable to the controller. Most controllers have clearly labeled terminals for T1 (Sensor 1, S1, Collector) and T2 (Sensor 2, S2, Store). NTC and Pt1000 sensors are not polarized – it only matters which terminal block they go into, not the order in which the two wires are connected.
Verify that the displayed temperature is realistic. Before starting the system, check that the values make sense. In a resting state (morning before the sun comes up), T1 and T2 should be close to ambient temperature. If T1 shows, for example, –40 °C or +200 °C, the sensor is damaged or wired incorrectly.

Typical Installation Mistakes – What I've Seen in Practice

Over years of practice, I've encountered a whole range of installation errors. Here are the most common ones, so you can avoid them:

Mistake #1: T1 Sensor on the Inlet Instead of the Outlet

This is probably the most common mistake beginners make. The collector has two connections – the inlet (cold side) and the outlet (hot side). If the sensor is on the inlet, it measures the temperature of the fluid that is about to enter the collector. In practice, this causes the controller to see a temperature 15–40 °C lower than the actual outlet temperature. The pump either doesn't start at all, or starts with unnecessary delay.

Mistake #2: T2 Sensor Too High in the Tank

When a customer installs the sensor in the middle or upper pocket of the tank (thinking "the middle is best"), the controller measures water temperature that is typically 10–20 °C warmer than the water near the solar coil. Result: the pump starts late or not at all.

Mistake #3: Sensor Cable Routed Next to 230V Mains Wiring

Electromagnetic induction from a 230V cable causes interference in the sensor's signal cable. The controller then displays a temperature that oscillates ±2–5 °C for no apparent reason. Solution: route the sensor cable in a separate conduit, at least 10 cm from mains wiring.

Mistake #4: Sensor Without Thermally Conductive Paste

An air gap between the sensor and the pocket wall has thermal conductivity roughly 40 times lower than copper. The sensor reacts much more slowly and shows values 3–8 °C lower than reality. In practice, this can look like a slow system start every morning.

Mistake #5: Wrong Sensor Type

An NTC sensor connected to a controller calibrated for Pt1000 – or vice versa. The controller won't necessarily report an error (both types are passive resistive sensors), but the displayed temperature will be completely off. NTC at 60 °C shows a resistance of about 2.5 kΩ, while Pt1000 at 60 °C shows 1232 Ω. A controller calibrated for Pt1000 would interpret 2.5 kΩ as a temperature of –40 °C or would report a sensor error.

Sensors in Systems with Multiple Collectors and a Manifold

If your system has multiple solar collectors connected in parallel via a supply and return manifold – for example a solution using the industrial stainless steel manifold set with ball valves – the sensor situation requires a bit more thought.

With parallel connection of collectors (each collector has its own inlet and outlet pipe, which join at the manifold), place the T1 sensor on the collecting outlet pipe after the manifold – that is, past the point where the outlets from all collectors merge into a single flow. This way you measure the average outlet temperature from all collectors, which is the correct reference value.

With series connection (the outlet of the first collector goes to the inlet of the second, etc.), the T1 sensor belongs on the outlet of the last collector in the series – that's where the fluid is hottest.

In both cases, the sensor must be well insulated from the surrounding air – in the shorter section of piping after the collectors the fluid is still relatively hot, but the pipe is exposed to outdoor conditions (wind, rain), which can distort the measurement if the sensor isn't covered by insulation.

If you're working with a mounting frame for installing two collectors or planning to expand using a mounting frame for installing an additional collector, always plan the sensor cable routing before mounting the collectors – retrofitting on a finished assembly is significantly more complicated.

Functional Verification and Diagnostics After Installation

After completing the sensor wiring and before first starting the system, perform these checks:

Resistance check with a multimeter: Measure the resistance of each sensor directly at the controller terminals. For NTC10k at room temperature (~20 °C), the resistance should be about 12–13 kΩ. For Pt1000 at 20 °C, the resistance should be about 1078 Ω. If the resistance is 0 Ω → short circuit; if it's OL (overload/open circuit) → broken cable.
Check the displayed temperature: Turn on the controller and check the displayed values. Before heating up, T1 and T2 should be close to ambient temperature (±3 °C is acceptable).
Functional test: Cover the collector with a dark sheet, or temporarily disconnect sensor T1 and use a resistance substitution box to simulate a high collector temperature (e.g., 1232 Ω for Pt1000 ~ 60 °C) – the controller should start the pump.

When diagnosing problems, it's useful to have Common Faults of Solar System Accessories and How to Fix Them on hand – this article contains a more detailed overview of causes and solutions for various types of faults, including sensor issues.

Overflow Temperature Sensor (T3) – When You Need It

Besides the basic T1 and T2 sensors, some controllers support a third sensor – T3 or Tmax – to measure the overflow temperature in the tank or directly at the domestic hot water outlet. This sensor is used to protect against tank overheating. When the tank reaches the set maximum temperature (typically 75–80 °C), the controller stops the pump even though the collector is still hotter.

We place the T3 sensor in the upper part of the tank – unlike T2, where we're interested in the bottom zone, here we want to know the maximum temperature in the tank. This is especially important in summer, when the solar system operates at full power and, without protection, the tank could exceed 95 °C.

Special Situations: Sensors on Tube Collectors

Tube (vacuum) collectors have a different design than flat-plate collectors, and the location for the T1 sensor is handled a bit differently in them. Most tube collectors with heat pipe technology have an immersion pocket directly in the top manifold – the collecting head where vapor from the heat pipe tubes condenses. This is the correct location for T1.

Note: with direct-flow tube collectors, the fluid flows directly through each tube. In this case, the pocket is on the outlet connection of the manifold – just like with a flat-plate collector.

Tube collectors reach higher outlet temperatures than flat-plate ones – easily 100–120 °C in summer. It's therefore important that the T1 sensor is rated for these temperatures. Standard sensors for flat-plate collectors are rated up to 130–150 °C, which should be sufficient, but always check the sensor's technical data sheet.

Frequently Asked Questions (FAQ)

Do I have to use the original sensor from the controller manufacturer, or can I use any compatible one?

Controllers are calibrated for a specific sensor characteristic (NTC10k, Pt1000, etc.). What matters is that the replacement sensor has the same characteristic – not that it's the same brand. If you have a Resol DeltaSol BS/2 controller working with Pt1000, you can use any sensor with a Pt1000 characteristic and suitable protection rating. However, always check the controller manual for the specified sensor type, and if possible, choose a sensor certified for solar applications (resistant to at least 130 °C).

Can I extend the sensor cable with a regular telephone or network cable?

For temporary testing, yes; for a permanent installation, no. Telephone cables have thin conductors (0.2–0.3 mm²) with plastic insulation unsuitable for outdoor conditions and higher temperatures. Use a cable with a cross-section of at least 0.5 mm², with UV-resistant insulation (CYKY or a special sensor cable). Insulate the joints watertight and place them in junction boxes – not loosely exposed.

The controller shows a normal collector temperature, but the pump doesn't start. What should I check?

First, compare the displayed T1 and T2 values. If T1 is only 3–4 °C higher than T2 and the set switch-on differential is 6 °C, the pump correctly won't start – you need to wait until the collector heats up more. If T1 is, say, 80 °C and T2 is 30 °C, but the pump still isn't running, check: the controller settings (the maximum tank temperature might have been reached), the pump wiring and its fuse/protection function, and whether the controller is in manual/blocking mode. Faulty contacts at the controller terminals are also a common cause.

Can the T2 sensor be installed on the outside of the tank instead of immersed?

Technically yes, but it's a significantly less accurate solution. A sensor glued to the tank's surface with thermally conductive paste and covered with insulation shows the temperature of the tank wall – which, in well-insulated tanks, is similar to the water temperature, but with a delay and a deviation of ±3–6 °C. For a typical residential system, this may be sufficient as an emergency solution, but for precise control and optimal performance, an immersion pocket is always better.

How can I tell if a sensor is damaged without special instruments?

The simplest way: disconnect the sensor cable from the controller and measure the resistance with an ordinary cheap multimeter. Set it to measure resistance (Ω) and touch the probes to both wires of the sensor cable. At room temperature (20–25 °C): NTC10k should show about 12,000 Ω, Pt1000 about 1,078 Ω. If the multimeter shows 0 (short circuit) or OL/1 (open circuit), the sensor or cable is damaged. If you measure a reasonable value but the controller still reports an error, the problem is probably in the terminal block's contacts.

How long does a temperature sensor last in a solar system?

A quality sensor from a reputable manufacturer will easily last 10–15 years without problems. The most common causes of premature failure are mechanical damage to the cable (e.g., during installation, or from rodents gnawing in the attic), corrosion of contacts in damp environments (which is why watertight joints matter), and thermal aging of the cable insulation if the sensor is routed without a conduit directly along a hot pipe. A sensor is a cheap and easily replaceable item – if in doubt, replacing it with a new probe is more sensible than a long diagnostic process on the old one.

Conclusion

Installing a temperature sensor for a solar collector may seem like a trivial matter on the surface – insert the probe into the pocket, connect the cable. In reality, correct sensor placement and wiring is one of the key conditions for the whole solar system to work efficiently and safely. Sensor T1 belongs at the collector outlet (at the outlet of the overall manifold, for multiple collectors), sensor T2 in the lower third of the tank near the solar coil. The cable must be correctly sized, routed separately from mains power, and the joints must be watertight. Thermally conductive paste during installation is not optional – it's essential for an accurate and fast sensor response.

If you're planning to install or expand a solar system, related to choosing a mounting frame, connecting collectors, or complete accessories, also check out other articles in our Knowledge Center – for example Mounting a Solar Collector Frame Step by Step, Antifreeze for Solar Systems – How to Choose the Right Composition and Concentration, or Manifold in a Solar System – What It's For and When You Need One. The right sensors in the right places are an investment that pays off by making the most of every hour of sunshine.

Do you have a question about this topic?

Can't decide, or dealing with a specific situation in your home? Write to us - we're happy to advise.