Connecting a Vaillant atmoTEC and turboTEC boiler to a hot water storage tank

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Connecting a Vaillant atmoTEC and turboTEC boiler with a hot water cylinder – complete technical guide

Low-temperature boilers from the Vaillant atmoTEC and turboTEC series are among the most widespread heating appliances in Central Europe. They are reliable, well serviced, and have a long history of proven operation in thousands of households in Slovakia and the Czech Republic. However, when a customer decides to connect such a boiler to a hot water cylinder, questions arise that are not at all obvious – what kind of cylinder, what size, how to connect it, where to place it, how to set the heating priority, and everything else that needs to be considered when designing the whole system. This article addresses exactly this topic – comprehensively, with concrete figures and experience from real practice.

Why a cylinder at all? The principle of boiler operation with and without a cylinder

Vaillant atmoTEC and turboTEC boilers in their basic configuration are not combi boilers – they do not have a built-in instantaneous heater or a second heat exchanger for domestic hot water (DHW) preparation. Their primary task is heating. This means that if you also want hot water from the same boiler, you need to connect an external cylinder to it. That is exactly why the product category "option to connect a cylinder" exists – boilers in this category are structurally and electrically prepared for such a connection.

In practice, it works like this: the boiler has an input on the control board for a cylinder sensor (an NTC sensor or a mechanical cylinder thermostat). When the temperature in the cylinder drops below the set value, the boiler automatically switches its output to heating the cylinder and temporarily dampens or stops the heating circuit – this is referred to as domestic hot water heating priority. Once the cylinder reaches the required temperature, the boiler returns to heating mode.

The advantages of this solution compared to instantaneous water heaters or combi boilers are clear: a larger volume of available hot water at once, more stable temperature without fluctuation during simultaneous draw-off at multiple points, and the possibility of using the cylinder with an external heat pump or solar collector in the future.

Design differences between atmoTEC and turboTEC from the cylinder's perspective

Both series – atmoTEC and turboTEC – are low-temperature boilers with a maximum flow temperature of around 75–80 °C, which is sufficient for heating a hot water cylinder. The difference lies mainly in the type of combustion:

atmoTEC is an atmospheric boiler – it draws combustion air from the room and exhausts flue gases through a conventional chimney. It does not require a sealed flue path, but it needs a functional chimney and sufficient ventilation of the boiler room.
turboTEC is a turbo (forced draught) boiler – combustion takes place in a sealed chamber, and both air and flue gases are led through a twin-pipe concentric system (coaxial flue) directly through a wall or roof. It does not require a conventional chimney.

From the cylinder's perspective, both types are equally capable – they have the same hydraulic connection principle, the same connectors for the cylinder sensor, and the same heating priority control principle. The difference during installation with a cylinder therefore lies not in the hydraulics, but in the flue gas solution and boiler placement.

Specific models available in this category, designed precisely for operation with a cylinder, include the Vaillant atmoTEC plus VU CZ/SK 200/3-5 with an output of 20 kW, the Vaillant atmoTEC plus VU CZ/SK 240/3-5 with an output of 24 kW, and the Vaillant atmoTEC plus VU CZ/SK 280/3-5 with an output of 28 kW. From the turboTEC series there are the Vaillant turboTEC plus VU CZ/SK 122/3-5 (12 kW) and the Vaillant turboTEC plus VU CZ/SK 202/3-5 (20 kW).

Hydraulic connection of the cylinder to a low-temperature boiler – diagram and principle

When connecting a cylinder to a boiler, there are basically two fundamental connection schemes – direct (gravity-fed or pump-driven) and indirect (via a heat exchanger in the cylinder). In practice, with Vaillant atmoTEC and turboTEC boilers, indirect connection via the cylinder's built-in heat exchanger is used almost exclusively. The reason is simple: a cylinder with a coil-type heat exchanger (a so-called cylinder with an internal exchanger) is cheaper, safer, and does not require an extra plate heat exchanger to separate the circuits.

The connection works as follows: the boiler's primary circuit (heating circuit) splits into two branches – one branch goes to the heating system (radiators, underfloor heating), the other goes to the coil of the cylinder's heat exchanger. This cylinder branch has its own circulation pump and either a three-way valve or a separate switching pump. Based on the signal from the cylinder sensor, the boiler controls when the cylinder pump should run, and thus when the cylinder is heated.

What is required on the boiler side

The boiler must have an input for the cylinder sensor – on atmoTEC plus and turboTEC plus boilers (the /3-5 series), this input is available as standard on the control board. Power supply for the cylinder pump from the boiler depends on the specific control board – some variants have a direct 230 V output for the cylinder pump, others require an external relay or module. In any case, it is essential to consult the service documentation for the specific model.

Important: on atmoTEC/turboTEC /3-5 series boilers, the control board is VRC-compatible, which allows the system to be extended with external Vaillant controllers (e.g. the calorMATIC series). This compatibility has a major impact on how the cylinder will be controlled in combination with heating.

What is required on the cylinder side

The cylinder must be equipped with an internal heat exchanger (single or double coil). The exchanger should be powerful enough for the boiler to heat the cylinder within a reasonable time – for a typical family house, this means a heating time of 30–45 minutes for a 20 °C temperature drop in a 150–200 litre cylinder.

The cylinder must also have an immersion NTC sensor installed – usually in the lower third of the cylinder. This sensor measures the temperature and sends a signal to the boiler's control board. If the cylinder does not have a pocket prepared for an immersion sensor, some manufacturers offer sensors with a pipe clamp, but this is a less accurate solution.

The cylinder must also have a safety group with a membrane expansion vessel for the cold water circuit and a safety valve set typically to 6 bar for the potable water circuit.

Sizing – what boiler output for what cylinder?

This is the question most often underestimated by customers. The relationship between boiler output and cylinder volume is not linear, and also depends on the surface area of the exchanger inside the cylinder. An approximate calculation is as follows:

The power required to heat the cylinder is calculated using the formula: P = m × c × ΔT / t, where m is the mass of water (kg = litre), c is the specific heat capacity of water (4.186 kJ/kg·K ≈ 1.163 Wh/kg·K), ΔT is the temperature difference (typically from 15 °C to 55 °C = 40 K) and t is the required heating time in hours.

Example: a 200-litre cylinder, heated from 15 °C to 55 °C in 45 minutes (0.75 h): P = 200 × 1.163 × 40 / 0.75 = 12.4 kW. The boiler must be able to deliver at least this output to the cylinder's exchanger (taking into account exchanger efficiency, typically boiler output × 0.8–0.9).

The graph shows several practical conclusions, which we confirm time and again in practice:

For a 100–150 litre cylinder, a turboTEC 122/3-5 with an output of 12 kW is sufficient – an ideal combination for 2–3-person households or apartments.
For a 200 litre cylinder, an atmoTEC 200/3-5 or turboTEC 202/3-5 with an output of 20 kW is suitable.
For a 250–300 litre cylinder, reach for the atmoTEC 240/3-5 (24 kW) or atmoTEC 280/3-5 (28 kW).
Cylinders over 300 litres are less common with low-temperature boilers – consultation is recommended here, as the heating time may be at the limit of comfort.

We go into more detail on this topic in the article What hot water cylinder volume do I need for a low-temperature boiler.

Cylinder heating priority – how it works in practice

Cylinder heating priority is a function that ensures that when the cylinder needs to be reheated, the boiler temporarily stops supplying heat to the heating circuit and focuses its entire output on heating the cylinder. In the case of Vaillant atmoTEC and turboTEC plus series boilers (/3-5), this function is implemented directly in the control board.

In practice, this means that in a household with radiators, the radiators may cool slightly during the cylinder heating period (20–45 minutes). In a correctly sized system, this situation is almost imperceptible, because the accumulation effect of the radiators and the building structure dampens the temperature drop. A problem could arise with a very tightly sized heating system or with underfloor heating covering large areas – in that case it is better to choose a softer priority or a time-based priority.

The control board allows you to set:

Cylinder charging temperature – usually 55–65 °C. At 60 °C there is a good compromise between hygiene requirements (protection against legionella) and energy efficiency.
Sensor hysteresis – by how many degrees the temperature must drop before cylinder heating starts. Typically 5–8 K.
Maximum priority time – how long cylinder heating can block space heating. If the cylinder is not fully heated within this time, the boiler switches back to heating.

The setup topic is covered in detail in the article Setting the cylinder heating priority on a low-temperature boiler. We recommend reading it if you are setting up the system for the first time.

Cylinder placement relative to the boiler – what experience shows

The cylinder should ideally be close to the boiler – the maximum length of the cylinder circuit should not exceed 5–8 metres (straight pipe length). Every additional metre means higher heat losses in the primary circuit and a longer time before the boiler water reaches the cylinder's exchanger.

In practice, the best option is when the cylinder is placed next to the boiler – in a boiler room, utility room, or bathroom. In family houses, this is almost always possible. The situation is more difficult in apartment buildings, where the cylinder must be located in a bathroom on a different floor from the boiler. In such cases, the pipework is insulated with thicker insulation, and a more powerful cylinder circuit pump is chosen.

The cylinder must always be installed vertically – this ensures correct temperature stratification inside the cylinder. Horizontal placement of the cylinder disrupts stratification and leads to poorer utilisation of the cylinder's content.

Electrical connection – cylinder sensor, pump, communication

The electrical part of connecting a cylinder to a Vaillant atmoTEC/turboTEC boiler is usually simple, but a few principles must be followed:

Cylinder sensor (NTC 10 kΩ) is connected to the corresponding terminal on the boiler's control board – usually labelled AF or ZF (remote sensor/cylinder sensor). Polarity does not matter, and the cable can be up to 10 m long without significant resistance measurement issues.
Cylinder pump is connected either directly to the control board (if the board has a 230 V / max. 100 W output for the cylinder pump), or via an auxiliary relay. Vaillant boilers in the /3-5 series mostly have a direct output for the cylinder pump – check the documentation for the specific boiler.
Controller (e.g. Vaillant calorMATIC) communicates with the boiler via the eBUS bus line – a two-wire cable without polarity. When an external controller is connected, the cylinder functions are transferred to the controller, which usually has more advanced settings.

Note: all electrical work must always be carried out or checked by an authorised person (an electrician with the relevant qualification). Vaillant boilers are Class I appliances – they must be grounded and connected via a suitable circuit breaker with a residual current device.

Typical problems from practice and how to avoid them

Over the years of working with Vaillant atmoTEC and turboTEC systems combined with a cylinder, we have seen several recurring mistakes. Here are the most common ones:

Undersized boiler output: a customer bought a 250-litre cylinder for a turboTEC 122 (12 kW). Result: heating the cylinder takes 80 minutes, and the heating circuit is completely without heat during this time. Solution: correct sizing according to the formula above.
Missing insulation of the cylinder circuit pipework: without insulation of the primary pipe between the boiler and the cylinder, 15–25% of the transferred heat is lost. Every pipe in the cylinder circuit must be insulated with at least 19 mm of rubber insulation.
Incorrect placement of the cylinder sensor: a sensor in the upper third of the cylinder causes the boiler to stop heating prematurely – the lower part of the cylinder remains cold. The correct position is the lower third to the middle of the cylinder.
Missing cylinder safety group: the cylinder must have a safety group on the potable water side (check valve, membrane pressure vessel, safety valve, pressure gauge cock fitting). Omitting this group is a serious installation error and can lead to damage to the cylinder or water leakage.
Cylinder without an anode: a steel cylinder with an enamelled tank requires a magnesium anode to protect it against corrosion. The anode must be checked every 2 years. Neglecting this leads to premature perforation of the cylinder.

Further faults and their diagnostics can be found in the article Common faults of low-temperature boilers with a connected cylinder and their solutions.

Atmospheric vs. turboTEC – which is better for combination with a cylinder?

The choice between an atmospheric and a turbo boiler when combined with a cylinder depends on several factors. From the perspective of the cylinder heating output itself, there is no fundamental difference between them – both types can heat the cylinder equally quickly. The difference lies elsewhere:

Installation conditions: atmoTEC requires a chimney and sufficiently ventilated spaces. turboTEC can also be installed where there is no chimney – through a wall or roof. For renovating flats without a chimney, turboTEC is practically the only option.
Control and modulation: turboTEC plus (/3-5 series) offers smooth output modulation, which can be an advantage when heating the cylinder – the boiler adapts to the current heat demand and does not overshoot the cylinder as much.
Efficiency: both series have similar efficiency (90–92%); low-temperature boilers are not condensing, so they are comparable in this respect.

A more detailed analysis can be found in the article Atmospheric vs. turbo low-temperature boiler – which to choose with a cylinder.

Expanding the system – solar collector, heat pump, bivalent operation

One of the great advantages of the boiler + cylinder system is the possibility of future expansion with an additional heat source. A cylinder with a double heat exchanger (two coils inside) allows a boiler and a solar collector to be connected simultaneously – the upper coil is for the boiler, the lower (cooler zone) is for the solar system, which operates with a lower carrier fluid temperature.

With Vaillant atmoTEC and turboTEC boilers, solar integration is common practice – Vaillant offers its own auroTHERM solar systems with compatible cylinders and controllers. The auroMATIC controller coordinates solar gain and top-up heating by the boiler so that the boiler only kicks in when solar energy is insufficient.

For future heat pump connection, the situation is more complex – air source heat pumps work with a lower flow water temperature (45–55 °C) and work better with larger cylinders (300–500 litres). In a bivalent connection, the boiler serves as the backup source and the heat pump as the primary one – this combination requires a bivalent cylinder and more sophisticated control.

Installation procedure – brief overview of the steps

Installation of a cylinder to an existing Vaillant low-temperature boiler must always be carried out by a professional installer with the relevant qualification. Amateur installation can lead to gas leaks, boiler failure, or an invalid warranty. Overview of steps for the professional:

Check the output of the existing boiler against the planned cylinder volume – if the boiler is not sufficient, it needs to be replaced.
Hydraulic design: determine the cylinder installation location, the routing of the primary pipework, and the placement of the cylinder pump and the three-way valve or switching relay.
Mechanical installation of the cylinder (anchoring, venting, safety group on the DHW side).
Carry out the primary pipework of the cylinder circuit by soldering or press-fitting, install the pump and expansion vessel for the primary circuit.
Connect the cylinder sensor to the boiler's control board, connect the cylinder pump.
Degas the system, pressure test the primary circuit (minimum 1.5x operating pressure), check the tightness of the cylinder's potable water circuit.
Set the boiler parameters: cylinder charging temperature, hysteresis, priority time. First test cycle of cylinder heating in the presence of the technician.
Hand over the documentation to the customer (boiler manual, cylinder manual, technical data sheet for the cylinder circuit).

A detailed installation procedure with photographs is described in the article Installing a low-temperature boiler with a cylinder connection step by step.

Energy efficiency of the boiler + cylinder combination

A low-temperature boiler with a cylinder is not as energy efficient as a condensing boiler, but compared to instantaneous water heaters or electric water heaters, it is considerably more economical. Actual consumption depends on:

Cylinder heat losses – a modern cylinder loses 1–2 kWh per 24 hours. Old cylinders without good insulation can have losses of up to 4–5 kWh/day.
Frequency of hot water draw-off – if a household draws DHW evenly throughout the day, the cylinder stays at temperature and losses are minimal. If DHW is drawn only in the morning and evening, the cylinder cools down in between.
Cylinder temperature settings – every 10 °C lower cylinder temperature reduces the cylinder's heat losses by approximately 30%.

More on energy optimisation can be found in the article Saving energy with a combination of low-temperature boiler and hot water cylinder.

Frequently Asked Questions (FAQ)

Can I connect a cylinder to an older Vaillant atmoTEC or turboTEC boiler that did not previously have one?

Yes, this is possible, provided that the boiler's control board has an input for the cylinder sensor and an output for the cylinder pump. On older models (the /3-3 series or earlier), it may be necessary to replace the control board or use an external cylinder thermostat that switches the boiler via a system input. We recommend consulting an authorised Vaillant service centre, who will check the specific model.

What cylinder temperature is optimal – from both a hygiene and energy-saving perspective?

From a hygiene perspective, the hot water cylinder temperature must reach at least 60 °C at least once every 24 hours – this temperature kills legionella. For normal operation, setting the cylinder to 55 °C is sufficient, but once a day (or once a week according to local regulations) heating to 70 °C is recommended (thermal disinfection). Lower temperatures (below 50 °C) increase the long-term risk of legionella growth, especially if the cylinder is not being drawn from.

Why doesn't the cylinder manage to heat up fully even though the boiler is working?

The most common reason is a clogged or damaged cylinder heat exchanger – limescale deposits on the exchanger dramatically reduce heat transfer. Another reason may be an insufficiently powerful cylinder pump (low flow rate), air in the cylinder circuit, or a boiler temperature set too low. Detailed diagnostics can be found in the article Common faults of low-temperature boilers with a connected cylinder and their solutions.

Does the cylinder need its own expansion vessel?

Yes – the cylinder on the potable water side (hot water for the household) must have its own membrane expansion vessel dimensioned for potable water (red colour, food-grade certification). The heating circuit's expansion vessel is NOT suitable for potable water and cannot serve as the cylinder's expansion vessel. The size of the DHW expansion vessel depends on the cylinder volume – as a rule of thumb, 10–15% of the cylinder volume is used.

How long does a cylinder last and what needs to be checked regularly?

A quality enamelled cylinder with a regularly replaced magnesium anode lasts 15–20 years. The anode must be checked every 2 years – if it is worn down to less than half of its original thickness, it needs to be replaced. In addition to the anode, the safety group seal, the safety valve (venting at least once a year), and the function of the temperature sensor are checked. A detailed service plan can be found in the article Maintenance and servicing of a low-temperature boiler with a cylinder – what and when to check.

Can I connect a cylinder to a turboTEC 122/3-5 with only 12 kW output?

Yes, the Vaillant turboTEC plus VU CZ/SK 122/3-5 is fully adapted for use with a cylinder – it has all the necessary inputs on the control board. Its 12 kW output is ideal for cylinders up to 150 litres in a 2–3-person household. With a larger cylinder, the heating time will be longer, but functionality is preserved. For a 4–5-person family with a 200-litre cylinder, we recommend at least 20 kW, i.e. the Vaillant turboTEC plus VU CZ/SK 202/3-5 model.

Conclusion – when is the atmoTEC/turboTEC + cylinder combination the right choice?

The combination of a Vaillant low-temperature boiler with a hot water cylinder is a proven and reliable solution with decades of verified practice in hundreds of thousands of installations. It is suitable for family houses as well as apartment buildings, where the customer wants comfortable and predictable hot water draw-off without compromises in pressure fluctuation or waiting for heating. A correctly sized cylinder with well-set boiler parameters works for years without problems and with minimal operating costs.

The key to success is correct sizing from the start – boiler output, cylinder volume, exchanger surface area, and heating priority settings must all be in harmony. For selecting specific models, see the article How to choose a low-temperature boiler with the option to connect a cylinder, which contains detailed selection criteria and practical recommendations for various types of households.

Models such as the Vaillant atmoTEC plus VU CZ/SK 240/3-5 or the Vaillant atmoTEC plus VU CZ/SK 280/3-5 provide enough output for larger households even with a more voluminous cylinder, while remaining simple, serviceable appliances that any authorised Vaillant technician in Slovakia can repair when needed, without waiting for special parts.

Do you have a question on this topic?

Can't decide or dealing with a specific situation in your household? Write to us - we're happy to help.