Bosch Tronic electric boiler with WD 120 water heater – how the combination with a water heater works
Bosch Tronic electric boiler with WD 120 water heater – how the combination with a hot water tank works
One of the most frequent questions we deal with when selling and installing Bosch Tronic Heat 3500 electric boilers is connecting them with an external storage domestic hot water heater – specifically with the Bosch WD 120 tank or similar indirect-heating storage tanks. This is a combination that has proven successful in family houses, holiday cottages, and smaller commercial premises. At the same time, it is a system that needs to be correctly sized, hydraulically connected, and set up correctly – otherwise heating and water heating may end up "stealing" output from each other, the water temperature in the tank may not be reached, or the boiler may operate inefficiently with short cycles.
In this article, we will look at the whole topic comprehensively: what the WD 120 is, how indirect heating works, how to design boiler output for both purposes at once, what the hydraulic wiring diagram looks like, what boiler settings are needed, and what to address during installation. We are drawing on real installations – we have seen dozens of them, and each one brought some new insight.
What is the Bosch WD 120 and why this particular tank
The Bosch WD 120 is a domestic hot water (DHW) storage tank with a volume of 120 litres, designed for indirect heating – that is, via a built-in heat exchanger (heating coil) through which hot water from the boiler flows. The tank has no electric heating element of its own; all its heat output is supplied by the primary heat source, in our case the electric boiler.
The tank is available as a floor-standing model with the hot water inlet from the boiler at the top or bottom, depending on the configuration. It has a built-in thermostat that signals to the boiler or tank pump when heating is needed. A volume of 120 litres is more than sufficient for a standard family of 3–4 people under normal consumption; for larger families or households with higher hot water consumption, a larger tank (160, 200 litres) should be considered.
Why the WD 120 specifically with an electric boiler? Because Bosch Tronic Heat 3500 electric boilers are equipped with a direct output for controlling DHW preparation – they have an input for a tank sensor (NTC sensor or thermostatic contact) and an internal time program that allows priority charging of the tank. This means the whole system is controlled intelligently from one place, without the need for external controllers.
The principle of indirect water heating and its advantages over direct electric heating
Indirect heating works by the boiler heating water in the primary (heating) circuit, which then circulates through the heat exchanger in the tank, transferring heat to the domestic water. The domestic water and the heating water never mix – they are separated by the wall of the heat exchanger. The advantage is that ordinary mains pressure water enters the tank directly from the water supply network, without any risk of contamination by the heating medium.
Let's compare this with a direct electric heater (a boiler with an electric heating element): this typically has an output of 2–4 kW, and heating the tank takes longer. With indirect heating from an electric boiler with an output of 9 or 12 kW, you can charge a 120-litre tank in 20–35 minutes, which is significantly faster and is much appreciated during the morning peak (when the whole family showers).
Another advantage: a single heat source – the boiler – serves both purposes. You don't need a separate electric boiler with its own fuse and supply line. In addition, the Bosch Tronic boiler can intelligently alternate between DHW preparation and heating according to the set priority.
Output balance – how many kW do you need for the house and for hot water?
This is the most important question when sizing the system. A mistake we see repeatedly: a customer buys an Bosch Tronic Heat 3500 9 kW electric boiler, the house has a heat loss of 7 kW, and they think that "2 kW extra" will be enough to heat the water too. In practice, it simply doesn't work that way.
During priority DHW heating, the boiler briefly disconnects the heating circuit and concentrates its entire output on the tank. The charging time for the tank at 9 kW and a 120-litre tank (heating from 15 °C to 55 °C) is approximately:
t = (m × c × ΔT) / P = (120 kg × 1.163 Wh/kg·K × 40 K) / 9,000 W ≈ 0.62 h ≈ 37 minutes
During this time, heating does not work. At an outdoor temperature of –10 °C and in a well-insulated house, this is not a problem – the house has thermal inertia, and the indoor temperature will not drop by more than 0.5–1 °C per hour. However, in a less insulated building, or with longer charging times (larger tank, lower boiler output), this can become uncomfortable. That is why we recommend designing the boiler output with a reserve of at least 20–30% above the building's actual heat loss.
Practical recommendations for combination with the WD 120:
- House with heat loss up to 4 kW: Bosch Tronic Heat 3500 6 kW – the minimum usable variant, tank charging takes approx. 55 minutes
- House with heat loss of 5–7 kW: Bosch Tronic Heat 3500 9 kW – recommended combination for a family house of 3–4 people
- House with heat loss of 8–11 kW: Bosch Tronic Heat 3500 12 kW – suitable for larger houses or less insulated buildings
- House with heat loss above 12 kW: Bosch Tronic Heat 3500 15 kW – large family houses, renovations of older houses
You can find out how to calculate your house's heat loss in more detail in the article What output of a Bosch Tronic electric boiler do I need for my house in this Knowledge Centre.
Hydraulic wiring diagram for electric boiler + WD 120 tank
Correct hydraulics are the alpha and omega of a functional system. The Bosch Tronic Heat 3500 boiler is equipped with a circulation pump in the primary circuit. The DHW tank is fed by a separate branch with a tank pump (a smaller pump, typically 25-40, for example a Grundfos UPS 25-40 or similar). The tank pump is controlled directly by the boiler via a relay or via a three-way valve.
There are two main wiring configurations:
Variant A – three-way switching valve (3/2 motorised valve): The boiler pump pushes water into the three-way valve, which directs the flow either to the heating circuit or to the tank, depending on the signal from the boiler. During priority tank charging, the heating circuit is closed off. This is a simpler solution, suitable for smaller systems.
Variant B – separate tank pump connected to the primary circuit: The boiler has its own pump for the heating circuit, and the tank has its own separate pump. Both pumps are hydraulically connected to the boiler in parallel, but the boiler controls them with priority – when charging the tank, it stops the heating pump and switches on the tank pump. This is a more flexible variant, suitable for more complex systems (for example, when the heating circuit has additional branches with a thermostatic controller).
Setting up the Bosch Tronic boiler to work with the tank
Boilers in the Bosch Tronic Heat 3500 range have several key parameters in the menu that need to be set correctly when connecting a DHW tank. Let's look at each of them:
1. Activating DHW preparation in the boiler menu
In the basic setting, the boiler only operates in heating mode. If you want to activate the tank function, you must switch on DHW preparation in the service menu (the "DHW" or "DHW preparation" parameter). Once switched on, an option appears for setting the target tank temperature and the time program.
2. Target tank temperature
The standard recommendation is 55–60 °C. A lower temperature (below 50 °C) carries a hygiene risk – at temperatures of 30–50 °C, Legionella pneumophila bacteria can multiply. A higher temperature (60–65 °C) is safer from a hygiene point of view, but the risk of limescale in the heat exchanger increases. The Bosch Tronic boiler also has a thermal disinfection function – once a week, it heats the tank to 70 °C, which eliminates any potential bacterial source.
3. Tank priority
This is a fundamental setting. You can choose:
- Absolute DHW priority – heating is completely switched off while the tank is charging. The tank charges quickly, but room temperatures may drop slightly in winter.
- Parallel operation – the boiler tries to supply both at the same time, but output is shared. The tank charges more slowly, and heating runs continuously. This requires a higher boiler output.
- No DHW priority – the tank is only charged when heating doesn't require output. We do not recommend this – it may lead to insufficient water heating on extremely cold winter days.
For most households, we recommend absolute DHW priority with a time program – for example, morning tank charging between 5:30 and 6:30, and a second charge in the afternoon between 14:00 and 15:00.
4. Tank sensor
The WD 120 is equipped with a sensor pocket (immersion sleeve) for an NTC sensor. The boiler must be connected to the tank via a sensor cable (2-core, length depends on distance – typically no problem up to 10 m). The sensor monitors the actual temperature in the tank, and the boiler automatically starts or stops charging according to the set hysteresis (usually ±3–5 °C).
Installation requirements and practical experience from installations
Several practical rules must be followed during the physical installation of the boiler + tank combination. We are drawing on installations where we dealt with various complications:
Distance between the boiler and the tank: Ideally, both devices should be in the same boiler room or utility room, and the pipe connection (heat exchanger circuits) should not exceed 3–5 metres. Longer piping increases heat loss and extends the charging time. For every 5 metres of uninsulated DN 22 piping, you lose roughly 30–50 W – this is not dramatic, but over 10 metres of piping, this amounts to tens of kWh of unnecessarily consumed electricity per year.
Pipe insulation: All pipes carrying hot water between the boiler and the tank must be insulated – a minimum of 20 mm of foam polyethylene or rubber insulation. In practice, this is sometimes forgotten, and customers are then surprised why their tank cools down quickly.
DHW tank safety valve: On the water supply side of the tank (cold water), there must be a safety valve set to a maximum of 6 bar, and a mixing/pressure-reducing valve if the mains pressure exceeds 4 bar. The safety valve must have a discharge pipe leading to the sewer – during heating, the water expands and drips from the valve (this is normal). Customers sometimes think a dripping safety valve indicates a fault – but this is actually a common physical phenomenon.
Expansion vessel on the DHW side: This detail is sometimes omitted during installation, but it is important. If there is a check valve in the DHW piping (and there should be, to prevent cold water flowing back into the network), a closed system is created in which water expands and pressure rises during heating. Without an expansion vessel on the DHW side, the safety valve drips much more often and its service life is shortened. We recommend an 8–12 litre expansion vessel on the pressure side of the tank.
Limescale: In areas with hard water (above 20 °dH), it is recommended to install a water softener or at least a polyphosphate dosing unit before the tank. Limescale deposits on the heat exchanger and gradually reduces its efficiency – in extreme cases down to 30–40% of the original value. A 3 mm layer of scale on the heat exchanger causes a loss of over 30% of thermal efficiency. More details on hydraulic wiring can be found in the article Expansion vessel and hydraulic wiring of a Bosch Tronic electric boiler.
Electrical installation – power supply for both devices
Depending on the output, the Bosch Tronic Heat 3500 electric boiler is either single-phase (4 kW and 6 kW, max. input 27 A) or three-phase (9 kW, 12 kW, 15 kW). The WD 120 tank, in a configuration without its own electric heating element, has no direct power supply – it is passive from an electrical installation point of view. The tank pump (P2) and any three-way valve are powered from the boiler's terminal block, where the Bosch Tronic Heat 3500 has separate outputs (230V AC) for controlling external devices.
The maximum current draw of the boiler-controlled outputs is usually 1A (230V), which is fully sufficient for the tank pump (typical input 20–60 W) or a motorised valve. You can find more about the electrical connection in the article Connecting a Bosch Tronic electric boiler to a single-phase vs. three-phase network in this Knowledge Centre.
Time program and comfort – how to set tank charging in practice
The Bosch Tronic Heat 3500 boiler includes a built-in digital timer with the option of setting daily and weekly programs. For the combination with the WD 120 tank, we recommend the following programming schemes according to the household's lifestyle:
A family with a regular morning routine: Program the tank to charge at 5:00–6:30. The tank charges before the morning shower routine, then the boiler switches back to heating. A second charge around 16:00–17:30 before evening showers. At night, the tank is not heated, which saves energy.
Cottage or holiday property: For weekend use, set the boiler so that the tank starts heating 2 hours before arrival (controlled via a GSM module or smart thermostat). During the week, it is recommended to keep the tank at a minimum temperature of 45 °C (against Legionella), rather than switching off heating completely.
Off-peak electricity rate: If you have a contract with a discounted night-time electricity rate, program the primary tank charging into the night window (e.g. 22:00–6:00). A 120-litre tank at 60 °C can cover morning DHW needs even without daytime charging – the heat losses of a well-insulated tank (energy efficiency class B or better) are only 60–90 Wh per hour.
Legionella and domestic hot water hygiene
This topic is often underestimated in practice, yet it is crucial for the health of users. Legionella pneumophila is a bacterium that multiplies in water at temperatures of 25–45 °C. A DHW tank, where water remains at a low temperature for longer periods, is an ideal environment for it to multiply.
Recommended measures for the WD 120 tank in combination with an electric boiler:
- Set the target tank temperature to at least 55 °C (bacteria do not multiply at this temperature)
- Activate the thermal disinfection function in the boiler – once a week (e.g. overnight from Tuesday to Wednesday), heat the tank to 70 °C for 15 minutes
- Regularly (once a year) flush the tank at 70 °C and check the magnesium anode
- If you do not use the tank for more than 2 weeks, drain it or set a one-off heating cycle to 70 °C before use
The magnesium anode in the WD 120 protects the tank against corrosion. Replacement is recommended every 2 years, but this depends on water hardness and aggressiveness. More about regular maintenance can be found in the article Maintenance and servicing of the Bosch Tronic electric boiler – what and how often to check.
The most common problems with the boiler + tank combination and their solutions
From practice, we know of several recurring complications you should be prepared for:
The tank does not heat up to the required temperature: There can be several causes – an incorrectly connected sensor, a boiler temperature set too low (note: the boiler temperature must be at least 10–15 °C higher than the required tank temperature), a heat exchanger clogged with limescale, or insufficient boiler output. Check each of these causes systematically.
The tank takes too long to charge: Either the boiler output is undersized, or the tank pump has too low a flow rate. The optimal flow rate through the heat exchanger for a 120-litre tank is usually 8–12 l/min. Check the pump speed setting and any air in the tank branch.
Heating drops significantly while the tank is charging: This is normal with absolute DHW priority, but if charging takes too long (over 60 minutes), there is a problem with output or hydraulics. The solution is to increase the boiler output or switch to parallel operation (if the boiler output allows it).
The DHW tank's safety valve keeps dripping: If it only drips during heating, this is normal. If it drips constantly, the mains water pressure is too high (check and possibly install a pressure-reducing valve), or there is no expansion vessel on the DHW side.
You can find a systematic troubleshooting procedure in the article Common faults of Bosch Tronic Heat electric boilers and their solutions.
Comparing scenarios – with a WD 120 tank vs. a combi boiler vs. an instantaneous water heater
Customers sometimes ask why not buy a boiler with a built-in DHW preparation function (combi) or an electric instantaneous water heater. Let's compare:
| Solution | Advantages | Disadvantages |
|---|---|---|
| Electric boiler + WD 120 tank | Large DHW reserve, fast heating, hygienically safe, centralised control | Higher investment cost, more space needed, correct hydraulics required |
| Combi boiler (gas/electric with instantaneous DHW preparation) | Compact solution, unlimited DHW flow with sufficient output | Lower comfort with simultaneous draw-off at multiple points, more expensive for electric versions |
| Direct electric boiler (with heating element) | Simple installation, low equipment cost | Slow heating (2–4 kW element), needs an additional power supply, separate circuit breaker |
| Instantaneous electric water heater | Instant hot water, compact, no tank | Huge input (6–27 kW), problematic for an average household, requires a strong connection |
For family houses with an electric boiler, combining it with a WD 120 or similar tank is clearly the best solution in terms of comfort, hygienic safety, and energy efficiency.
Frequently Asked Questions (FAQ)
Can I connect a WD 120 tank to a Bosch Tronic Heat 3500 4 kW electric boiler?
Technically, yes, but it's on the edge of usability. At an output of 4 kW and a 120-litre tank, charging from 15 °C to 55 °C takes about 83 minutes. During this time, heating does not work (with priority tank charging). For a smaller house up to 50–60 m², this may be sufficient, but consider the 6 kW version or a smaller tank (80 litres) instead. We recommend consulting a heating designer before making a decision.
Do I need to call a plumber for the WD 120 tank, or can I connect it myself?
The WD 120 tank itself falls into the category of pressurised water appliances, and its installation should be carried out by a certified plumber. The electric boiler, in turn, requires connection by an authorised person (an electrician authorised under Decree No. 508/2009 Coll.). Combining both devices and their hydraulic interconnection requires an experienced heating engineer. Installing the electric boiler by a layperson may result in loss of warranty. You can find more about the requirements in the article Installing the Bosch Tronic Heat 3500 electric boiler – procedure and requirements.
What is the ideal hysteresis of the tank thermostat?
The Bosch Tronic boiler usually sets the tank thermostat hysteresis to 3–5 °C. This means that if you set the tank temperature to 55 °C, the boiler will start charging when it drops to 50–52 °C and stop once it reaches 55 °C. A larger hysteresis (5–8 °C) reduces the number of switching cycles and is gentler on the boiler, but the tank cools down more before a new charging cycle. A smaller hysteresis (2–3 °C) ensures more stable water temperature, but the boiler switches on more often. For households, we recommend 4–5 °C.
How long does hot water last in the WD 120 tank without heating?
The WD 120 tank has 50 mm thick polyurethane thermal insulation. Heat losses at a temperature of 60 °C and an ambient temperature of 20 °C are approximately 60–90 Wh per hour (depending on the specific unit and the condition of the insulation). A tank filled at 60 °C cools by roughly 4–6 °C over 8 hours. Hot water is therefore usable even after a whole night without heating – which confirms that night-time charging (with a cheaper electricity tariff) and morning draw-off is a realistic and practical scenario.
Is an expansion vessel also needed on the DHW (domestic water) side?
Yes, and this is one of the most frequently overlooked details. If there is a check valve in the installation (which is standard), the hot water has nowhere to expand to during heating, and pressure rises. The safety valve does discharge the excess water, but you lose a few litres of water with every heating cycle, and the safety valve wears out faster. An 8–12 litre expansion vessel with a pre-charge pressure of 3 bar (set according to the mains pressure – usually pre-charge pressure = mains pressure minus 0.3 bar) eliminates this problem. Remember that the expansion vessel on the heating side and the expansion vessel on the DHW side are two different components!
Can I combine the Bosch Tronic electric boiler with a tank other than the WD 120?
Yes, the boiler is not exclusively tied to the WD 120 tank. What matters is that the tank has a sensor pocket for an NTC sensor (compatible with the boiler's range: typically 10 kΩ at 25 °C), a heat exchanger with sufficient heat transfer area, and a volume suited to your consumption. Bosch, Buderus, Junkers, Dražice, or Regulus tanks are commonly used in practice. Always check that the heat exchanger's parameters are compatible with the boiler's output. A tank with too small a heat exchanger will not be able to accept the boiler's full output, and the charging time will be extended.
Conclusion – key takeaways for planning your system
The combination of a Bosch Tronic Heat 3500 electric boiler with a WD 120 domestic hot water tank is proven, reliable, and comfortable. It works well when the boiler output is correctly sized (with a reserve for both circuits), when the hydraulics are properly designed (tank pump, safety valves, expansion vessels), when the boiler parameters are correctly set (target temperature, priority, hysteresis, thermal disinfection), and when the entire installation is entrusted to experienced professionals.
If you're deciding on the output, take a look at the full range of Bosch Tronic electric boilers on the Bosch Junkers Tronic category page. If you're choosing between models, we recommend the article How to choose a Bosch Tronic Heat 3500 electric boiler – what to watch out for as well as Bosch Tronic Heat 3500 vs. other electric boilers – comparing models and outputs. A correctly designed system will serve you reliably for 15–20 years and ensure comfortable heating and enough hot water throughout the winter.
Do you have a question on this topic?
Can't decide, or dealing with a specific situation in your household? Write to us – we'll be happy to help.