Setting the differential temperature in solar regulation: how to correctly configure switching

We are still preparing products.

But you can look at other categories.

Differential temperature in solar regulation: the foundation that determines the performance of the entire system

When you look at a solar system from the customer's perspective, everything seems simple: collectors capture solar radiation, the heated fluid flows into the tank, the water heater is heated, and you have hot water. But when you take a closer look—at the regulation, at exactly when the pump turns on and off, at why the system sometimes runs unnecessarily or, on the contrary, cannot keep up—you will find that the correct setting of the differential temperature is the quiet decision-maker that determines whether your solar system works efficiently or just unnecessarily wears out the circulation pump and wastes energy.

In this section of the Knowledge Center, we will focus specifically on this parameter: what is differential temperature, why it has two values (on and off), how to correctly set them for different types of systems and installations, and what errors we see most often in practice. If you are about to configure the regulation for the first time, or you are looking for the reason why your system is not working as it should, read on.

What is differential temperature and why it exists

Differential temperature (shortened to ΔT or delta-T) is the temperature difference between two points in the system—usually between the temperature sensor on the collector (output from the solar field) and the sensor in the tank or water heater (lower zone). The regulation continuously compares these two values and, based on their difference, decides whether to turn on or off the circulation pump.

At first glance, it might seem: "If the collector is warmer than the tank, let the pump run. If not, let it stop." But reality is more complex. The collector heats up and cools down slowly, the tank is not evenly mixed, the sensors have their inertia, and if you reacted to every temperature fluctuation, the pump would turn on and off every few seconds. This destroys the pump, destabilizes the system, and the result is paradoxically worse than if the pump had not run at all.

Therefore, solar regulations work with two differential temperature values:

ΔT on (T-on) – the minimum temperature difference at which the regulation turns on the pump
ΔT off (T-off) – the minimum temperature difference at which the pump is still running; if it drops below this value, the regulation stops the pump

Between T-on and T-off there is so-called hysteresis—a band in which the regulation "holds" the decision and does not change it. Without this hysteresis, the system would oscillate.

The graph above illustrates a typical temperature difference curve during the day. The curve rises in the morning as the collector begins to heat up. At the moment when ΔT exceeds the T-on value (green line, e.g. 8 °C), the regulation turns on the pump. The pump runs until ΔT drops below T-off (orange line, e.g. 3 °C). The band between them is hysteresis—the "safe zone" of regulation.

Switch-on differential temperature (T-on): how to set it correctly

Switch-on differential temperature is the value that the temperature difference between the collector and the tank must reach to start the pump. In practice, it usually ranges from 5 to 12 °C, with a typical recommendation from most regulation manufacturers being 6 to 8 °C.

Why not set T-on to 1–2 °C?

This is a typical beginner's mistake. The logic is "the sooner the pump turns on, the more energy I can extract." The problem is that with a very low switch-on differential temperature:

The pump starts even before the collector has actually heated up to a useful temperature—in the early morning hours with weak radiation
Cold fluid from the pipe flows into the tank before the collector has had time to heat up—the tank can even cool down slightly
The system oscillates: the pump turns on and off every few minutes, the sensor on the collector fluctuates, and the system's performance is unstable
The lifespan of the circulation pump is dramatically reduced due to a high number of starts

In practice, I have seen systems set to T-on = 2 °C, where the pump started up to 200 times in one day. The customer was wondering why their pump only lasted two seasons.

Why not set T-on too high (e.g. 15–20 °C)?

On the other end of the spectrum are customers who set T-on to 15 °C "just to be sure." The result is that the system does not start during cloudy weather or in transitional periods when it could still effectively collect energy. Collectors reach temperatures 10–12 °C higher than the tank—which is usable—but the regulation does nothing.

The correct setting of T-on depends on several factors:

Pipe length: the longer the pipe between the collector and the tank, the higher the T-on (the pipe contains cold fluid that must be moved to the tank when the pump is turned on; for short distances, 5–6 °C is sufficient, for distances over 10 m I recommend 7–9 °C)
Volume of the collector and pipe: a larger volume of fluid in the system = higher switch-on ΔT
Type of collector: vacuum collectors have lower heat losses, heat up faster, and can work with a lower T-on (5–6 °C); flat collectors require a slightly higher value (7–8 °C)
Orientation and slope: systems with an ideal orientation (south, 35–45°) can work with a lower T-on

Shutoff differential temperature (T-off): hysteresis and system protection

Shutoff differential temperature is the value that the temperature difference between the collector and the storage tank must drop below for the regulation to turn off the pump. A typical value ranges between 2 and 4 °C, with the most common recommendation being 3 °C.

It is important to understand that T-off must always be lower than T-on. The difference between them forms the hysteresis. For example, if you set T-on = 8 °C and T-off = 3 °C, the hysteresis is 5 °C. This means the pump turns on when ΔT reaches 8 °C and turns off only when ΔT drops to 3 °C.

Why is too low T-off a problem?

If you set T-off = 0 °C or even a negative value (some regulators allow this), the pump will run even when the fluid from the collector is practically the same temperature as the storage tank. The efficiency of heat transfer is almost zero, the pump runs unnecessarily, consumes electrical energy, and wears out.

Why does too high T-off reduce efficiency?

On the contrary, if you set T-off = 6 °C (i.e., the same or close to the T-on value), the hysteresis is too small or non-existent. The pump turns on at ΔT = 8 °C and turns off at ΔT = 6 °C – this happens very quickly, before the system has time to transfer a sufficient amount of heat. The result is again oscillation.

From practical experience, I recommend the following basic setting as a starting point for most single-family homes:

T-on = 7 °C
T-off = 3 °C
Hysteresis = 4 °C

This setting works reliably for systems with 2–4 flat collectors, piping up to 15 m, and a standard 200–300 liter storage tank. For larger systems or specific conditions, individual optimization is necessary.

Maximum storage tank temperature: protection against overheating

In addition to differential temperatures, each solar regulator also sets the maximum allowable temperature of the storage tank (T-max of the tank). When the tank reaches this temperature, the regulator turns off the pump regardless of how large the temperature difference is between the collector and the tank.

A typical setting for T-max of the tank is 60–75 °C for standard TÚV systems. Reasons:

At temperatures above 60 °C, bacteria such as legionella are effectively destroyed in the tank (biological disinfection)
Temperatures above 80 °C can damage plastic components of the tank or the anode
At temperatures above 90 °C, there is a risk of evaporation of the antifreeze mixture in the collectors and overheating of the system
Certain types of tanks (with polyurethane insulation) are limited to 80 °C

For combined systems (TÚV + heating support), the T-max of the tank can be set higher, but it should never exceed 90 °C without special measures (expansion tank, safety valve, venting).

Maximum collector temperature: protection against overheating

A less known, but equally important parameter is the maximum collector temperature (T-max of the collector or collector cooling function). This function works opposite to the usual logic: if the temperature of the collector exceeds the set value (e.g., 120–130 °C), the regulator turns on the pump regardless of the fact that the tank is full and ΔT is not met.

Why? When the tank is full and the sun is shining brightly, the collectors can overheat. The temperature in the collector plane can reach 150–180 °C, and in the case of vacuum collectors, even 200 °C or more. At such temperatures, the antifreeze mixture (propylene glycol) breaks down, forms deposits, loses corrosion inhibitors, and can "boil" into vapor – this causes pressure increase and can lead to failure of the expansion tank or safety valve.

The collector cooling function thus turns on the pump and "pumps" the hot liquid from the collector into the tank – the tank does get overheated, but the collector is saved. The setting of T-max of the collector depends on the type of antifreeze mixture (see the manufacturer's technical sheet), but a typical value is 120–135 °C.

Practical settings on specific regulators

Euroster 813 Solar

On the Euroster 813 Solar, the differential temperature is set directly via the display menu. The regulator works with two sensors (collector and tank) and allows setting T-on and T-off independently. The standard factory setting is T-on = 8 °C and T-off = 4 °C – for most single-family homes, this is a reasonable starting point.

Setting procedure for Euroster 813 Solar:

Enter the menu by pressing the SET button for a long time
Go to the dT-on parameter (turn-on differential temperature) and set the desired value (6–8 °C for normal conditions)
Go to the dT-off parameter (turn-off differential temperature) and set it 2–4 °C below the T-on value
Set the T-max of the tank (I recommend 65 °C for standard TÚV)
Set the T-max of the collector according to the antifreeze mixture manufacturer's data (usually 120–130 °C)
Save the settings and monitor the system's behavior for the first 2–3 sunny days

The Euroster 813 Solar does not allow for PWM pump speed regulation – it operates only in the on/off mode. This should be kept in mind during setup, as systems with continuous pump speed regulation can work with a lower T-on (the pump starts slowly and gradually accelerates).

Control Stations ZPS 6, ZPS 16 and ZPS 28

Control stations ZPS 6, ZPS 16 and ZPS 28 are more complex solutions that integrate control, circulation pump, flow meter, thermometer and other components into one unit. The differential temperature is set in the same way as with a separate controller, but the advantage is that ZPS stations have a built-in flow meter and allow monitoring of the actual system performance in kW, which is very useful when adjusting the settings.

Practical experience from field work: during the installation of ZPS 16 with three vacuum collectors on a family house in Považská Bystrica, we first set T-on = 7 °C and T-off = 3 °C. The system worked well, but during the first week of monitoring the flow, we noticed that in the morning between 7:00 and 9:00 the pump oscillated – it turned on, the tank was slightly cooled by the cold fluid from the pipe (route approx. 12 m), ΔT dropped, the pump turned off, the collector heated up and the cycle repeated. Solution: increasing T-on to 9 °C. After this change, the system operates without oscillations and the annual yield increased (less losses due to unwanted mixing).

Seasonal optimization of settings

Differential temperature is not a parameter you set once and forget. The optimal setting can vary slightly depending on the season and weather conditions.

Summer

In summer months, the radiation is strong, the collectors heat up quickly and the tank is relatively cold in the transitional period (if you have used hot water during the night). The system works with large ΔT and the standard setting is usually fine. It is important to monitor the T-max of the tank and possibly reduce it if the tank regularly reaches the maximum temperature before the collector power is actually needed.

Spring and autumn

In the transitional period, the radiation is weaker and the collectors heat up more slowly. If T-on is set too high, the system will not turn on on days with partial cloud cover, even though it could collect usable energy. I recommend reducing T-on by 1–2 °C in this period (e.g. from 8 °C to 6 °C) and monitoring whether the system starts to oscillate.

Winter

In winter, most solar systems for DHW operate at minimum power or not at all. It is important to check the anti-freeze protection – the controller should have an anti-freeze function set (if the collector temperature drops below +4 °C, the controller briefly turns on the pump and transfers hot fluid from the tank to the collector, thus preventing freezing). Differential temperature in winter is not a priority, the more important is the anti-freeze setting.

Influence of sensors on the accuracy of differential measurement

Differential control is only as good as the sensors are. And it is precisely here that we see many problems in practice, which customers attribute to poor controller settings, while the actual cause is the sensors.

Placement of the sensor on the collector

The sensor should be placed on the outlet pipe from the collector (upper pipe in natural circulation, outlet in the direction of flow in forced circulation), directly on the collector or on a welded-on housing. Not on the supply pipe in the boiler room, not on the outside wall, not on a pipe 2 meters away from the collector.

Sensor placement error: a customer in Prievidza installed a solar system himself and placed the collector sensor on the roof structure next to the collector, not on the outlet from it. The sensor measured the air temperature near the collector, not the fluid. As a result, the controller started the pump with a significant delay (air heats up later than the fluid in the collector) and the system operated inefficiently throughout the morning.

Placement of the sensor in the tank

The tank sensor should be placed in the lower part of the tank – typically at a height of 1/4 to 1/3 of the tank height from the bottom. Reason: the tank is thermally stratified (the bottom is colder, the top is warmer). The controller should measure the cold bottom layer, because it is precisely there that the fluid from the collector is directed. If the sensor were placed in the upper part, the controller would "see" hot water and the pump would turn on late or not at all.

Exception: some controllers (e.g. in ZPS control stations) allow setting two tank temperatures – a bottom sensor for differential control and a top sensor for T-max protection. This solution is technically the most accurate, but requires a tank with two sensor housings.

Calibration and sensor checks

PT1000 sensors (the most commonly used type) usually have an accuracy of ±1 °C when properly connected. Incorrect wiring (oxidized contact, incorrect cable length, poor insulation) can cause deviations of 3–5 °C or more. If you are unsure about the accuracy of the sensor, check the resistance of the sensor at room temperature: a Pt1000 at 20 °C should have a resistance of 1078 ohms. A deviation of more than 5 ohms indicates a problem. For more information on this topic, see the article Common faults in solar regulation: error messages, pump outages, and inaccurate sensors in this Knowledge Centre.

Differential temperature in systems with multiple tanks or zones

In larger installations, we encounter systems where solar collectors supply multiple tanks (e.g., a TÜV tank + a tank for floor heating) or where the TÜV tank is connected to a boiler tank. Controlling the differential is more complex in such cases.

The basic principle remains the same, but the regulation must address priority: which tank is heated first? Most regulations for these cases prioritize the TÜV tank (heating it first when the temperature drops below the set limit), while the heating tank or combined tank is secondary.

More complex control stations are suitable for such systems. For example, the Control Station ZPS 28 is specifically designed for larger systems with multiple supply points, where simple differential thermostats are insufficient. For more information on the differences between these solutions, see the article Euroster 813 Solar vs. control stations ZPS: comparison of functions and applications.

With multiple tanks, it is important to set the differential temperatures for each zone individually and to consider that the pump operates in one circuit at a time – if the regulation switches to the second tank, the flow will change and the temperature in the pipe may briefly fluctuate. To minimize this effect, it is advisable to increase the hysteresis (the difference between T-on minus T-off) to 6–7 °C in multi-zone systems.

Adjusting the settings after system startup: step-by-step procedure

The initial setting of the differential temperature is always just the beginning. The optimal setting is only found after monitoring the system in real operation. Here is a practical procedure I recommend:

Step 1 – Basic setting: Set T-on = 7 °C, T-off = 3 °C, T-max of the tank = 65 °C, T-max of the collector = 125 °C. This is your starting point.
Step 2 – First few days of monitoring: During the first 3–5 sunny days, monitor the regulation display every hour. Record when the pump turns on, what the temperature of the collector and tank is at the time of activation, and when it turns off.
Step 3 – Identifying oscillation: If the pump turns on and off more than 3–4 times per hour during sunny weather, increase T-on by 1–2 °C.
Step 4 – Identifying delayed start: If you notice that the collectors are clearly hot (visible from the maximum collector temperature), but the pump does not turn on, check ΔT and possibly reduce T-on by 1 °C.
Step 5 – Monitoring during transitional periods: Test the settings on cloudy days and in spring/autumn. During this time, it is most important to have T-on set low enough to utilize every available amount of energy.
Step 6 – Recording final values: Once you find the optimal setting, record it (also in the service log or on a label near the regulation). In the event of a power outage or regulation reset, you will know what it was set to.

A more detailed guide for the first startup and physical wiring can be found in the article Installation of solar system regulation: pump wiring, sensors, and expansion tank.

Most common mistakes when setting the differential temperature in practice

Over the years of working with solar systems in Slovakia, we have seen several errors that repeat themselves over and over:

Setting T-on = T-off: A regulation without hysteresis oscillates. T-on must always be at least 3–4 °C higher than T-off.
Ignoring the T-max of the collector: Some customers turn it off or do not set it at all because "it is not important." It is one of the most important protective functions.
Changing settings during operation without recording: A customer changes T-on from 8 to 5 °C, the system starts to oscillate, calls a technician, the technician spends an hour searching for the problem – and in the end, finds out that someone changed the setting. Always record any changes.
Incorrect sensor placement – see the previous section.
Inconsistent settings in systems after reconstruction: A customer replaces the tank with a larger one, but leaves the regulation settings unchanged. A larger tank heats up more slowly, ΔT remains high for a long time – the system works efficiently, but the customer complains that the tank does not heat up quickly enough. Solution: reduce T-on by 1–2 °C for a larger volume.

Frequently asked questions (FAQ)

What is the ideal activation differential temperature for most single-family homes?

For single-family homes with 2–4 flat solar collectors, a 200–300-liter tank, and piping up to 15 meters, the standard setting is T-on = 6–8 °C and T-off = 2–3 °C. Vacuum collectors work well even at T-on = 5–6 °C. This setting is not universal – after the first few days of operation, I recommend fine-tuning according to the system's actual behavior.

Can I set T-off to 0 °C so that the pump runs as long as possible?

Technically, most regulations allow this, but it is not recommended. With T-off = 0 °C (or close to zero), the pump runs even when the temperature difference between the collector and the tank is practically zero – this means no heat transfer is taking place, but the pump consumes electrical energy and unnecessarily wears out. Set T-off to at least 2 °C.

Why does my pump run at night when there is no sun?

This is a sign of a poorly configured protection against night cooling (night radiation). At night, the tank may be warmer than the collector – if the regulation does not have a function for the minimum collector temperature or if the anti-frost protection is not properly set, the pump may start and transfer heat from the tank TO the collector, thus cooling the tank. Check the setting of T-min of the collector (it should be at least 5–10 °C) and activate the night cooling protection function if available.

How can I tell that the differential temperature is set incorrectly without monitoring the system all day?

A simple test: On a clear sunny day (summer, around 11:00), check the current temperature of the collector and the tank on the regulation display. If the collector is at least 8–10 °C warmer than the tank, but the pump is not running – T-on is likely set too high. If the pump runs every few minutes for only short periods (30–90 seconds) – T-on is too low or T-off is too close to T-on. Both situations are easily recognizable without long-term monitoring.

Can I set the differential temperature myself or is it a job for a professional?

Basic settings can be done by any technically capable customer – the control menu on regulations such as Euroster 813 Solar or ZPS control stations is relatively intuitive and the manuals are in Slovak. I recommend, however, to have the installation manual for the regulation available and at least basic knowledge of how a solar system works. More complex settings (multi-zone systems, priority schemes, integration with a boiler) and the first system startup should be carried out or at least checked by a professional. The topic of connecting with a boiler and a TÜV tank is also covered in the article How to connect the solar system regulation with a boiler or TÜV tank.

Do I need to change the differential temperature setting after replacing the collectors with a new type?

Yes, we recommend checking it. Different types of collectors (flat, vacuum tube, flat vacuum) have different thermal characteristics – different heating speeds, different heat losses, different stagnation points. Vacuum collectors heat up faster and have lower losses, so they usually allow a lower T-on setting. After replacing the collectors, always monitor the system for a few days and adjust the settings as needed.

Conclusion: differential temperature is the foundation, not an add-on

Correct setting of the differential temperature is the foundation of an efficiently operating solar system. It is not a technical curiosity for enthusiasts – it is a key parameter that determines how much energy your system actually extracts from solar radiation and how long the circulation pump will last.

From the experience with dozens of customers whose regulation settings we have fine-tuned, the results are always the same: properly set hysteresis reduces the number of pump starts by 60–80 %, increases the efficiency of heat transfer and prolongs the pump's lifespan. And all it takes is one hour at the regulation display and knowing what you are setting and why.

If you are unsure which regulation to choose for your system – whether a simple differential thermostat or a more complex control station – take a look at the comparison in the topic How to choose regulation for a solar system: differential thermostat vs. control station. And if you are looking for specific products suitable for your system, you will find an overview in the category of regulations for solar systems.

Do you have a question about this topic?

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