Converting Protherm Raja to Ray – Differences Between the Older and Newer Model

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Protherm Raja and Ray – the story of one evolution

When a customer comes into the shop and asks "I'd like a Protherm electric boiler, but I don't know whether to go for the Raja or the Ray," the answer is simple: the Raja no longer exists as a manufactured product. The Protherm Raja was the predecessor of today's Ray – in fact, it's the same basic concept of an electric boiler with modulated output, which underwent several generational changes over the years. Nevertheless, this question keeps coming up, because Rajas are still installed in thousands of households, service companies still work on them, and owners of older installations need to know what has changed and what they can expect from the newer Ray.

This article is written for those who are either looking for a replacement for their old Raja, or want to understand why Protherm moved on to the Ray, and how the two differ in terms of construction, electrics, control, and practical everyday operation. This isn't a marketing comparison – it's a technical analysis with real values and insights from installation practice.

A brief history: from the Raja to the Ray

The Protherm Raja appeared on the market in the first half of the 2000s as one of the first modulated electric boilers available on the Central European market at a reasonable price. The basic idea was ingenious for its time: instead of a simple on/off boiler with fixed output, you got a device that could adapt to the current heat demand. The Raja was available in several power variants – typically 6, 9, 12, 15, 18 and 24 kW – and controlled them by step-switching the heating elements.

Production of the Raja was gradually discontinued around 2016–2018 (depending on the market and distributor stock). It was replaced by the Protherm Ray, which has gone through several revisions – the current generation available today brings substantial improvements over the Raja not only in design, but also in electronics, communication protocols, and safety features.

Physical construction: what changed inside and out

Dimensions and weight

The Raja had relatively compact dimensions, but its steel casing gave a more massive, "technically industrial" impression. A typical Raja 18 kW measured approximately 400 × 650 × 170 mm (w × h × d) and weighed around 22–26 kg depending on the variant. The Ray, by comparison, is slimmer – the current Protherm Ray 18KE measures approximately 400 × 700 × 180 mm and weighs around 28 kg, so it's slightly heavier (larger heat exchanger and improved thermal insulation), but visually looks more elegant thanks to a cleaner front panel design.

Practical consequence: when replacing a Raja with a Ray in a boiler room, you usually don't need to change the position of the wall mounting brackets or the recess dimensions – the boilers are designed so that retrofitting is as easy as possible. However, you should check the height of the space, as the Ray is 30–50 mm taller for some power outputs.

Hydraulic circuit and heat exchanger

The Raja used a simpler tubular heat exchanger with straight heating rods immersed in water. The design was functional but limited the possibilities for hydraulic connections. The Ray moved to a more compact heat exchanger with better water circulation around the heating elements, which improved thermal efficiency and reduced the risk of localized overheating. The result is also quieter operation – the Raja was sometimes audible when switching power stages due to pressure surges, while the Ray is significantly quieter in this respect.

Important for servicing: the heating elements (heating rods) in the Ray are designed to be more accessible for replacement. On the Raja, replacing a rod was sometimes complicated due to the sealing and position, whereas on the Ray it can be done without completely removing the boiler from the wall.

Electrical section: power stages and control

Step vs. modulated power control

This is one of the most important differences between the generations. The Raja controlled output by switching discrete heating elements – for example, the Raja 18 kW could operate in steps of 3, 6, 9, 12, 15 or 18 kW. Each element was either on or off. This step switching caused characteristic jumps in output, which were also reflected in current draw – when switching from one stage to another, there was a short but noticeable surge on the grid.

The Ray retains the principle of step-switching the elements, but adds finer control using PWM (pulse-width modulation) at some stages, ensuring a smoother increase in output and less pronounced load jumps. The Ray's controller also more actively monitors the return water temperature (not just the outlet temperature) and optimizes cycling accordingly – the result is fewer switching cycles per hour and a longer relay service life.

Current requirements and fusing

The Raja 18 kW draws a nominal 26 A per phase at full output (at 3×400V). The same applies to the Ray 18 kW. The figures here are the same – physics doesn't change. The difference lies in how full draw is reached: the Raja jumped from zero to full output relatively quickly, which could cause problems with a weaker electrical connection. The Ray has a "soft-start" function implemented (gradual increase in output), which spreads the load increase over time and reduces the amplitude of the current surge.

In practice, this means that an older house with a less generously rated electrical installation, where the Raja occasionally tripped the circuit breaker on start-up, may run more reliably with the Ray – although, of course, the basic requirements for fuse protection and cable cross-section remain the same. You can find more details on electrical requirements in the article Connecting and Installing the Protherm Ray – electrical installation and fusing requirements.

Control and boiler intelligence

Raja – simple control with a thermostat

In its basic configuration, the Raja was controlled by an internal controller with a setting for the desired heating water temperature and an external room thermostat (230V contact input or voltage-free contact). Weather-compensated control was available only when using an external controller (typically Protherm Thermaster or another compatible unit). The communication protocol was proprietary – the Raja only "understood" certain controllers from the Protherm/Vaillant portfolio.

Ray – OpenTherm and eBUS natively

This is a revolutionary change. The Ray has native support for both the OpenTherm (OT) and eBUS protocols. OpenTherm is an open communication standard that enables bidirectional digital communication between the boiler and the controller. Over OpenTherm, the thermostat doesn't just tell the boiler "turn on/off" – it sends the required water temperature, and the boiler reports back its current status, error messages, outlet temperature, etc.

The result is dramatic in terms of comfort and savings: the Ray with an OpenTherm controller operates with much finer modulated output, maintains temperature with an accuracy of ±0.5 °C instead of ±2–3 °C with a regular thermostat, and shortens the overall heating time while being more efficient. The weather compensation curve is built directly into the Ray, and the controller sets it automatically based on the outdoor temperature (when a sensor is connected). You can read more about this setting in the article Setting and Programming the Protherm Ray – control, weather compensation curve, OpenTherm.

Power variants: Raja vs. Ray – what's available today

The Raja was manufactured in various power variants depending on the market – in Slovakia, the most common models were 6, 9, 12, 15, 18 and 24 kW. The Ray moved to a somewhat different power scale. The current lineup includes:

Protherm Ray 6KE – 6 kW, suitable for small apartments and low-energy houses
Protherm Ray 9KE – 9 kW, a popular variant for houses up to 80–100 m²
Protherm Ray 12KE – 12 kW, suitable for houses of 100–130 m² with good insulation
Protherm Ray 14KE – 14 kW, an intermediate step for more demanding conditions
Protherm Ray 18KE – 18 kW, for larger houses or older buildings

The 21, 24 and 28 kW power variants are part of the extended portfolio and are designed for large houses or commercial applications. The Raja had 15 and 24 kW variants that have no direct equivalent in the Ray (the Ray skips to 14 and 18 kW), which should be kept in mind when choosing a replacement. Choosing the right output is discussed in more detail in the article What Output Do I Need for a Protherm Ray – 6, 9, 12, 14, 18, 21, 24 or 28 kW?

Safety features – what the Ray adds

The Raja had the standard safety features of its time: a maximum temperature safety thermostat, a pressure gauge, a safety valve, and a low-pressure switch. These features are of course also present in the Ray, but the Ray adds several modern protective mechanisms:

Electronic flow sensor – the Raja relies on minimum water pressure as a flow indicator. The Ray has an electronic sensor that actively monitors whether water is actually circulating. If the pump becomes blocked, it shuts down the boiler before overheating occurs.
Frost protection – automatic activation of the boiler at temperatures below 5 °C; the Raja did not have this feature natively (it could only be achieved with an external controller).
Diagnostics and error codes – the Raja only displayed basic fault states via an LED indicator. The Ray has an alphanumeric display with readable codes (e.g. E01, E05, etc.) and an error history. This significantly simplifies servicing.
Pump anti-seizing protection – after 24 hours of inactivity, the Ray automatically briefly activates the pump, preventing it from seizing during long-term shutdown (e.g. in summer).

Practical replacement scenarios from the field

Scenario 1: Raja 18 kW in a family house from 1998

A homeowner in Trenčín called with a problem: his Raja 18 kW, installed in 2009, started showing unstable operation – sometimes it didn't heat up, sometimes it ran at full output without reason. Diagnostics revealed a damaged control module. Spare parts for the old Raja had been unavailable since 2021. He decided to replace it with a Protherm Ray 18KE.

Installation result: the boiler fit onto the same wall mounting points, and the ½" and ¾" hydraulic connections matched the existing piping. The electrical installation was 5×6 mm² with 32A circuit breakers – also suitable for the Ray. They replaced the old bimetallic room thermostat with an OpenTherm controller, and the customer immediately noticed improved comfort and estimates savings of about 12–15% on electricity consumption compared to before (although part of the savings is due to better control, not just the boiler itself).

Scenario 2: Raja 12 kW in an apartment with underfloor heating

An apartment unit in Bratislava, 85 m², complete underfloor heating, Raja 12 kW from 2011. The owner noticed that the boiler ran almost continuously even at 5 °C outside. The problem turned out to be a combination of reduced output from aged heating elements (sludge on the elements) and the primitive control of the old thermostat, which didn't allow setting weather compensation.

Solution: replacement with a Protherm Ray 12KE with an OpenTherm controller and outdoor sensor. Underfloor heating is ideal for weather compensation – the Ray can maintain water temperature at 35–45 °C instead of the 55–60 °C the Raja was set to without weather compensation. The result? Significantly more comfortable operation, no temperature swings, evenly warm flooring, and consumption dropped by an estimated 18% compared to the previous year.

Scenario 3: Raja 9 kW – replacement with a Ray of a different output

A customer from Martin inherited a house from her parents with a Raja 9 kW from 2007. In the meantime, the house had undergone façade insulation and window replacement – the original heat loss calculation of 9 kW was for the original state of the house; after insulation, the actual demand is around 6–7 kW. The logical step was to switch to a Protherm Ray 6KE, which, at the same electricity price, saves money because the boiler doesn't have to modulate an unnecessarily large output downward. A smaller boiler with higher average utilization operates more efficiently than a large boiler constantly running at minimum output.

What stays the same – and why that's a good thing

It's also worth mentioning what hasn't changed between the Raja and the Ray, since these are precisely the features that form the basis of the whole product line's popularity:

Wet electric heating – both boilers use heating rods immersed directly in the heating water (wet method), with no combustion, no emissions, and no flue. Operation is quiet and low-maintenance.
Closed expansion system – both operate with a closed circuit and a built-in expansion vessel (external for some outputs), which is standard for the European market.
Modularity of heating stages – the philosophy of multiple heating elements switched as needed remains the basic principle of both generations.
Hydraulic compatibility – the connections are standardized, which makes retrofitting easier.
Long service life – with proper maintenance (air venting, water pH check, cleaning), the Ray lasts just as long as the Raja – 15 years or more is not unusual. You can find more on this topic in the article Maintenance and Servicing of the Protherm Ray – what to check and how often.

Controller compatibility: what works and what doesn't

This is in practice one of the most frequently addressed questions. A customer has an old Raja with an external Protherm Thermaster or Therm TR 21 controller and asks whether the same controller can be connected to the new Ray.

The answer depends on the specific controller model:

Old bimetallic thermostats (voltage-free contact) – also work with the Ray, connected to terminals 1–2 (ON/OFF input). However, you won't benefit from OpenTherm control.
Protherm Thermaster – most old Thermasters communicate via the proprietary Protherm protocol. These controllers are usually NOT compatible with the Ray via digital bus. They can only be connected as an ON/OFF thermostat.
OpenTherm controllers (e.g. Vaillant VR 90, Honeywell evohome, Salus RT500RF, Netatmo Thermostat) – fully compatible with the Ray via the OpenTherm protocol. This is the recommended configuration.
eBUS controllers – the Ray supports eBUS communication, making it compatible with controllers from the Vaillant/Protherm group, e.g. the VRC 700.

Spare parts availability and serviceability

This is a purely practical but crucial question: if you have a Raja and it breaks down, what can you do? Protherm, as the manufacturer, guarantees spare parts availability typically for 10 years after production ends. For the oldest Rajas (production ended 2016–2018), this means spare parts should still be available through authorized distributors until 2026–2028. In practice, however, it's not that simple – some electronic components arrive on the market with delays or are only available as complete control boards (not individual components), which increases repair costs.

For the Ray, the situation is different – it's a current model still in production, and spare parts (heating rods, control board, pump, expansion vessel) are readily available. In addition, thanks to the model's widespread use, there is more repair experience in the service network.

Practical recommendation from the field: if your Raja is working and you just want to improve control, consider adding an OpenTherm controller with an ON/OFF input (works with the Raja via the contact). If the Raja shows faults in the heating elements or control electronics and is older than 12 years, replacing it with a Ray is usually economically worthwhile – repair cost + parts vs. a new boiler with an extended warranty and modern control. Common fault conditions and their solutions are discussed in the article Common Protherm Ray Faults – error codes, outages, and how to fix them.

Photovoltaics and the Raja/Ray – a fundamental difference

A topic that has become increasingly important in recent years is the integration of the electric boiler with photovoltaics. The Raja was not prepared for this at all – it had no input for output control from an external system. The only thing that could be done was to connect a PV inverter with an export management function to an output relay and simply turn the boiler on/off according to the energy surplus. Finer control – e.g. increasing the boiler's output proportionally to the amount of surplus solar energy – was not possible.

The Ray has an input for output control (typically via PWM or a digital input) that is compatible with modern PV controllers and energy managers (e.g. Kostal, SMA, Fronius Ohmpilot, iSelect). This allows smooth use of surplus energy from solar panels to heat the heating water – with variable output between minimum and maximum. This kind of integration is described in detail in the article Protherm Ray and Photovoltaics – how to use surplus energy from solar panels.

Noise emissions and operating comfort

Customers switching from the Raja to the Ray almost always notice one thing right away, within the first few days: the Ray is significantly quieter. The Raja made characteristic relay "clicks" when switching power stages and occasional hydraulic banging with sudden changes in flow. In some installations (weaker expansion vessel, short piping runs), this was quite loud and disruptive, especially at night.

The Ray addresses this issue on several fronts: better relay damping, slower stage switching with short overlaps between them, and a higher-quality hydraulic heat exchanger design. In practice, the Ray produces the sound of the pump (50–55 dB up close) and an occasional soft click when changing stages – nothing that would disturb sleep through the boiler room wall.

Installation differences and recommendations for installers

For installers who have worked with Rajas and are now switching to the Ray, it's good to know the following:

The Ray's wall mounting points are standardized according to the norm, but the spacing differs from the Raja by 20–40 mm – use the template included in the packaging.
The Ray's electrical connection is more clearly labeled – the terminal block is accessible without removing the front cover, which is appreciated during inspections.
The blocking terminal for dual-tariff HDO (ripple control) is a native feature of the Ray; on the Raja, this was sometimes handled with an external relay.
The direction of the hydraulic connection (inlet/outlet) is more clearly marked – on older Raja production runs, there were cases where the installer swapped the inlet and outlet because the labeling was unclear.
The Ray requires a minimum water pressure of 0.8 bar; the Raja tolerated slightly lower pressure. On older installations with lower pressure, check the expansion vessel and top up or replace it if necessary.

Conclusion before the FAQ: is it worth waiting or replacing now?

There's no universal answer to this question, but in practice: if the Raja is running reliably and the control suits you, there's no reason to replace it just because of its age. An electric boiler is a simple appliance – no combustion, no condensate, no flue – and with good water quality, the heating elements can last 15–20 years.

Replacement is recommended when: spare parts are unavailable, the repair cost would exceed 40–50% of the price of a new boiler, or when you're planning to modernize the system (PV panels, a new controller, weather compensation) and the Raja would be a limiting factor. In such a case, the Ray isn't just a replacement – it's a fundamental upgrade to the entire system.

Frequently Asked Questions (FAQ)

Can I replace a Raja with a Ray without changing the electrical installation?

In most cases, yes, if the Raja and Ray have the same output (e.g. Raja 18 kW → Ray 18KE). The cable cross-section and fusing remain the same, since the power draw is identical. You only need to check the condition of the cables and terminal block in the distribution board. If you're changing the output (e.g. installing a Ray 12KE instead of a Raja 18KE), you can size the fuse protection for a lower current, which is only an advantage.

Does an old Thermaster controller work with the Ray?

It depends on the variant. If the Thermaster communicated with the Raja via a digital bus (eBUS/proprietary Protherm protocol), it likely won't be fully compatible with the Ray in digital mode. It can be connected as a voltage-free contact (ON/OFF), but you'll lose modulation. When replacing the boiler, we recommend also investing in a modern OpenTherm controller – the difference in comfort and consumption is significant.

Why Ray 14KE and not 15KE like the Raja had?

When transitioning to the Ray, Protherm changed the power scale – 14 kW instead of 15 kW is the result of optimizing the heating elements and three-phase load distribution. In practice, a 1 kW difference has no impact on covering heat demand – if the Raja 15 kW suited you, the Ray 14KE will be a fully adequate replacement. For most houses, this difference falls within the margin of calculation values and you'll never notice it in operation.

How long does replacing a Raja with a Ray take in practice?

An experienced installer can complete the replacement in 4–6 hours, including venting the system, setting up the controller, and commissioning. The hydraulic and electrical connection usually takes about 1.5–2 hours – the rest consists of setup, checks, and an operational test. For a more complicated electrical installation (e.g. dual-tariff HDO, PV control), add another 1–2 hours.

Will I lose the warranty if I have the Raja repaired by an unauthorized service?

For the Raja, the warranty usually only applies during the first 2 years after installation – older units are out of warranty regardless of who performs the service. The new Ray is covered by the statutory 2-year warranty, and Protherm offers an extended warranty upon product registration. Repairs during the warranty period should be carried out by an authorized service center to keep the warranty valid. After the warranty period, any qualified electrician can service the unit.

Is the Raja environmentally outdated compared to the Ray?

Both boilers are equally "green" in terms of operational emissions – an electric boiler doesn't burn anything, so direct emissions are zero. The environmental footprint depends on the source of the electricity. The difference lies in conversion efficiency – the Ray, with modern control and weather compensation, typically consumes 10–20% less electricity for the same level of thermal comfort, which of course has both an environmental and economic impact. With a green electricity tariff (renewable sources), the advantage of electric heating is even greater.

Have a question on this topic?

Can't decide, or dealing with a specific situation in your household? Write to us – we'll be happy to help.