TUV Module for Electric Boiler: How Does Hot Water Preparation Work in an External Tank?

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Domestic Hot Water (DHW) Module for Electric Boiler: How Does Hot Water Preparation Work in an External Tank?

When a customer chooses the Attack Electric electric boiler, most attention naturally focuses on performance, control methods, and heating costs. Preparation of domestic hot water – abbreviated as DHW – remains in the shadow, although in a typical household it accounts for 30 to 50 percent of total thermal energy consumption. A proper DHW solution is not a detail: it is a systemic question that directly affects comfort, electricity consumption, and the overall installation cost. In this article, we will take a detailed look at how DHW preparation works in an external tank via the DHW module set, what it specifically contains, when it makes sense, and what you need to know before installation.

What is the DHW module and why the boiler does not prepare it by itself

Electric boilers Attack Electric – whether models of the Excellent or Easy series – are primarily designed as heat sources for the central heating system. The heating of water in a closed circuit (primary circuit) supplies radiators, floor heating, or other heating elements. Hot domestic water, however, is a different story: it is an open circuit, where water comes from the water supply, must meet hygiene requirements, and is regularly replenished with fresh cold water.

The boiler itself cannot heat DHW – for that, it needs either an integrated tank (some combined units have it inside) or an external tank (boiler) and control logic that switches energy from heating to DHW heating when needed. This is exactly what the DHW module set solves: it is a hardware and software set that is added to the Attack Electric electric boiler and allows it to control an external tank with an electric heating element or with a heat exchanger coil.

Two Principles of DHW Heating in an External Tank

Before we go into the specific module, it is important to understand that an external tank can be heated in two basic ways. The choice depends on what type of tank you have or plan to install.

Heating via a heat exchanger coil (indirect heating)

There is a metal coil (heat exchanger) built into the tank, through which hot water from the boiler's primary circuit flows. Heat from this water passes through the coil walls into the tank water – the utility water. Both circuits are physically separated, which is important from a hygiene point of view: heating water (which may contain corrosion inhibitors or other additives) never mixes with drinking water.

This principle is common in storage water heaters with one or two coils. The boiler must have sufficient outlet temperature (typically 60–75 °C) and a properly dimensioned heat exchanger to heat the required amount of water in a reasonable time.

Direct heating by an electric heating element in the tank

The tank has its own electric heating element (cartridge, rod heater) – similar to a standard boiler. The DHW module in this case controls the switching on and off of this element based on the temperature in the tank, while coordinating the entire heating with the heating boiler to avoid unnecessary overloading of the electrical connection or simultaneous power draw for heating and DHW.

A combination of both approaches is also possible: a tank with a built-in electric element and a heat exchanger coil (so-called combined tank) provides maximum flexibility – the primary circuit coil covers basic heating, the electric element serves as a booster or backup.

What the DHW module set for Attack Electric specifically contains

DHW module set is an accessory specifically designed for Attack Electric boilers (primarily the Excellent series). The set consists of several components that together form a complete solution for tank heating control:

Control module (electronic board or relay) – integrated into the boiler system, where it communicates with the main control unit. It receives a signal from the tank temperature sensor and, based on the set desired temperature, turns the heating on and off.
Tank temperature sensor – usually a submersible NTC sensor, which is placed into a well in the tank heater. It continuously measures the actual water temperature and sends the information to the boiler's control unit.
Connecting cable and terminal block – electrical connection between the boiler and the tank (for signal wiring of the sensor and possibly also for controlling the relay of the electric element).
Documentation and installation manual – wiring diagrams, parameter settings in the boiler's menu.

Depending on the configuration (indirect heating via coil or direct via electric element), the kit may also include a three-way switching valve with an actuator, which physically redirects the flow of water from the primary circuit either to the heating system or to the tank. This is a typical case when the tank does not have its own electric element and the heating is provided only by the coil heat exchanger.

How the control works: TÚV priority vs. heating

This is where the biggest part of the system's intelligence is hidden. In practice, a conflict arises: the boiler has limited power and cannot heat and heat the tank equally efficiently at the same time. Therefore, the control system works with so-called TÚV priority.

TÚV priority principle step by step

When the water temperature in the tank drops below the set value (for example, below 50 °C), the boiler's control system evaluates the need for TÚV heating as a priority. At that moment, the following sequence of events occurs:

1. The boiler shifts power from the heating circuit to the tank heating. In a system with a three-way valve, the valve switches and hot water begins to circulate through the tank coil. In a system with a direct electric element, the tank element receives a command to turn on (or the boiler's power may be temporarily limited for a short time to avoid simultaneous maximum draw).

2. Heating is temporarily paused or limited during TÚV heating – it depends on the specific settings and boiler model. In practice, heating a standard tank (150–200 liters) takes 30 to 90 minutes with sufficient boiler power.

3. When the temperature in the tank reaches the set value (hysteresis regulation, for example 60 °C), the boiler switches back to the heating mode. The three-way valve returns to the heating position.

Most models of Attack Electric Excellent allow setting a daily schedule for TÚV heating – for example, to prioritize heating at night during the low electricity rate (night tariff, two-zone meter), which is a very efficient solution in practice.

Which Attack Electric models are compatible with the TÚV module

The TÚV module is designed for boilers in the Attack Electric Excellent range. These boilers have a more advanced control unit capable of communicating with the tank temperature sensor and controlling external outputs (three-way valve, relay of the tank element). Specifically, these are:

Attack Electric Excellent 8 / 7.5 kW – suitable for smaller houses and apartments, where a tank with a volume of 80–120 liters meets the needs of 2–3 people
Attack Electric Excellent 15 kW – medium power, suitable for family homes, 150–200 liter tank for 3–4 people
Attack Electric Excellent 24 / 22 kW – the highest power in the range, suitable for larger houses or homes with higher TÚV consumption (5 or more people, 200–300 liter tank)

The model Attack Electric Easy 8 is a basic, compact boiler focused primarily on heating. Its control logic is simpler and direct support for the TÚV module with control of an external tank via the boiler electronics may not be available – in this case, it is necessary to check the current version of the boiler and consult with a service technician, or alternatively, solve the TÚV heating with a separate tank thermostat independent of the boiler.

Tank sizing: what volume is right?

One of the most common mistakes I encounter in practice is an undersized tank. The customer buys a 15 kW boiler and connects an 80-liter tank, which barely meets the needs of 2 people, although 4 people live in the house. Result: cold water on the third shower and a tank that heats up all day long.

An approximate rule for tank sizing is as follows:

1–2 people: 80–100 liter tank
2–3 people: 120–150 liter tank
3–4 people: 150–200 liter tank
4–6 people: 200–300 liter tank
6 or more people or higher consumption (whirlpool, large bathtub): 300–500 liter tank

In addition to the volume, the power of the coil heat exchanger also matters – the more powerful the coil, the faster the heating. Most tanks with a coil are dimensioned for 10–25 kW, which is sufficient without problems for a direct electric boiler with 15 kW power. For a more powerful boiler (22–24 kW), it is worth considering a tank with a larger or double coil for faster heat transfer.

The material of the tank is also important. For permanent living, I recommend enamelled tanks with a magnesium anode or stainless steel tanks – the lifespan is significantly longer than with cheap galvanized tanks. Enamelled tanks with an anode require anode replacement every 2–4 years (depending on water hardness), which is a simple and inexpensive maintenance compared to corrosion of the entire tank.

Hydraulic circuit diagram: boiler, tank, and heating system

Installation of TÚV module: what to know before installation

Installation of a TÚV module is not a task that an untrained person can handle without technical preparation. It is a job that combines electrical installation, heating hydraulics, and setting up the control system. In most cases, it is carried out by a qualified installer – and this is the correct approach, especially if the boiler is under warranty (installation by an authorized person is required).

Here is an overview of what needs to be considered when planning the installation:

Location of the tank

The tank should be placed as close as possible to the boiler, ideally in the boiler room or technical room. Every meter of pipe between the boiler and the tank means heat loss, longer heating time, and greater need for insulation. The connections for cold and hot water must be properly dimensioned – for tanks over 150 liters, I recommend pipe diameters DN 20 (3/4") or DN 25 (1").

Hydraulic connection

If the system uses a three-way switching valve (tank with coil without its own electric heating element), the valve must be properly connected: normal flow in the heating position, switching to the tank in TÚV priority mode. The valve actuator is powered from the boiler via the control module. It is important to observe the flow direction and install ball valves for the possibility of disconnecting the tank without the need to drain the entire system.

Electrical connection

The tank sensor is connected to the appropriate terminals on the boiler's control board (according to the diagram provided with the module). If the tank has its own electric heating element controlled by the module, this circuit must be properly protected and grounded. Never connect the tank's heating element to the same circuit breaker as the boiler itself without verifying the total power consumption – simultaneous draw may exceed the circuit breaker's capacity.

Setting parameters in the boiler menu

After the physical installation, the settings follow: desired tank temperature (typically 55–65 °C), hysteresis (difference between the start and stop temperature, usually 5–10 °C), a heating schedule for TÚV (e.g., heating during night tariff from 22:00 to 06:00), and possible limitation of the TÚV heating duration. A detailed setup procedure can be found in the article Control and setting of the electric boiler Attack Electric: how to save on electricity? in our Knowledge Center.

Legionella prevention: why the tank temperature of 60 °C is key

This point is sometimes underestimated, although it is critically important for health. The bacterium Legionella pneumophila multiplies in hot water tanks at temperatures of 25–50 °C. Below 20 °C, the bacteria are inactive, above 60 °C they die. Therefore, the rule is: the TÚV tank should be heated to at least 60 °C for at least 30 minutes at least once every 24 hours.

The control system of the Attack Electric Excellent boilers allows to program so-called thermal disinfection of the tank – automatic heating to 65–70 °C once per set interval (usually once a week). I recommend activating this setting always, especially if the tank has not been in operation for a longer time (holiday, empty house) or if the standard heating temperature is set to a lower, more economical value (55 °C).

This also leads to an important insight for power sizing: if a 8 kW boiler is heating a 200-liter tank from 10 °C to 65 °C, the heating time will be:\n Q = m × c × ΔT = 200 kg × 1,163 Wh/(kg·K) × 55 K ≈ 12 793 Wh ≈ 12,8 kWh. Considering the actual power delivered to the tank (assuming heat transfer efficiency via the coil is ~85 %), we need ~ 15 kWh, which means the heating takes ~ 15 000 / 8 000 ≈ 1 hour 52 minutes. With a 15 kW boiler, it takes less than an hour. For a night tariff (8 hours) it is acceptable even with a lower power, but it needs to be consciously planned.

Economics of the solution: when is the TÚV module more cost-effective than a standalone water heater

Customers ask: why not just buy a standard electric storage water heater (boiler) and have TÚV completely independent from the boiler? This is a legitimate question and the answer depends on the specific situation.

Advantages of the solution with a TÚV module and a tank with a coil (connected to the boiler):

One energy source for the whole house – the boiler, which is already running for heating, heats TÚV without an additional device
Opportunity to use the night tariff for both circuits simultaneously – set TÚV heating and heating during the time of cheap electricity
Centralized control via one boiler interface
Longer lifespan of the tank with a coil compared to a tank with a direct electric element (fewer on/off cycles, more even heating)

Advantages of a standalone electric boiler:

Simpler installation – no hydraulic connection with the boiler is needed
Independent operation – TÚV works even when the boiler is not in operation (summer, off-season)
Lower initial cost (less piping, no three-way valve)
Suitable for apartments and smaller spaces without a boiler room

In practice, for a family house with an Attack Electric Excellent boiler, I recommend the solution with a TÚV module and a tank with a coil whenever the boiler is running most of the year (i.e., not only during a short winter season). For cottages, apartments or houses where heating is seasonal only, a standalone boiler may be more practical and cheaper to operate.

Typical scenarios from practice: how customers solve TÚV with Attack Electric boilers

Scenario 1 – New family house construction, 4 people, floor heating: The customer installed an Attack Electric Excellent 15 kW with a TÚV module and a 200-liter tank with a coil. TÚV heating is programmed for the night tariff period (22:00–06:00). In the morning, the tank is fully heated, and the boiler covers heating during the day. The electricity consumption for TÚV has been shifted to the night tariff, which with a two-zone metering system brings a cost saving of approximately 25–35 % for TÚV compared to the day tariff.

Scenario 2 – Older building, radiators, boiler room renovation: The customer replaced an old gas boiler with an electric Attack Electric Excellent 24 kW. The original tank had its own 2 kW electric element – too slow for heating. The solution: a system coil (combination tank) was added to the tank, and the tank was integrated into the system with a TÚV module. The 22 kW boiler power via the tank coil heats 200 liters from 15 °C to 60 °C in about 30–35 minutes – convenient for 5 people. The backup electric element in the tank remained as an emergency backup in case of boiler failure.

Scenario 3 – Cottages area, seasonal use: The customer with an Attack Electric Easy 8 in the cottage solved TÚV practically: a simple 80-liter boiler with its own thermostat, connected to the water supply. The boiler handles only heating, TÚV operates independently. In the summer season, the boiler is turned off, the boiler runs only for TÚV. A simpler and cheaper solution for a cottage with irregular use.

Maintenance of the tank and what neglect can cause

The hot water tank is a vessel in which drinking water is used and is exposed to thermal and pressure cycles. Without regular maintenance, several problems can occur. In the article Maintenance and service of the electric boiler Attack Electric: what and when to check? you will find a more detailed overview of the entire service plan, here we focus on the tank itself.

Magnesium sacrificial anode – in enameled tanks, it protects the inner surface from corrosion. Check every 2 years, replace when it is worn down to less than 1/3 of the original diameter (typically every 3–5 years depending on water hardness). Hard water (limestone area) consumes the anode faster.
Descaling – in areas with hard water, limescale (CaCO₃) settles on the heating element and inside the tank. A 1 mm layer of limescale reduces heat transfer by 10–15 %, a 5 mm layer by 50 %. Descaling with acidic solutions (citric acid) or mechanical cleaning once every 2–4 years.
Pressure relief valve – the tank must have a functional pressure relief valve (usually 6–8 bar). The valve should be manually bled once a year (short opening) to verify functionality and prevent sediment buildup on the seat.
Tightness of connections and valves – during the annual inspection, check the seals of the cold and hot water connections, and address any dripping immediately.

Most Frequently Asked Questions (FAQ)

Do I have to replace the entire tank when installing the TÚV module?

Not necessarily. If your existing tank has a heat exchanger coil and a temperature sensor port, you can connect it directly to the system with the TÚV module. If the tank does not have a coil, you will need to replace it with a tank that has one – the TÚV module can control the tank's direct electric element only if the tank is equipped with an electrical connection and a sensor port. Before purchasing the module, consult a qualified installer about the compatibility of your tank.

Is the TÚV module compatible with all Attack Electric models?

The TÚV module is primarily designed for the Attack Electric Excellent series. Models Excellent 8/7.5 kW, Excellent 15 kW, and Excellent 24/22 kW support it within their control electronics. The Attack Electric Easy 8 model has a simpler control system and direct support for the TÚV module via the boiler electronics may not be available – in such cases, TÚV can be solved either with a separate water heater with its own control system, or by consulting a technician regarding the specific boiler version.

How do I set the optimal tank temperature for energy savings?

From a hygiene perspective, the tank temperature must not drop below 55 °C for long periods – at lower temperatures, the risk of Legionella bacteria growth increases. For regular operation, I recommend setting the target temperature to 60 °C, which is a good compromise between hygiene safety and energy consumption. If you have the thermal disinfection function activated (weekly heating to 65–70 °C), you can lower the regular operating temperature to 55 °C and save 5–10 % on TÚV costs. Never set the tank to a permanent temperature below 50 °C.

What happens to heating during TÚV heating?

During active TÚV priority, the heating circuit is temporarily paused or limited – this depends on the specific boiler settings and the duration of the tank heating. In practice, this means that radiators or floor heating will not receive heat for 30–90 minutes. In a building with good thermal mass (well-insulated house, floor heating with a massive thermal storage floor), this pause is completely unnoticeable. In less insulated houses with radiators, the room temperature may drop by 0.5–1 °C during this period. If TÚV heating takes place at night (which is the optimal setting), this effect is not an issue at all.

Can I use the TÚV module in combination with solar heating?

Yes, this is one of the most practical solutions. A tank with two coils (upper and lower coil) allows connecting solar collectors to the lower coil (primary solar circuit) and the Attack Electric boiler to the upper coil (boost heating). The TÚV module controls the boiler so that it only intervenes in heating when solar energy is insufficient – meaning the sun heats the tank for free and the boiler only boosts the remaining part when needed. This combination can almost completely cover TÚV with solar energy during the summer period and significantly reduce overall electricity consumption. For a more detailed installation procedure, I recommend consulting an installer with experience in both systems.

What is the lifespan of a tank with a coil and when should it be replaced?

A glazed tank with regular anode replacement can last 15–25 years. Stainless steel tanks are practically maintenance-free and can last 25–40 years under normal conditions. It is time to replace the tank when: visible corrosion appears on the outer shell (a sign of internal problems), the tank has repeated water leaks at the connections, the internal glaze is seriously damaged (red-brown water color), or the tank has a crack in the weld. Never attempt to repair the tank yourself – pressure vessels with hot water must always be in a safe condition.

Conclusion: TÚV module as part of a well-planned installation

Preparing hot water is not a question that should be treated as an additional problem after the boiler installation. It is a part of the overall heating system concept – and the earlier the customer understands this, the more cost-effective and practical the final installation will be. The TÚV module set for Attack Electric Excellent boilers is an elegant solution that turns one boiler into a comprehensive heat source for heating and TÚV, with intelligent control, the possibility to use night tariffs, and long-term operational economy.

If you are planning to install an electric boiler and need help choosing the right model and accessories, also see the articles How to choose an Attack Electric boiler: power, model and accessories and Attack Electric Excellent vs. Easy: what are the differences and which one to choose? – both are available in our Knowledge Center. The right choice from the start will save nerves, time, and money for years to come.

Do you have a question about this topic?

Not sure how to decide or dealing with a specific situation in your home? Write to us – we are happy to help.