Stepped Output Control in Thermona THERM EL Boilers – How It Works and Why It Matters
Stepped Output Control in Thermona THERM EL Boilers – How It Works and Why It Matters
When a customer decides on an electric boiler, most of their attention goes to the overall output of the unit – how many kilowatts the boiler can deliver to the system. Much less attention is paid to how the boiler regulates this output, i.e. how it behaves during normal, everyday operation when the building does not need full power. And this is exactly the area where a good boiler differs significantly from an average one. Stepped output control in the Thermona THERM EL series is one of the key technical features that determines whether heating will be comfortable, economical and long-lasting. In this article, we will look in detail at what stepped control actually means, how it specifically works in THERM EL boilers, why it is beneficial for the user, and what practical consequences it has on consumption, comfort and the lifespan of the unit.
What is stepped output control and why is it different from a gas boiler
At the outset, it is useful to understand the basic principle. An electric boiler heats water using electric resistance elements – coils or rod-shaped resistors, colloquially called "heating elements" or "heating cartridges". Each such element has a fixed power rating – for example 2 kW, 3 kW or 5 kW. The boiler cannot smoothly vary the output of a single heating element the way a modulating burner in a gas boiler can. Instead, it regulates output by switching individual heating elements on or off in various combinations. This technique is called stepped output control.
With a gas boiler with a modulating burner, you can have smooth control ranging, for example, from 20% to 100% of the rated output – the burner burns stronger or weaker depending on the current demand. An electric boiler without step control would only work in a binary way: either all elements are running at full power, or they are completely off. The result would be significant temperature fluctuations, frequent switching on and off (so-called short cycling), and unnecessarily high consumption. Stepped control fundamentally improves this – the boiler can adapt to the building's actual heat demand at any given moment.
How stepping is implemented in the Thermona THERM EL series
Thermona THERM EL boilers are available in several power variants – from a compact 8 kW model up to more powerful variants intended for larger buildings. Each model has several heating elements (usually three or more), which are switched on progressively – hence the term "steps". The boiler's control electronics monitor the current water temperature in the boiler, compare it with the set target temperature, and based on the difference between them, decide how many heating elements to switch on.
Specifically, it works like this: if the boiler is cold and the water temperature is significantly below the target value, all elements switch on at once – the boiler operates at full power. As the temperature approaches the set value, the control disconnects one or two elements – the boiler operates at reduced power. When the temperature reaches the target value, the elements disconnect completely, or only one maintenance element remains on, or the boiler switches to standby mode. This whole process repeats cyclically, with each cycle being substantially longer and smoother than simply switching the entire boiler on and off at once.
The result is that the boiler in practice operates at different power levels depending on the current outdoor temperature and the building's heat losses. In the transitional season (spring, autumn), one or two stages are sufficient, while during severe frost, full power is switched on. This behavior is transparent to the user – there is nothing to set manually, the electronics do it automatically.
Number of stages in individual THERM EL models
The THERM EL series includes models with different total outputs and different numbers of stages. For a better understanding, it is important to know how the stepping is specifically implemented:
- Thermona THERM EL 8 – a boiler with a rated output of 8 kW. It has three heating elements, and by combining their switching on and off, it allows operation at several power levels. It is ideal for smaller new-build houses or well-insulated homes with low heat loss.
- Thermona THERM EL 15 – a medium model with an output of 15 kW, suitable for typical family houses with a heat loss of up to 12–14 kW. Three heating elements with different power ratings allow finer stepping.
- Thermona THERM EL 23 – output of 23 kW, intended for larger family houses or smaller commercial buildings. More stages allow very flexible output adjustment.
- Thermona THERM EL 30 – 30 kW, typically for larger buildings or a combination of heating and intensive domestic hot water heating.
- Thermona THERM EL 38 – a powerful 38 kW model for demanding applications. The highest number of control stages in the series, which means very fine output adjustment even with large power differences between warm and cold weather.
In practice, this means that, for example, the THERM EL 23 model with three elements of 7–8 kW each can operate at levels of approximately 7–8 kW, 14–16 kW and 23 kW (plus a zero stage). Models with differently sized elements (for example 2 + 3 + 5 kW) can achieve even more combinations of switched-on elements – for example 2, 3, 5, 5+2, 5+3, 2+3 kW, etc. The manufacturer chooses element combinations so that the steps are distributed as evenly as possible.
Why stepping matters for operating economy
Here we come to the practical side of things that concerns every homeowner: how much does it cost? Electricity is more expensive per kilowatt-hour compared to natural gas or wood. That is why it is extremely important that the boiler does not waste energy – that it does not consume more than is needed at any given moment.
A boiler without stepping would work as follows: outside it is 5°C, the building needs 6 kW, but the boiler has only one stage – 15 kW. It switches on at full power, very quickly overheats the water, switches off, the water cools down, the boiler switches on again... This phenomenon is called short cycling and has several negative consequences:
- Increased heat losses – with each cycle, part of the thermal energy is lost to the surroundings of the boiler, to the pipes, to the expansion vessel. The shorter the cycle, the more percentage points are lost.
- Uneven room temperature – the system oscillates between overheating and undercooling. Radiators are either too hot or cold. Thermal comfort is lower.
- Mechanical wear – every time a heating element switches on, it is a thermal shock for it (a cold element suddenly heats up to operating temperature). Frequent short cycles shorten the lifespan of both the elements and the electromagnetic relays.
- Electromagnetic interference and load on the electrical grid – every time an element switches on, it is a sharp current surge. With very frequent switching, this can also be a problem for the home electrical installation.
Stepped control minimizes all these problems. When the boiler operates at one or two stages that correspond to the current heat demand, cycles are longer and smoother. The water temperature stays within a narrow band around the set value. The system is stable, comfortable, and wear is minimal.
Cooperation with weather-compensated control and a room thermostat
The boiler's stepped control works best in combination with other control tools. Thermona THERM EL supports connection to a weather-compensated controller and a room thermostat, which creates a two-level control system:
Room thermostat (or a programmable controller, or a smart thermostat) determines whether the boiler should be heating at all – i.e. if the room is already warm enough, it switches the boiler off completely regardless of the outdoor temperature. When the room temperature drops below the set value, the thermostat activates the boiler.
Weather-compensated control works on the principle of monitoring the outdoor temperature – based on a set curve (the so-called heating curve or weather compensation curve), it determines what temperature the water in the boiler should have at a given outdoor temperature. For example: at an outdoor temperature of 0°C, the water in the boiler should be 55°C, at –10°C it should be 65°C, at +5°C 45°C is sufficient. When the outdoor temperature changes, the control automatically adjusts the required water temperature, and therefore also how many stages the boiler operates on.
In combination with weather-compensated control, stepping makes even more sense: the boiler does not have to fight to maintain a high fixed temperature, but maintains a variable temperature depending on the weather. In the transitional season – which, even in Central European conditions, is most of the heating season – one or two power stages are sufficient. Full power is really only used during severe frosts.
The effect of stepping on heating element lifespan
This is a topic that customers mostly do not consider when purchasing, but they very quickly start caring about it if something breaks. Heating elements are a consumable part of every electric boiler – even though they are designed for a long service life, their durability depends on the operating conditions. The greatest enemy of heating elements is thermal stress – i.e. repeated rapid heating and cooling.
With stepped control, this heating is more gradual and the cycles are longer. Instead of hundreds of short switch-ons per day, you have dozens of long cycles, during which the element heats up slowly, works at a stable temperature for a longer period, and then cools down slowly. This is much more favorable for the material of the heating resistor.
In addition, Thermona THERM EL uses so-called heating element rotation – the control electronics alternately switch different elements on as "first" and "last", so that wear is evenly distributed among all elements. Without this function, the first element (the one that is always switched on first) would age considerably faster than the others.
Practical examples from the field – how stepping manifests in real operation
To give the topic a concrete shape, we will describe several typical scenarios encountered in practice.
Scenario 1: A 140 m² family house, good insulation, transitional season
The house has a heat loss of approximately 7 kW at a design temperature of –12°C. The owner has installed a Thermona THERM EL 15. In October, when it is 8–12°C outside, the house only needs 2–3 kW to maintain an indoor temperature of 20°C. The boiler operates at one stage (the smallest output), the cycles are long – the element switches on for 20–30 minutes, then switches off for a similarly long time. Electricity consumption is minimal, comfort is maximum, the temperature in all rooms is stable. The owner doesn't even notice the boiler ran all day, because there's no impact on temperature or noise.
Scenario 2: The same house during January frosts
It is –10°C outside, the house needs full power – 7 kW or more if the house has been unheated for several hours (for example during a night setback). The boiler switches on all three elements, operating at full power of 15 kW. Once the house reaches the required temperature, the control disconnects two elements, and the boiler maintains 20°C on just one stage – because the heat loss of this house at –10°C is approximately 7 kW, and one stage is sufficient to cover it. If the boiler had no stepping, it would have to switch the full 15 kW on and off very frequently just to maintain an average heat output of 7 kW.
Scenario 3: An apartment building or larger property with a THERM EL 38 boiler
A larger building with multiple apartment units, heat loss of 30 kW at –15°C. The Thermona THERM EL 38 has enough stages to cover the entire range from minimal load (spring/autumn, night, low-occupancy weekend) to maximum load (severe frost, morning warm-up after a night setback). The building manager will notice, above all, that electricity is consumed evenly throughout the day, not in short peaks. This can also have a positive impact on the calculation of the load profile and possibly on the tariff.
Scenario 4: Night setback and morning warm-up
A programmable thermostat sets a night setback to 17°C. At 5:30 in the morning, the daytime heating program starts – the required temperature jumps to 21°C. The house is cold, and the difference between the actual and required temperature is large. The boiler switches on all stages and operates at full power to warm the house as quickly as possible. As the rooms approach the target temperature, the stages are gradually disconnected, and the boiler smoothly transitions into maintenance mode. This transition is considerably smoother and more comfortable than with a simple on/off system.
Stepping and tariff optimization – off-peak tariff and thermal storage
An interesting topic that some customers do not fully take advantage of is the combination of stepped control with a low (off-peak/night) electricity tariff. If the customer has a suitable tariff schedule (e.g. D7 or D8 in Slovakia), they can set the boiler to operate at a higher output during the off-peak period and heat the water in the storage tank to a higher temperature. During the day, when the tariff rises, the boiler operates less intensively, or only to maintain temperature. Stepping plays a role here in that the boiler can very precisely control what output it uses when charging the storage tank and during normal operation.
For those considering this strategy, it is also worth reading the article Setting up and commissioning the Thermona THERM EL boiler – first start-up step by step, which describes in detail the programming options and setting of time programs.
What stepping cannot fix – system limits and correct sizing
It is important to be honest: stepped control is an excellent tool, but not a cure-all. It only makes sense if the boiler is correctly sized – that is, if its output corresponds to the actual heat loss of the building. If the boiler is significantly oversized (for example 38 kW for a house with a heat loss of 6 kW), even stepping cannot fully compensate for that. The minimum output of the boiler (one stage) may still be too high for normal operation, cycles will be short, and the system will operate inefficiently.
Conversely, if the boiler is undersized, it will constantly operate at full power without being able to maintain the required temperature. Stepping will not improve anything in this case, because the boiler will never reach the required temperature and the stages will never disconnect.
Therefore, choosing the correct boiler output is an absolute basis – stepping is then a "bonus" that further enhances this correct sizing. If you are not sure what output you need, we recommend reading the article What output of the Thermona THERM EL electric boiler do I need for my house? or How to choose an electric boiler Thermona THERM EL – output, space size and other criteria.
Electrical requirements and fusing for stepped switching
For completeness, we should also touch on the technical side of the electrical installation. Stepped switching of the elements has a direct impact on the current load on the home electrical installation. When each element switches on, there is a current surge – not dramatic (resistive load is more favorable than a motor load), but still a short-term increase in the drawn current. With a correctly designed installation, this is not a problem – the circuit breaker is sized for the maximum power draw of the boiler, i.e. for the full load of all stages at once.
In practice, this means that during stepped operation, the boiler loads the installation most of the time to only 30–70% of its rated capacity, which is favorable both for the electrical grid and for the circuit breaker. More information on electrical installation requirements can be found in the article Installation of the Thermona THERM EL electric boiler – procedure, electrical installation and inspection requirements.
Diagnostics and faults related to output control
If the stage control stops working correctly, there can be several causes. The most common problems from field service experience:
- Failure of one heating element – the boiler continues operating on the remaining elements, which reduces the total available output. This manifests as the house not heating up sufficiently even with seemingly correct settings. The THERM EL control electronics usually signal the fault with an error code.
- Temperature sensor fault – if the water temperature sensor (NTC sensor) reports incorrect values, the control switches elements on and off at the wrong times. The boiler may operate continuously at full power or not switch on at all.
- Welded relay contact – with very intensive short cycling (for example due to a significantly oversized boiler), the relay contact that switches a given element can become "welded" in either the closed or open position. This results in either a permanently switched-on element (overheating, with the safety thermostat disconnecting the boiler) or a permanently switched-off element (reduced output).
- Control board problems – less common, but they do occur. This manifests as irregular, unpredictable control behavior.
For a detailed overview of error codes and their solutions, we recommend the article Common faults in Thermona THERM EL boilers – causes, error codes and solutions.
Stepping compared to other control technologies
For a complete picture, it is worth comparing stepped control with alternatives that exist on the market:
- Frequency inverters – some powerful electric systems use a frequency inverter for smooth control of the heating element's output. This is a technology more related to heat pumps than to resistive boilers. For a standard electric boiler, it is economically unjustified.
- Triac (transistor) regulators – some boilers regulate output by phase cutting – the heating element receives only part of each phase of the alternating current, reducing its effective output. This method is smooth, but it produces electromagnetic interference (EMI) and is less suitable for a typical home installation.
- Pulse-width modulation (PWM) – a similar technique, where the element is switched on and off very rapidly with varying on-ratios. This too produces interference and has limited use.
Compared to these alternatives, stepped control is simpler, more reliable and cheaper to manufacture and maintain. It has no special electronic components that could easily fail. Control via relays (or solid-state relays – SSR) is a proven and durable technology. For home electric boilers in the 8–38 kW power class, this is a standard and proven control method, which, combined with correct sizing and weather-compensated control, gives excellent results.
Frequently asked questions about stepped output control in Thermona THERM EL boilers
Can I manually set how many stages the boiler operates on?
In normal operation, no – the boiler automatically controls the number of active stages based on the difference between the set and current water temperature. This is neither desirable nor necessary. Some of the boiler's service menus allow a technician during installation to set limits or disable specific elements (for example if one is damaged and awaiting a replacement part), but this is not a function for the regular user.
Is stepped control a replacement for a thermostat? Do I still need a room thermostat?
No, stepping and the room thermostat are two different, mutually complementary systems. Stepping regulates the boiler's output based on water temperature. The room thermostat regulates the boiler's operation based on the air temperature in the room. Both systems together create comfortable and economical heating – without a room thermostat, the boiler could keep heating even when it is warm outside and the house already has enough heat.
Does the number of stages affect the boiler's noise level?
Resistive heating elements do not produce noise by themselves. The boiler may be audible when the relay switches on (a short click) and during water circulation (the pump). With more active stages, the flow through the boiler is higher, which may slightly increase the sound of the circulation pump – but in practice, the difference is almost imperceptible.
What happens if one heating element burns out? Will the boiler stop working?
No, the boiler will continue to operate – on the remaining elements and stages. Of course, the total available output will be reduced. The control electronics signal the fault with an error code, so the customer will be informed of the problem. Depending on the severity of the winter and the house's heat loss, the house may continue to be heated, just with a smaller reserve. The element should be replaced as soon as possible – see the article on maintenance and service.
Can stepped control help when combined with photovoltaic panels?
Yes, and quite significantly. In a photovoltaic system where the boiler is directly powered by surplus energy from PV panels, it is beneficial if the boiler can operate at different power levels – depending on how much energy the PV system is currently producing. Many smart energy management systems (e.g. surplus management systems) send the boiler a signal "take this much", and stepped control makes this possible precisely. Not every combination is straightforward (it depends on the specific management system), but the principle is correct and works in practice.
How long does one control cycle last during normal operation?
This depends on the size of the system, the volume of water in the system, the set hysteresis, and the current heat load. In normal operation, we are looking at 10 to 40 minutes per cycle (switch on – reach temperature – switch off – drop – switch on...). With a correctly sized system with sufficient water volume (possibly with a hydraulic separator or storage tank), the cycles are longer, which is desirable. If the cycles were shorter than 5–7 minutes, it is a sign that the system is oversized or has too small a water volume.
Conclusion: Stepping is not a detail, it is the basis of proper heating
Stepped output control is, at first glance, a technical detail. In reality, it is one of the pillars of why an electric boiler can be an economical and comfortable heat source despite the higher price of electricity compared to gas. Boilers in the Thermona THERM EL series are designed so that this stepping works automatically, reliably and without user intervention – but it is good to know what happens behind the scenes, so you can correctly set up, fine-tune and, if necessary, diagnose the system when something is not working as expected.
If you are facing a choice of a specific model, I have found that customers who understand the principle of stepping also choose the correct output much better – because they know that the boiler will not operate "at full power or not at all", but will continuously adapt. And this leads them to more sensible and successful decisions. For choosing the correct model from the THERM EL range, I recommend the article How to choose an electric boiler Thermona THERM EL – output, space size and other criteria, where all relevant parameters are discussed in detail.
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