Bosch Tronic Heat 3500 and solar water heating – combination with WD storage tank

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Bosch Tronic Heat 3500 and solar water heating – combination with a WD storage tank

Combining the Bosch Tronic Heat 3500 electric boiler with solar domestic hot water heating is a solution that is becoming increasingly common in practice – and not without reason. It is a well thought-out connection of two heat sources, where the solar system does the "heavy lifting" during the summer months and the electric boiler reliably provides supplementary heating during transitional periods or when the sun is not enough. This article looks at the technical background of such a combination, what a WD storage tank is, and how the whole system is designed, connected and operated in practice – including specific figures, diagrams and scenarios we have seen in real installations in Slovakia.

Why combine an electric boiler with solar heating?

At first glance, it may seem that an electric boiler and a solar system are two separate things that have little in common. The opposite is true. In the context of a combined system, the electric boiler is primarily a backup and supplementary heating source – it works when solar energy is not sufficient to cover the demand for hot water or heating. Thanks to this approach, a household can significantly reduce its electricity consumption, since during the summer the solar system can cover 60 to 80% of the annual hot water demand (depending on collector area and location).

The Bosch Tronic Heat 3500 is suitable for this task for several reasons: it has smooth output control, a clean hydraulic layout, and easy integration into external control systems. Moreover, its robust construction does not require complicated servicing, and the electric boiler can work as a purely backup source for several months a year with essentially no significant load.

What is a WD storage tank and how does it work?

A WD storage tank (from the German Wärmespeicher mit Doppelregister, or in the Slovak context also referred to as a tank with two heat exchangers) is a domestic hot water storage tank equipped with two separate coil or plate heat exchangers. Each exchanger forms a separate circuit:

Lower heat exchanger – connected to the solar collector circuit. Solar energy heats the tank from the bottom up, making use of natural thermal stratification.
Upper heat exchanger – connected to the boiler (in our case, the Bosch Tronic Heat 3500 electric boiler). It heats the upper volume of the tank, from where domestic hot water is drawn.

The operating principle is simple: the solar circuit heats the tank throughout the day according to the available energy. If the temperature in the upper part of the tank drops below the set value (typically 45–55 °C), the electric boiler switches on and heats the upper exchanger. Thanks to stratification, it does not need to heat the entire tank volume – only the upper part, which saves energy and time.

Typical WD storage tank volumes for family houses: 200 litres (2–3 people), 300 litres (3–4 people), 400–500 litres (5 or more people, or a combined system with space heating). For systems with the Bosch Tronic Heat 3500 and a solar circuit, volumes of 200 to 400 litres are most common.

Hydraulic wiring diagram – how the whole system works together

Integrating the Bosch Tronic Heat 3500 into a solar system with a WD storage tank requires proper hydraulics. The solar circuit (collectors → pump station → lower tank heat exchanger → back to collectors) is functionally separated from the electric boiler circuit (boiler → upper tank heat exchanger → back to the boiler). Both circuits are hydraulically independent – they only share heat transferred through the exchanger walls.

In practice, most installations look like this:

The solar system has its own control unit (solar controller, e.g. Bosch Solar 6000 or Resol), which controls the solar circuit pump based on the temperature difference between the collector and the lower part of the tank.
The Bosch Tronic Heat 3500 is connected to the upper tank heat exchanger. Its built-in thermostat or an external controller (e.g. Bosch CR10H) monitors the temperature of the upper part of the tank and starts the boiler as needed.
Domestic hot water is drawn from the top of the tank – where the water is always hottest (thermal stratification).
Cold water is fed into the tank from the bottom, keeping the temperature in the lower zone lower, which allows the solar circuit to transfer heat more efficiently (the lower the temperature in the tank, the higher the efficiency of the collectors).

Choosing the right electric boiler output for a combined system

One of the most important questions when designing such a system is: what output should the electric boiler have when part of the heating is taken over by the solar system? The answer is not trivial and depends on several factors.

In a purely storage-tank-based DHW heating system (without space heating), a simple rule of thumb applies: for a 200-litre tank, an output of 2–4 kW is ideal (heating in 1–2 hours), for 300 litres 4–6 kW, and for 400 litres 6–9 kW. Since the solar system covers most of the demand in summer, the electric boiler works mainly during transitional periods (spring, autumn) and in winter months.

If the electric boiler serves both space heating and DHW preparation at the same time (a common scenario with the Bosch Tronic Heat 3500), the output is designed according to the building's heat loss – and the DHW tank is just one of the control priorities. In such cases, the following are commonly installed:

Electric boiler BOSCH TRONIC HEAT 3500 6 kW – for small houses and flats (up to 80 m²) with good insulation, where solar covers most of the DHW
Electric boiler BOSCH TRONIC HEAT 3500 9 kW – for medium-sized houses (80–120 m²) with a combined system, where the boiler serves as a backup for heating as well
Electric boiler BOSCH TRONIC HEAT 3500 12 kW – for larger family houses (120–160 m²) or buildings with a higher DHW standard

For purely backup DHW heating (where space heating is handled by another source), the Electric boiler BOSCH TRONIC HEAT 3500 4 kW is sufficient, heating a 200–300-litre tank within an adequate time – especially if the control system is set for night-time heating during the cheap tariff.

For a more detailed guide on choosing the output, see the article What output of the Bosch Tronic electric boiler do I need for my house in this Knowledge Centre.

Control and regulation of the combination – how to set it up intelligently

This is perhaps the most important part of the whole topic. Good control determines whether the system runs economically, or whether the electric boiler unnecessarily "competes" with the solar system and consumes electricity when it would be enough to wait a few hours for solar energy.

Basic control logic

A properly set-up system works according to these priorities:

Solar heating has priority – the solar controller starts the collector circuit pump whenever the collector temperature is 5–8 °C higher than the temperature in the lower part of the tank.
The electric boiler starts only when the tank temperature drops below the set limit – typically 45–50 °C. This value is set on the boiler controller or on an external tank thermostat.
Time program – the boiler can have a time program set so that it only runs during the night tariff or only in the morning before the peak DHW draw-off. This makes use of cheaper night-time electricity (off-peak tariff).
Hysteresis – the boiler does not restart until the tank temperature drops a further 3–5 °C below the set limit. This prevents short, frequent cycles.

The Bosch Tronic Heat 3500 allows the connection of an external controller (e.g. Bosch CR10H or equivalent OpenTherm devices). An external tank temperature sensor can be easily connected via the terminal block, and the controller takes care of the rest. Details on the wiring can be found in the article Connecting and setting up the control system of the Bosch Tronic Heat 3500 electric boiler.

Special case: DHW priority vs. space heating

If the electric boiler supplies both space heating (underfloor, radiators) and DHW via a WD storage tank at the same time, DHW heating priority must be set. Most controllers handle this automatically: when the tank temperature drops below the minimum, the boiler switches its output to tank heating for 30–60 minutes, while space heating draws on the building's thermal inertia in the meantime. Once the tank is heated, output is returned to space heating.

Practical scenarios from real installations

Scenario 1: New build 120 m², 4 m² solar, 300 l WD storage tank

A customer in the Trenčín region, collectors facing south, tilt 45°, 4-member household. Installed a Bosch Tronic Heat 3500 9 kW as a backup source for both DHW and space heating (underfloor). WD storage tank 300 litres. Result after the first year of operation: the solar system covered 68% of the annual DHW demand. The electric boiler worked mainly from October to March, and in June–August it only switched on for a few days during longer overcast periods. Annual electric boiler consumption for DHW: approx. 850 kWh; using the night tariff (DD1), total annual electricity costs for DHW dropped by 71% compared to the previous solution (instant water heater).

Scenario 2: Older house after insulation upgrade, 90 m², 6 m² solar, 200 l WD storage tank

A customer in the Banská Bystrica region, boiler room renovation, switch from gas to electric boiler + solar. Installed a Bosch Tronic Heat 3500 6 kW. 200-litre tank – sufficient capacity for a 3-member household. A larger-than-standard solar area of 6 m² compensates for the shorter sunny season in a location at higher altitude. Result: solar covered 74% of DHW, with the boiler supplementing only during the winter half of the year. The customer particularly appreciated that in summer the electric boiler did not switch on at all, and the DD1 night tariff fully handled it even in November and March.

Scenario 3: Cottage with irregular use, 150 l WD storage tank, mini-solar 2 m²

A customer wanted a minimalist solution for a cottage used weekly. Bosch Tronic Heat 3500 4 kW + a small 2 m² solar system + 150-litre tank. The controls were set so that the boiler starts every Friday evening (before the family arrives) and heats the tank to 55 °C. Solar heating tops it up over the weekend, and by Monday morning the tank is full of hot water again. This scenario shows that even a minimal solar area makes sense when combined with a smart boiler time program.

Installation and hydraulic requirements

When installing a combined system, there are several things that often cause problems in practice if overlooked:

Expansion tank and safety valve – each circuit (solar and boiler) must have its own expansion tank and safety valve. The solar circuit operates at higher temperatures (up to 130–160 °C during stagnation), so it must be designed differently from a standard heating system.
No mixing of circuits – the solar antifreeze fluid (propylene glycol mixture) must not come into contact with the drinking water in the tank. A WD storage tank with coil heat exchangers ensures this, but when choosing a tank it is necessary to verify that the exchanger meets the drinking water standard (EN 14861).
Pipe insulation – solar piping on the roof or outdoors must be insulated with UV-stable insulation, while boiler piping in the boiler room can use standard mineral wool or foam insulation.
Air venting – both circuits must have functional automatic air vents, ideally at the highest point of each circuit.
Check valves – prevent thermosiphon circulation at night, which could cause the tank to lose heat through the collectors.

More detailed installation requirements for the Bosch Tronic Heat 3500 can be found in the article Installing the Bosch Tronic Heat 3500 electric boiler – procedure and requirements.

Economic analysis – is it worth it?

The question of return on investment is legitimate in the context of a solar + electric boiler combination. Let's break it down with specific figures.

An average 4-member household consumes approx. 2,500–3,500 kWh of thermal energy per year for DHW heating. At an electricity price of approx. €0.22/kWh (D2 tariff), this would amount to €550–770 per year for electricity for DHW (excluding losses). With the DD1 night tariff (average savings of 30–40%) and a solar system covering 65–75% of DHW needs:

Annual electric boiler consumption for DHW: approx. 700–900 kWh
Of which in the night tariff (60–70% of consumption): approx. 450–630 kWh × €0.15/kWh = €67–95
The rest in the day tariff: approx. 200–300 kWh × €0.22/kWh = €44–66
Total annual electric boiler costs for DHW: €111–161

Investment costs for a solar system (2–4 m² of collectors, controller, pump station, installation): €2,500–4,000. WD storage tank 300 l: €800–1,300. Bosch Tronic Heat 3500 electric boiler: depends on output, approx. €400–900. Total investment: €3,700–6,200 (excluding grants and subsidies). Annual savings compared to electricity without solar: €400–600. Simple payback period: 7–12 years. If a subsidy is taken into account (e.g. Green Households 2+, where collectors can receive a grant of €350–700/m²), the payback period shortens to 5–8 years.

The long-term outlook, however, is clear: electricity prices are rising, a solar system has no moving parts on the collectors (only the pump) and has a lifespan of 20–25 years. The Bosch Tronic electric boiler, if regularly maintained, lasts 15–20 years. The combination therefore makes economic sense especially from a long-term perspective.

Setting up and optimising for the night tariff

Combining solar heating with the night tariff is a triple win. In summer, solar heats the tank during the day. The electric boiler has a time program set so that it only starts when the tank temperature is low AND the night tariff is active at the same time (e.g. 22:00–6:00 in DD1). Result: the boiler runs cheaply at night, the tank is full of hot water in the morning, and solar keeps it warm during the day.

In winter, when solar does not provide enough energy, the boiler simply works more – but still prefers night-time hours. You can find out more about setting up the night tariff for the Bosch Tronic in the article Bosch Tronic electric boiler and the night tariff – how to save on electricity.

Common installation and operating mistakes

From experience, we know that certain mistakes recur in combined systems. Here are the most common ones:

Boiler set to too low a tank temperature – if you set the boiler to heat the tank to only 40 °C (seemingly economical), you risk Legionella bacteria growth. The recommended minimum DHW temperature is 55–60 °C, with regular weekly disinfection heating to 70 °C.
Missing check valve in the solar circuit – at night, the collectors cool the tank if a check valve is not installed or if the solar controller does not have a function to protect the tank from cooling.
Oversized tank without sufficient boiler output – a 500-litre tank with a 4 kW boiler would take 8–10 hours to heat, which is uneconomical and practically unusable.
Undersized tank with a large solar area – if the solar system has 6 m² of collectors but the tank is only 100 litres, the collectors overheat (stagnation) and the system's lifespan is shortened.
Incorrect control priority – the boiler running at the same time as active solar heating, which wastes energy. The controller must correctly block the boiler during active solar heating.

Legionella and hygienic heating – a mandatory topic

For every DHW storage system, hygienic safety is crucial. The Legionella pneumophila bacterium multiplies at temperatures of 25–50 °C. DHW tanks are therefore at risk if not operated correctly.

Preventive measures in the combined Bosch Tronic + solar + WD storage tank system:

Tank temperature kept above 55 °C in the upper zone – boiler set to a minimum tank temperature of 55 °C
Weekly hygienic heating – most controllers (including those for the Bosch Tronic) allow setting automatic hygienic heating to 70 °C once a week (e.g. Sunday night)
Thermostatic valve at the DHW outlet from the tank – mixes hot water (70 °C) with cooler water to a safe temperature (max. 55–60 °C at fixtures), preventing scalding

Frequently Asked Questions (FAQ)

Can I connect the Bosch Tronic Heat 3500 directly to a WD storage tank without a solar system?

Yes, the Bosch Tronic Heat 3500 electric boiler can be connected to a WD storage tank even without a solar circuit – the lower heat exchanger simply remains unused or is shut off. In that case, the tank works as a standard boiler, heating from the upper heat exchanger. If you plan to add a solar system later, the tank will already be ready for it, without needing to be replaced.

What WD storage tank volume do you recommend for a combination with solar?

For a 3–4-member household with a collector area of 3–5 m², a 250–300-litre tank is the standard. For a larger solar area (6–8 m²) or a system combining DHW and space heating, we recommend 400–500 litres. A tank that is too small causes the collectors to overheat (stagnation), while a tank that is too large heats up slowly, making the boiler work inefficiently in winter months.

Can the solar system overheat the tank in summer to the point of causing a fault?

Yes, this is a real scenario with incorrect design. If the tank reaches its maximum temperature (usually 90–95 °C) and DHW draw-off is not sufficient to cool it, the solar circuit enters stagnation – the collectors overheat to 150–200 °C, the glycol boils, and the system is put under stress. The solution is a properly sized expansion tank in the solar circuit, stagnation-resistant piping (steel or copper pipes without plastic components near the collectors), and possibly a thermal protection function for the tank in the solar controller (night-time cooling via the collectors).

How much will I save per year if I add solar heating to an existing Bosch Tronic?

It depends on the location, collector area and DHW consumption. As a rough figure: a solar system with 3–4 m² of collectors for a 4-member household in Slovakia saves approx. 1,200–1,800 kWh of electricity per year for DHW heating. At a price of €0.20–0.25/kWh, this represents €240–450 per year. The payback period depends on the total investment costs and any subsidies, but realistically it is 6–10 years for the whole system.

Do I need special electrical protection for the combination of boiler + solar controller?

The solar controller (pump station + controller) typically has a power input of only 50–100 W and does not require special protection – a standard 230 V socket is sufficient. The Bosch Tronic Heat 3500 electric boiler, on the other hand, requires its own circuit breaker and RCD according to the installed output (e.g. 3×16 A for 9 kW, 3×20 A for 12 kW). Both systems are electrically independent and do not affect each other. Details can be found in the article Installing the Bosch Tronic Heat 3500 electric boiler – procedure and requirements.

Is it possible to combine the Bosch Tronic Heat 3500 with an air-to-water heat pump and solar at the same time?

Yes, such triple hybrid systems exist – a heat pump as the main heat source, solar for DHW, and an electric boiler as a peak backup source. However, this requires a properly designed hydraulic system (hydraulic separator, priority valves) and coordinated control. In such a system, the electric boiler works the least and acts as a "safety net" during extreme frosts or a heat pump outage. The WD tank is then an ideal hub for all three heat sources.

Conclusion – does this combination have a future?

The combination of the Bosch Tronic Heat 3500 with solar water heating and a WD storage tank is a technically well-proven solution with several irreplaceable advantages: simple installation, proven component reliability, the option to build the system gradually (first the boiler, later solar), and transparent control. It is not a solution for everyone – it requires sufficient solar potential at the location, a suitably oriented roof, and a willingness to invest in a long-term project. But with proper design and installation, it is one of the most economical solutions for hot water preparation for family houses in Slovakia.

The Bosch Tronic Heat 3500 electric boiler range is available in output variants from 4 to 15 kW – an optimal combination can be found for every house size and every type of solar installation. If you are considering such a system, we recommend starting with correctly sizing the solar area and the storage tank, and only then choosing the boiler output – in this order, not the other way around.

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.