Setting up and programming the Protherm Ray – control, weather compensation curve, OpenTherm

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Setting up and Programming Protherm Ray – Control, Weather Compensation Curve, OpenTherm

The Protherm Ray electric boiler is one of those appliances where installation makes up only a smaller part of the overall work. Most of the effort – and at the same time most of the real potential for energy savings – lies in properly setting up the controls. The boiler does work "out of the box" with minimal intervention, but without thorough programming it operates unnecessarily energy-intensively, reacts imprecisely to weather changes, and oscillates needlessly between on and off. In this article we'll cover every important aspect of setup – from the first start-up, through the weather compensation curve, to OpenTherm communication with an external controller.

First start-up and basic menu orientation

The Protherm Ray features a color graphic display with an intuitive interface. Unlike older Protherm Raja boilers, where settings were made via simple rotary controls and numeric codes, the Ray has a full menu in Slovak (and several other languages). After first switch-on, the boiler goes through an initialization sequence, checks input parameters (voltage, phases for the three-phase model) and displays a home screen with current values: boiler water temperature, set target temperature, outdoor temperature (if an outdoor sensor is connected), and domestic hot water temperature if applicable.

You access the service menu by holding a button combination as described in the manual (usually Home + down arrow for 5 seconds). The service menu is divided into several parameter groups – D.0xx for basic settings, D.1xx for weather compensation control, D.2xx for domestic hot water, D.3xx for communication protocols, and D.5xx for diagnostics and fault history. Each parameter has a numeric identifier, a description, and a value range. When changing a parameter, always confirm with OK, otherwise the value will not be saved.

The first thing to set during installation is the language (parameter D.000), units (°C vs. °F – leave it on Celsius), time zone, and daylight saving time. Next, it's important to set the type of heating system: low-temperature (underfloor heating, max 45–55 °C), medium (panel radiators or convectors, 55–70 °C), or high-temperature (older cast-iron radiators, 70–80 °C). This choice affects the preset temperature limits and the behavior of the weather compensation curve.

Temperature control – modes and their practical differences

The Protherm Ray supports three basic heating control modes, which differ in what quantity serves as the reference value for calculating the required boiler water temperature.

Fixed temperature (manual mode)

The simplest, and at the same time the most energy-costly way of operating. You set a specific boiler water temperature (for example 65 °C) and the boiler tries to maintain this temperature regardless of outdoor conditions. When it's +10 °C outside and the house is already warm, the boiler still keeps the water at 65 °C and overheats unnecessarily. This mode only makes sense for simple installations without an outdoor sensor, for short-term testing, or for hot water tanks where a fixed temperature is desirable for hygienic reasons (Legionella prevention at 60 °C).

Room control (room thermostat)

The boiler operates based on a signal from a room thermostat. The thermostat can be connected either as a simple ON/OFF switch (eBUS terminals or potential-free terminals), or via the OpenTherm protocol (see below). In ON/OFF mode the boiler either runs at full power or is switched off – there is no power modulation. The advantage is simplicity and compatibility with all thermostats on the market. The disadvantage is energy waste and temperature fluctuation in the room.

Weather compensation control (control based on outdoor temperature)

This is the most efficient and at the same time the most sophisticated mode that the Protherm Ray natively supports. The boiler monitors the outdoor temperature via an external sensor (NTC 10k sensor mounted on the north-facing wall of the house, at least 2 m above ground, out of reach of direct sunlight) and, based on the so-called weather compensation curve, automatically calculates what boiler water temperature is needed to keep the house warm. The result is smooth and proactive control – the boiler starts heating more before the room temperature drops, which significantly improves thermal comfort and reduces energy consumption.

Weather compensation curve – what it is and how to set it

The weather compensation curve is a mathematical relationship between the outdoor temperature and the required boiler water temperature. It says: "If it's X degrees outside, the boiler should have the water at Y degrees." This relationship is linear and is defined by the slope (steepness of the curve) and, optionally, an offset (parallel shift).

Slope of the weather compensation curve – parameter D.100

In the Ray boiler menu, the weather compensation curve is primarily set via parameter D.100, which determines the slope of the curve. Values typically range from 0.2 to 4.0 in steps of 0.1. The higher the slope, the more strongly the boiler reacts to a drop in outdoor temperature by increasing the water temperature. Practical reference values:

Slope 0.3–0.6: Underfloor heating, new builds with passive or low-energy standards, houses with very good insulation (insulation thickness 20+ cm). The boiler operates with water temperature of 28–45 °C.
Slope 0.7–1.0: Panel radiators in a well-insulated house, renovated house with added insulation. The boiler operates with water temperature of 45–60 °C.
Slope 1.1–1.5: Older panel or cast-iron radiators, houses with medium insulation (masonry without insulation or with a thin layer). The boiler operates at 60–75 °C.
Slope 1.6–2.0+: Old cast-iron systems, poorly insulated buildings, cottage-type houses. High temperatures needed even in mild outdoor conditions.

Parallel curve shift – parameter D.101

The slope of the curve defines how quickly the water temperature changes with changes in outdoor temperature. The parallel shift (D.101) moves the entire curve up or down without changing the slope. If you have the correct slope but the house is still 1–2 °C too cold, shifting the curve up by +5 °C will solve this problem. In practice, the parallel shift is used for fine-tuning after the correct slope has been set.

Practical example: An installer set a slope of 0.9 for a renovated family house from the 1980s with 10 cm of added insulation. After the first week of operation, the customer reported that the outer rooms (north side) were still 1.5 °C cooler than the thermostat requested. Instead of changing the slope, it was enough to increase the parallel shift from 0 to +7 °C – the curve was raised evenly and the problem was solved without any further intervention.

Effect of the room sensor on the weather compensation curve

The Protherm Ray also supports combined control – weather compensation + room sensor (parameter D.102). In this mode, the weather compensation curve determines the base water temperature, but the boiler also corrects it based on the deviation of the room temperature from the set value. If the room is warmer than set, the boiler lowers the water temperature below the curve value. If the room is cooler, it raises it. This combination is the best in terms of efficiency and we recommend it in all installations where technically possible.

Maximum and minimum boiler water temperature – protective limits

Besides the curve itself, you also need to correctly set the protective temperature limits:

D.120 – maximum boiler water temperature: For underfloor heating set 45 °C (many underfloor systems have a thermostatic valve, but the limit in the boiler is an additional safeguard). For panel radiators 70–75 °C, for cast-iron systems up to 80 °C. The Protherm Ray 6KE and larger models have a hardware limit of 80 °C.
D.121 – minimum boiler water temperature in weather compensation mode: Usually 20–25 °C. This prevents the boiler from lowering the water temperature so much in mild weather that the circulation pump circulates cold water and causes thermal stress.
D.130 – frost protection temperature: Default 8 °C. The boiler automatically switches on if the boiler water temperature drops below this value, even in holiday (eco) mode.

Programming the time schedule

The Protherm Ray has a built-in weekly programmer, which allows you to set different temperature modes for each day of the week with 30-minute resolution. You access it via the main menu → Schedule → Heating.

The available daily modes are:

Comfort (day): Full required temperature, for example 21 °C in the room or a defined boiler water temperature according to the curve.
Eco (night/absence): Reduced temperature, typically 3–5 °C lower than comfort. The boiler doesn't shut off entirely, it just operates at a lower target output.
Anti-frost: Frost protection only, the boiler maintains the minimum.

A typical weekly program for a working family: Weekdays – Eco from 23:00 to 6:00, Comfort from 6:00 to 8:00, Eco from 8:00 to 15:30 (when everyone is at work/school), Comfort from 15:30 to 22:30, Eco from 22:30 to 23:00. Weekend – Comfort from 7:00 to 23:00, Eco at night.

Important note regarding weather compensation control: the boiler needs a certain amount of time (so-called lead time) to reach the required water temperature before morning. Parameter D.160 (comfort period lead time) is set in minutes – for a well-insulated house, 30 minutes is usually enough, while an older building with greater thermal inertia may need 60–90 minutes. If you wake up to a cold apartment in the morning, increase this parameter.

OpenTherm – what it is and why it matters

OpenTherm is a digital communication protocol between the boiler and the thermostat. Unlike a classic ON/OFF thermostat, which only tells the boiler "switch on" or "switch off," OpenTherm communicates bidirectionally and transmits a whole range of information: required boiler water temperature, actual temperature, required output, boiler status, error codes, and other parameters.

The practical difference is significant. With a standard ON/OFF thermostat, the boiler runs at full power (or a fixed set temperature) every time it switches on, and when the thermostat reaches the required room temperature, the boiler switches off. It cycles between on and off. With OpenTherm, the thermostat continuously tells the boiler: "I need you to raise the water temperature to 52 °C" – and the boiler adjusts its output accordingly. The result is smooth operation, fewer cycles (which directly affects the lifespan of the heating elements in an electric boiler), better thermal comfort, and typically 5–15% lower energy consumption.

Activating OpenTherm on the Ray boiler

OpenTherm communication is activated in the service menu via parameter D.300. Set the value to 1 (or "OpenTherm" depending on the firmware version). The terminals on the boiler are marked OT+ and OT– (or BUS and COM in some diagrams). Two wires from the OpenTherm thermostat are connected to these terminals – with OpenTherm, polarity is usually not critical (the protocol is tolerant), but we recommend following the manufacturer's markings.

Wiring: For OpenTherm, a standard two-core shielded cable of 2×0.75 mm² or 2×1 mm² with a maximum length of up to 50 meters is sufficient. For longer runs (large houses, boiler in the basement) use 2×1.5 mm². Ground the shielding on only one side (usually at the boiler) to avoid ground loops.

After activating OpenTherm, we recommend verifying communication via diagnostics D.500 – the boiler should display the ID of the connected thermostat and the communication status (OK). If communication is interrupted or the thermostat is not compatible, the boiler automatically switches to a backup ON/OFF mode and shows a warning icon on the display.

Thermostats compatible with OpenTherm for the Ray

The Protherm Ray is compatible with most thermostats supporting the OpenTherm standard version 2.2+. Verified models from practice include: Honeywell/Resideo T6/T4 OpenTherm series, Siemens RDG and RDE series, Salus RT and iT series, Danfoss TP and CF series with OT, Nest Learning Thermostat (via an OT adapter), Netatmo Smart Thermostat, Tado Smart Thermostat, and others. Always check for explicit OpenTherm support before purchasing – a plain wifi thermostat usually does not support OpenTherm.

eBUS – alternative communication protocol

Besides OpenTherm, the Protherm Ray also supports the proprietary eBUS protocol, used by the Vaillant Group's own controllers (Protherm belongs to this group) – for example the Vaillant VRC 700, VRC 720, or the older VRC 470. eBUS is a two-wire, half-duplex protocol powered directly from the bus. If you plan to use a controller from the Vaillant VRC series, eBUS is a better choice than OpenTherm, because it transmits more specific information. For other thermostat brands, OpenTherm remains the preferred choice.

Domestic hot water (DHW) settings

The Protherm Ray does not have a built-in DHW tank, but supports connecting an external tank with a heating coil. The boiler controls the tank via an output relay (terminals BK1/BK2) and measures the tank temperature via an NTC sensor connected to terminals S1/S2 of the D.2xx group.

Key parameters for DHW:

D.200 – required tank temperature: We recommend 55–60 °C for hygienic reasons (Legionella prevention). Do not set below 55 °C for regular tank use.
D.201 – tank hysteresis: The temperature difference at which the boiler starts heating. Typically 5 °C – if you set the target to 60 °C and the hysteresis to 5 °C, the boiler will start heating at 55 °C and stop at 60 °C.
D.202 – DHW priority: If DHW priority is set (value 1), the boiler temporarily interrupts heating when there is a demand for tank heating (typically for 30–40 minutes) and focuses on heating the water. Once the required tank temperature is reached, it returns to heating. We recommend keeping this setting.
D.205 – anti-Legionella function: Once a week (day and time configurable), the boiler heats the tank to 65 °C regardless of the normal setting. We recommend activating this for all tanks over 100 liters.

Smart Grid and PV Boost function – using photovoltaics

Modern firmware versions of the Protherm Ray support the Smart Grid Input (SGI) function – a digital input that switches the boiler into a special power mode when it receives a signal from an external system. This function is primarily used in conjunction with photovoltaics and an energy manager – this topic is discussed in detail in the article Protherm Ray and Photovoltaics – How to Use Surplus Energy from Solar Panels.

In short: parameter D.700 activates the SGI input. When a logic level of 1 is present on the input, the boiler raises the tank temperature (or boiler water temperature) by a set number of degrees (D.701) above the normal required value. This way the boiler consumes the photovoltaic surplus instead of exporting it to the grid at a low feed-in price. If you have PV on your house and are considering the Protherm Ray 9KE or a larger model, this setting can significantly improve the economics of the whole installation.

Output control – power stages and modulation

An electric boiler cannot modulate its output smoothly like a gas condensing boiler, but the Protherm Ray addresses this limitation with intelligent power stages. Each model has a different number of heating elements, and the boiler switches between and combines them to achieve the smoothest possible control:

Protherm Ray 6KE: typically 3 power stages (2+2+2 kW)
Protherm Ray 9KE: 3 stages (3+3+3 kW) with three-phase supply
Protherm Ray 12KE: 4 stages (3+3+3+3 kW)
Protherm Ray 14KE: 4–5 stages
Protherm Ray 18KE: 6 stages of 3 kW each

Parameter D.010 allows you to limit the maximum output of the boiler – for example, if you know your house needs a maximum of 12 kW even at -15 °C, set the limit on the 12KE boiler so the remaining elements are not used. This makes sense from the point of view of electrical installation fusing or contractual limits on the connection point.

Circulation pump settings

The Protherm Ray has integrated control of the circulation pump (D.4xx parameter group). This is an area that many installers neglect, yet it has a significant impact on the efficiency of the system.

Parameter D.400 sets the pump mode: constant speed, proportional pressure, or constant pressure. For underfloor heating (a low-temperature system with a large surface area), we recommend proportional pressure – the pump automatically reduces its output when heat demand is low, which saves electrical energy and reduces noise. For radiator systems with thermostatic valves, constant pressure is suitable. Parameter D.401 sets the target pressure or required speed.

D.410 – pump overrun: the time the pump runs after the boiler switches off. Default is 3 minutes. For systems with underfloor heating with large thermal inertia, you can increase it to 5–7 minutes so that the heat from the water is distributed evenly.

Diagnostics and fine-tuning after installation

After setting all parameters, we recommend performing a trial run lasting at least 3–5 days before handing the system over to the customer. Monitor the following indicators:

Boiler water temperature vs. outdoor temperature: Verify that the weather compensation curve produces reasonable values under different outdoor conditions (ideally test at at least 3 different outdoor temperatures).
Cycling frequency: If the boiler cycles more than 4–6 times per hour, something is wrong – too much output, incorrectly set hysteresis, or a hydraulic problem.
Return vs. flow temperature: The temperature difference ΔT should be 5–15 °C. Too small a ΔT indicates excessive flow, too large a ΔT indicates low flow (clogged filter, air in the system, incorrect pump setting).
OpenTherm communication status: Diagnostics D.500 – check that communication is stable without dropouts.

An advantage of the Ray boiler over older Raja models is the fault history stored in memory (D.520 – the last 10 events with timestamps), which greatly simplifies diagnostics. You can find more about error codes in the article Common Protherm Ray Faults – Error Codes, Outages, and How to Fix Them.

Changing parameters without access to the service menu – what the customer can do themselves

Not all settings are available in the service menu, which is protected by a PIN code. The customer has access to:

The required room temperature (if a room thermostat is connected via the Ray)
Switching between Comfort/Eco/Anti-frost modes
Setting the time program
The required DHW temperature (within the allowed range set by the installer)
Holiday mode (from–to date, temperature)

We recommend explaining these functions to customers when handing over the system, and strongly advising them not to switch off the boiler with the main power switch (only to lower the temperature or activate holiday mode). Completely switching off the boiler in winter can lead to water freezing in the pipes during a prolonged absence if frost protection is disabled.

Firmware update

The Protherm Ray allows firmware updates via the service interface (BUS connector) using the Protherm Toolkit software on a technician's laptop. After an update, it is recommended to check and, if necessary, reset all service parameters, since some updates reset certain parameter groups to factory values. Always write down or photograph the current settings from the service menu before updating.

Frequently Asked Questions (FAQ)

How do I know if the weather compensation curve is set correctly?

A correctly set curve means the room temperature is stable without significant fluctuations during the day regardless of outdoor conditions, and the boiler does not operate in short cycles. Practical test: at an outdoor temperature of -10 °C, the boiler water temperature for a typically insulated house with panel radiators should be around 65–70 °C. At 0 °C, around 55–60 °C. If the values are significantly higher and the house is hot, the slope is too steep. If the house is cold even at maximum slope, either the curve isn't enough, or there's an issue with the house's insulation, insufficient boiler output, or the system's hydraulics.

Can I connect the Protherm Ray to a regular wifi thermostat?

Yes, most wifi thermostats on the market work as an ON/OFF switch and connect to the potential-free terminals of the boiler. This works reliably, but the boiler will not modulate its output – it will only be switched on and off according to the signal from the thermostat. If you want modulation and full efficiency, choose a wifi thermostat with explicit OpenTherm support (Netatmo, Tado, Salus iT600). These are priced from €80 to €200, and the investment pays off through lower energy consumption.

What does the "OpenTherm comm. error" message on the display mean?

The boiler has lost communication with the OpenTherm thermostat. Check the physical cable connection (OT+/OT– terminals on both the boiler and the thermostat), and check that the thermostat is powered and functional. Common causes also include oxidized terminals or a loose cable. If the cable is fine and the thermostat responds, check in the thermostat's settings whether the OpenTherm output is enabled. Once communication is restored, the boiler automatically resumes normal operation.

Is it possible to control the Protherm Ray remotely – via mobile phone or the internet?

The Protherm Ray does not natively have a wifi or ethernet module. Remote access is achieved via an external OpenTherm or eBUS thermostat with internet connectivity (e.g. Netatmo, Tado, Vaillant VRC 720 with wifi). These thermostats communicate with the boiler via OpenTherm and are also accessible via a mobile app. An alternative is installing the proprietary Protherm VR 900 module (myVAILLANT), if the specific boiler version supports it – check the documentation for your firmware version.

The Protherm Ray shows an incorrect outdoor temperature – what should I check?

The outdoor sensor is an NTC 10 kΩ sensor. Check its location – the sensor must be on the north-facing wall, out of reach of direct sunlight, ventilation grilles, air conditioning, or other heat sources. Check the sensor's resistance: at 20 °C it should be around 12 kΩ, at 0 °C around 32 kΩ, at -10 °C around 57 kΩ. A large deviation indicates a damaged sensor (replacement is simple and cheap – standard NTC 10k). Also check the integrity of the wiring – outdoor connections must be in waterproof boxes.

Can the Protherm Ray boiler also be connected to a smart home system (KNX, Loxone)?

Direct native integration via KNX is not available, but there are solutions: using an OpenTherm-KNX gateway (for example Produal OTD-KNX), or control via a smart thermostat compatible with the given system. Loxone most often communicates with the boiler via a Loxone Tree thermostat, which controls the boiler via OpenTherm. Some integrators also use a simple relay connection for larger systems, where Loxone takes over the entire control and only gives the boiler an ON/OFF signal – in this case OpenTherm must be disabled in the boiler and the boiler water temperature set manually.

Conclusion – why correct setup determines efficiency

The Protherm Ray is a technically sophisticated boiler with a wide range of tools for precise and efficient control. But as with any tool, it depends on how you use it. A correctly set weather compensation curve, under equivalent conditions, reduces electricity consumption by 8–20% compared to a fixed temperature. OpenTherm communication adds another layer of efficiency and comfort. The combination of both approaches with an intelligent time program is the maximum you can do for the efficiency of an electric boiler without altering the building itself.

If you're considering the right output for your house, also read the articles How to Choose a Protherm Ray Electric Boiler – Output According to House Area and What Output Does a Protherm Ray Need – 6, 9, 12, 14, 18, 21, 24 or 28 kW? For details on electrical connection, see the article Connection and Installation of the Protherm Ray – Requirements for Electrical Wiring and Fusing.

The range includes models from the Protherm Ray 6KE for smaller houses and apartments, through medium outputs such as the Protherm Ray 12KE, up to more powerful variants such as the Protherm Ray 18KE for larger family houses. Each of these boilers supports all the described control functions – they differ only in output and the number of power stages.

Do you have a question on this topic?

Can't decide, or are you dealing with a specific situation in your home? Write to us - we'll be happy to help.