Connecting Vitotron to a system with a boiler or DHW tank

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Connecting Vitotron to a system with a boiler or DHW tank – complete technical guide

The Viessmann Vitotron electric boiler is now a fairly common choice not only for low-energy new builds, but increasingly also for renovations, where the owner wants to get rid of a gas boiler or has simply opted for electric heating as the main or supplementary heat source. However, a large proportion of customers forget when ordering, or only realize later, that the boiler alone is not enough – they also want domestic hot water (DHW). And that's where a whole set of questions arises: How do I connect Vitotron to a boiler/tank? What kind of tank? Where do I put the sensor? What about the 3-way valve? Can the boiler heat the tank and provide heating at the same time?

This article will guide you through the whole topic – from a basic understanding of the principle, through wiring diagrams, component selection, priority settings, to concrete practical examples. I'm writing it the way I would explain it to a customer who comes into the shop with a house floor plan and asks "so what exactly do I need".

Why Vitotron alone doesn't solve DHW

Vitotron is exclusively a heating boiler – it heats water in the heating circuit (central heating). It does not have a built-in domestic hot water tank, nor a built-in heat exchanger for DHW, so if you want hot water from the tap as well as from the heating boiler, you need to connect an external tank to it. This is not a disadvantage at all – on the contrary, an external tank can have exactly the volume you need, and the system is more flexible than combined appliances.

Vitotron outputs a heat carrier (water in the heating circuit) at a temperature typically of 45–75 °C. This water passes through a heat exchanger in the tank and transfers heat to the drinking water. The tank can be any indirect tank (boiler with a heat exchanger), sized according to the household's needs.

Two basic ways to connect a DHW tank to Vitotron

1. Connection via a 3-way switching valve (standard solution)

This is the most widespread and most comfortable method. A 3-way switching valve (three-way valve) is an electromechanical device that controls where the heated water from the boiler goes – either to the heating circuit (radiators, underfloor heating), or to the DHW tank. The boiler alternately heats the tank and heats the house depending on which has the higher priority.

That's why Viessmann offers Vitotron versions with an integrated 3-way valve and tank sensor – these are the models Viessmann Vitotron 100 0.4–8.0 kW with 3-way switching valve and tank sensor and Viessmann Vitotron 100 1.0–24.0 kW with 3-way switching valve and tank sensor. These versions have the valve directly in the boiler body, which greatly simplifies installation and eliminates errors when wiring an external valve.

2. Connection with an external valve or without a valve (simpler, but limited)

In simpler systems – for example, if you have a DHW tank with its own electric heating element (electric boiler) and Vitotron is used only for heating – you don't need a switching valve at all. The tank is heated by its own electricity, Vitotron provides heating. This solution is cheaper to implement but more expensive to operate, because the tank cannot be heated by cheaper energy from the boiler – you pay for two electrical appliances at once.

Another option is a system with an external 3-way valve, where the valve is not part of the boiler. This is suitable when you have a Vitotron without a valve (Vitotron 100 0.4–8.0 kW or Vitotron 100 1.0–24.0 kW) and are adding a tank afterwards. The external valve is technically the same, only the installation is somewhat more complicated and requires correct electrical connection to the boiler's control unit.

How DHW heating priority works

When Vitotron is set with the tank in DHW priority mode (which is common and recommended), it works as follows: a temperature sensor placed on the tank measures the water temperature in the tank. If the temperature falls below the set value (typically you set a target temperature of 55–60 °C to protect against legionella), the boiler receives an instruction – switch the 3-way valve to the tank. The valve switches, the boiler starts heating the tank at full power. When the tank reaches the required temperature, the valve switches back to the heating circuit and the boiler continues heating.

This alternation happens automatically without operator intervention. The user only sets the desired temperatures and time programs. The whole system is controlled by the Vitotron control unit (depending on the model, either Vitotronic 200 or a controller integrated directly in the boiler).

Important detail: while the tank is being heated, space heating stops. In a larger house with a small power reserve, this can be noticeable – so correct tank sizing is key. More on this topic in the section What Tank to Choose and What Volume.

Tank sensor – where to place it and what it's for

The tank sensor is an immersion temperature sensor (usually NTC 10k or a type compatible with the Viessmann controller), which is inserted into an immersion sleeve (pocket) on the tank. The correct sensor position is essential for the correct operation of the whole system.

The sensor should not be at the very top of the tank, because the water there is always the hottest and the system would think the tank is sufficiently heated even though the bottom part is still cold. The correct position is at 1/3 of the tank height measured from the bottom – i.e. in the middle-lower zone. This is exactly where the hot and cold water layers meet, and this position gives a realistic picture of the tank's condition.

For tanks with two immersion sleeves (common in tanks for bivalent systems), the Viessmann sensor is usually connected to the lower sleeve. The upper sleeve is for a possible second sensor (e.g. for a solar controller or safety thermostat).

The electrical connection of the sensor to the boiler is made using a two-core cable (CYKY 2×1.5 is recommended, or a shielded cable if the run is longer than 10 m or passes near power cables). The cable connects to the boiler's terminal block at predefined terminals – for models with an integrated valve, these terminals are directly brought out and labeled in the wiring diagram in the boiler's documentation.

Which DHW tank to choose for Vitotron – volume, heat exchanger, material

Tank volume

The tank volume depends on the number of people in the household and the daily hot water consumption. Approximate values for family houses:

1–2 people: 80–120 liters
3–4 people: 150–200 liters
5–6 people: 200–300 liters
6 or more people, or higher consumption: 300–500 liters

With an electric boiler in DHW priority mode, an important rule applies: the larger the tank, the longer the heating takes, and the longer space heating is stopped. Therefore, the compromise between tank size and boiler power is key. In practice, with a boiler power of 8 kW and a 200 l tank, the heating time from 15 °C to 60 °C is around 60–90 minutes (depending on the size of the heat exchanger in the tank). Heating does not operate during this time.

In apartments with a boiler power of 3–4 kW and a tank of 100–120 l, heating is faster, but you should still expect 45–60 minutes of interrupted heating. If the apartment is well insulated and the temperature doesn't drop quickly, this interruption is not a problem. In an older house with worse insulation it can be noticeable – in that case consider a tank with its own electric heater as a backup.

Tank heat exchanger

The tank must have an indirect heat exchanger – a coil or jacket-type exchanger, through which water from the heating circuit flows and transfers heat to the drinking water without mixing the circuits. Tanks differ in the size of the heat exchanger (surface area in m²) – a larger exchanger means faster heating, which is desirable.

For Vitotron, a tank with a heat exchanger of at least 0.6–0.8 m² for a volume of 150–200 l is suitable. Higher boiler power allows a tank with a larger heat exchanger to be used more efficiently.

Tank material

On the market there are enamelled tanks (most common, good price/lifespan ratio, require a magnesium anode) and stainless steel tanks (longer lifespan, higher price, no anode needed). For family houses, an enamelled tank with regular anode replacement (every 2 years) is fully sufficient. I recommend stainless steel tanks where the water is aggressive or there are longer periods without operation.

Vitotron with an integrated 3-way valve versus an external valve – which is better

I have covered this topic in more detail in a separate article Vitotron with a 3-way valve and tank sensor – when it's worth it, here I will focus on the practical differences during installation.

When installing a Vitotron with an integrated valve, tank installation is significantly simpler:

There is no need to buy the valve separately (and choose the correct type and DN).
The electrical connection of the valve to the boiler is internal – there's no need to deal with external cables and terminals.
The control logic is fully built in – you only need to set the desired tank temperature.
Fewer potential wiring errors, fewer components that can fail.
Warranty and service are clearer – everything is from one manufacturer.

An external valve is a solution for additional installation of a tank to an existing Vitotron without a valve. It is feasible, but requires exact adherence to the wiring diagram and the use of a compatible valve (Viessmann sells the appropriate accessories). A layperson should not attempt this – an installer is needed.

From an economic point of view, the version with an integrated valve is typically 10–15% more expensive, but you save on the external valve and extra work – in the end it comes out similar or cheaper.

Specific installation examples from practice

Case 1: New build – family house, 4 people, underfloor heating + 200 l tank

The customer built a new house with a heat loss of approx. 7 kW. They chose the Viessmann Vitotron 100 1.0–24.0 kW with 3-way valve and tank sensor, operated at 8–10 kW depending on the outdoor temperature. Connected to it was an indirect tank of 200 l with a 0.8 m² coil.

The sensor was installed in the immersion sleeve at a height of 70 cm from the bottom of the tank (tank height 160 cm, i.e. exactly at 1/3). The set tank temperature was 58 °C, hysteresis 5 °C (heating starts when the temperature drops to 53 °C). Heating time of the tank from cold was approx. 70 minutes at a boiler power of 10 kW. During tank heating, underfloor heating does not operate, but since underfloor heating has enormous thermal inertia, the room temperature does not drop by more than 0.5 °C – the customer didn't notice anything.

Case 2: Apartment renovation, 2 people, 3-room apartment, 120 l tank

The old gas boiler was replaced with a Vitotron 100 0.4–8.0 kW (basic model without a valve, since the customer originally had only one circuit). The 120 l DHW tank had its own 2 kW electric heating element as backup. Vitotron only heated the heating system.

After a year of operation, the customer decided to also connect the tank to Vitotron so as not to operate the tank on more expensive direct electricity. An external Viessmann 3-way valve DN 20 was added, connected according to the boiler documentation to the terminals for an external valve. A tank sensor was added and set to 55 °C. Result: the tank is heated with cheaper energy via the boiler (even cheaper during the night tariff), while the electric heater in the tank serves only as an emergency backup (set to 65 °C, so it normally never activates).

Case 3: Larger family house, 6 people, 300 l tank, bivalent system

House 200 m², heat loss 15 kW. Vitotron was used as a supplementary source alongside a heat pump. The 300 l DHW tank was bivalent – the upper heat exchanger for the boiler/heat pump, the lower one for solar panels. The tank sensor for the boiler was connected to the upper sensor, the solar controller had its own sensor for the lower zone. In this case, Vitotron mainly served in winter as a top-up heater for the tank when the solar system was insufficient.

An interesting fact from this project: the customer originally wanted a 500 l tank, but when we did the calculation, we found that the heating time for a 500 l tank via a 15 kW boiler would be almost 3 hours – which would be impractical in terms of heating interruption. A 300 l tank was heated in approx. 100 minutes, which was acceptable.

Electrical and hydraulic connection requirements

Hydraulic connection

The DHW tank (boiler circuit) is connected to the system in parallel with the heating circuit via a 3-way valve. Correct pipe sizing between the boiler and the tank: for boiler power up to 12 kW, DN 20 (3/4") is sufficient, for higher power DN 25 (1"). It is important that the tank has its own circulation pump for the boiler circuit, or that the tank is connected so that the system pump can serve the tank. With an integrated 3-way valve in the boiler, the tank is connected directly after the boiler, before the branch to the heating circuit.

A safety valve must be installed on the tank on the drinking water side (set to 0.6 MPa = 6 bar), an expansion vessel for the drinking water circuit (not for the heating circuit!), and a non-return valve on the cold water supply pipe. These elements are part of the plumber's installation and must not be omitted.

Electrical connection of the sensor and valve

The tank sensor (NTC) is connected with two wires, without polarity, to the boiler's terminal block. The cable must not be run together with power cables – this is a low-voltage signal cable, and interference could cause incorrect temperature measurement. I recommend a minimum distance of 15 cm from power cables, or the use of shielded cables for longer runs.

The (external) 3-way valve is usually powered by 230 V AC, controlled by the boiler via a relay. The boiler switches the valve's power supply depending on whether DHW priority is active. You will find the exact terminal block diagram in the Vitotron documentation – always follow it, as the wiring differs slightly between versions with integrated and external valves.

Setting up the Vitotron boiler for operation with a tank

After physically connecting everything, configuration follows. The specific steps depend on the controller version, but the principle is the same across all Vitotron 100 models.

In the controller menu, the tank (DHW) function is activated – the required tank temperature is set (usually 55–60 °C, for preventive thermal disinfection 70 °C, but this is a one-off program, not normal operation). Next, the hysteresis is set – the temperature difference at which heating starts. I recommend a hysteresis of 5 °C (heating starts when the temperature drops 5 °C below the set value).

Important setting: the time of thermal disinfection. DHW tanks are a potential breeding ground for Legionella pneumophila bacteria if the water temperature does not exceed 60 °C for a long time. Vitotron allows you to program automatic tank heating once a week to 70 °C – this function is called thermal disinfection. Set it to a time when the household does not use water (for example at night or in the morning before 6:00).

Other settings that affect the boiler's cooperation with the tank:

Maximum tank heating time – protection against the boiler getting stuck heating the tank indefinitely (e.g. in the event of a sensor fault). Set to 90–120 minutes.
Tank vs. heating priority – with full tank priority, heating stops for the entire duration of heating. With so-called parallel operation, the boiler can alternate output between circuits, but this requires a system with its own pumps on each circuit.
Tank temperature limit – the maximum temperature to which the boiler heats the tank (protection against overheating and energy savings).

Vitotron and the DHW tank with a night electricity tariff

One of the advantages of an electric boiler is the ability to work with a night tariff (NT), which is significantly cheaper. If you have a NT arrangement with your electricity supplier (usually 8 hours at night, in some tariffs also during certain hours in the day), you can program the boiler so that the tank is heated primarily during NT hours.

In practice, this means that by the end of the NT period (e.g. at 6:00), the tank must be fully heated. Vitotron can achieve this if you set the required tank heating time to 4:00–5:00 and the tank has sufficient volume (with 200 l and an 8 kW boiler, the heating time is approx. 60–80 minutes). The rest of the NT period can be used by the boiler to heat storage heaters (if present) or it simply does not run.

When planning, it's good to know that a larger accumulation tank (e.g. 400 l) allows more thermal energy from NT to be stored and consumption to be spread across the whole day. This is a widespread strategy among Vitotron owners – they combine a larger DHW tank with higher boiler power and benefit from the price difference between the day tariff and NT. More on operating economics can be found in the article Vitotron operating costs – how much does heating with an electric boiler cost.

Tank protection and safety elements

A DHW tank must be equipped with several safety elements that are legally required (STN EN standards and Decree No. 508/2009 Coll.):

Safety valve on the cold water supply pipe – protects the tank from exceeding pressure. Set to 6 bar (0.6 MPa). The safety valve must be properly drained – it must not be capped or blocked.
Non-return valve before the tank – prevents hot water from flowing back into the distribution system.
Expansion vessel for the drinking water circuit – when the tank is heated, the water expands in volume, and this vessel absorbs the volume increase. Note: this vessel is different from the heating system's expansion vessel! It must be certified for contact with drinking water.
Thermal disinfection – organizational and technical measure against Legionella, described above.
Tank safety thermostat – some tanks have one built in, serving as a backup in case of a control failure and preventing the tank from overheating above 95 °C.

Sizing boiler power for simultaneous operation with DHW

This is one of the most common mistakes I see in practice: the customer chooses a boiler correctly sized for the house's heat loss, but forgets about DHW. The boiler's power must cover not only the house's heat loss, but also tank heating within a reasonable time. More on choosing boiler power can be found in the article What power Vitotron do I need for my house or apartment.

Practical rule: for a typical family house with a heat loss of 8 kW and a 200 l tank, we choose a boiler of at least 10–12 kW. When the boiler heats the tank (DHW priority), the entire output goes to the tank – the house is temporarily not heated. If the boiler were only 8 kW, the tank heating time would be longer and the house would cool down more. A power surplus of 2–4 kW above the house's heat loss is recommended for a system with a tank.

DHW tanks with a larger heat exchanger (larger coil surface) heat up faster at the same boiler power – so it makes sense to invest in a higher-quality tank with a larger heat exchange surface if you want to minimize heating outages during DHW heating.

Most common mistakes when installing a tank with Vitotron

From my practice I have identified several recurring mistakes that either cause the system to malfunction or unnecessarily increase operating costs:

Tank sensor in an unsuitable position – if it's too high, the system thinks the tank is cold (the upper layer cools faster) and unnecessarily heats a fully charged tank. If it's too low, the tank may be charged only at the top while the bottom remains cold, leading to insufficient DHW comfort.
Tank without its own circulation pump – in systems with a single pump for the whole boiler circuit, the tank and heating must alternate via the valve, which works fine. But if the tank is connected on its own pump and this pump doesn't switch on in sync with the valve, circulation without flow can occur and the boiler may overheat.
Tank too small for high DHW consumption – an 80 l tank for a family of 4, where everyone showers for 15 minutes every morning, simply isn't enough. Result: cold water after the first two showers and an unhappy customer.
Thermal disinfection not activated – a silent problem that doesn't show up immediately, but can cause health problems after years. Always activate it.
Missing expansion vessel for the drinking water circuit – the tank's safety valve then drips with every heating cycle, which is not normal and leads to premature wear of the safety valve.
Mixing up the sensor signal cable with the valve power cable – I see this mistake relatively often with an external valve and additional installation. Result: damaged boiler control electronics or a non-functional sensor.

Viessmann EKCO.TM – an alternative for simpler systems

If you're looking for a simpler solution without the need for an external tank with a heat exchanger, it's worth mentioning the Viessmann EKCO.TM 10–30 – an electric boiler from a different product family, suitable for systems where DHW is handled by another appliance (e.g. a heat pump or solar system) and Vitotron only serves to top up the heating water. EKCO.TM does not have an integrated valve, but it is a compact and reliable device for purely heating applications.

Frequently asked questions (FAQ)

Can I connect any DHW tank to Vitotron, or do I have to use a Viessmann tank?

Vitotron is a hydraulically standard device – the outlet and return connections are common threaded or flanged fittings. So you can connect any indirect DHW tank from any manufacturer to Vitotron, as long as the tank has a sensor pocket (immersion sleeve) of a size compatible with the Viessmann sensor (M10 or according to specification), or you use an adapter sensor. The boiler's warranty is not affected by this – the boiler doesn't care what tank is connected, only the correct electrical signal from the sensor matters.

What if the tank sensor is not connected, or fails?

If the tank sensor fails, Vitotron will usually display an error code on the controller's display. In such a case, the system either does not heat the tank at all, or – depending on the settings – heats the tank to the maximum set temperature for a fixed time. A faulty disconnected sensor will therefore be immediately visible as an error. Under no circumstances should you turn on the boiler without a connected sensor if you have the DHW tank function activated – there is a risk of either the tank not heating up, or the valve remaining stuck in the tank position for a long time.

Do you have a question about this topic?

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