Storage Tank with One or Two Heat Exchangers – Which to Choose

We are still preparing products.

But you can look at other categories.

Tank with one or two coils – which to choose and why it matters

When a customer chooses a boiler and tank set, most attention usually goes to the boiler's output, brand, or price. The question of how many heat exchanger coils the hot water tank has usually comes up as an afterthought – or not at all. Yet this decision can fundamentally affect not only hot water comfort, but also the return on the entire investment, system flexibility, and future expansion options.

This article covers this topic in depth: what a coil in a tank is, how one and two coils work, when a single coil is really sufficient and when a second coil is not just an advantage but a genuine necessity. Read it in full – especially if you're planning an installation in a family house where solar panels or a heat pump might appear in a few years.

What is a heat exchanger coil in a tank and how does it work

A domestic hot water (DHW) tank is essentially an insulated vessel in which water is heated and stored for household use. The tank itself, however, doesn't produce heat – it receives it from an external source. And it is precisely for this heat transfer that coils are used.

A heat exchanger coil in a tank is usually realized as a spiral coil made of copper or stainless steel pipe, submerged in the water inside the tank. Hot water from the boiler (or another source) flows through this coil. Heat passes through the coil wall into the tank volume and heats the domestic water, while the two circuits remain physically separated – the water from the boiler and the drinking water never mix.

The size of the coil (more precisely, its heat exchange surface area, given in m²) directly determines how quickly the tank can absorb heat from a given source. This is especially crucial for low-temperature sources such as solar collectors or heat pumps – these do not supply heat at high temperatures, and therefore need a larger coil for efficient heat transfer.

Tank with one coil – who it's for

A tank with one coil is the basic and most widespread configuration. It has one heat exchanger coil, usually placed in the lower part of the tank, through which water from the boiler flows. It's a simple, reliable and cheaper solution that fully suffices for most households where the only heat source is a gas or other type of boiler.

The typical heat exchange surface area of the lower coil in tanks of 100–200 l ranges from 0.8 to 1.6 m². For a powerful gas boiler operating with heating water temperatures of 60–80 °C, this is more than sufficient – such a boiler can heat up the tank quickly (100 l in 20–30 minutes at full output).

An example of a well-designed set with a single-coil tank is Protherm Aqua Complet Panther 25KOO + FE 120 BM – an atmospheric Panther 25 kW boiler combined with the FE 120 BM tank. The FE 120 BM tank has one coil and a volume of 120 litres, which is quite a comfortable combination for a 3–4-person household with typical hot water consumption. A similar setup is the Protherm Aqua Complet Panther 25KTO + FE 120 BM set, which is the turbo version of the same boiler.

A single-coil tank is the right choice when:

you have only one heat source (gas boiler, electric boiler, controlled pellet boiler),
you do not plan to install solar collectors or a heat pump in the near future,
you're looking for an economical and simple solution with minimal maintenance requirements,
you have limited space and a two-coil tank would be unnecessarily large or heavy.

Tank with two coils – what extra it brings

A tank with two coils has – as the name suggests – two heat exchanger coils. The lower coil is usually larger and intended for a low-temperature source (solar, heat pump). The upper coil is smaller and connects to the boiler or another backup heat source capable of working with higher temperatures.

This arrangement is based on a physical principle: temperature stratification occurs in the tank – the lower layer of water is cooler, the upper is warmer. Solar collectors supply heat precisely to the lower, cooler section, where the temperature gradient is most favourable for them and the larger-area coil can efficiently transfer heat even at a relatively low collector temperature (e.g. 45–55 °C). The boiler then only "tops up" the upper layer to the required temperature (usually 55–60 °C) when solar isn't enough – for example on cloudy days or in the evening.

This principle of dual heating is extremely energy-efficient. In summer, a solar system can cover practically 100% of DHW demand, 50–70% in spring and autumn, and at least 10–20% in winter. The boiler only turns on when the water temperature in the upper part of the tank drops below the set value. Result: lower gas consumption, longer boiler lifespan (fewer switching cycles), and lower operating costs.

Heat exchange surface area: why it's crucial for solar

Here's one of the most common mistakes in practice. A customer buys a two-coil tank, but the lower coil has too small a heat exchange surface area. Result: solar does work, but the tank heats up more slowly than it could, and the overall system efficiency is low.

The basic rule for solar systems: the heat exchange surface area of the lower coil should be at least 0.1 m² per m² of solar collector. In practice, this means that with a collector area of 4 m² (typical for a family of 4), you need a coil with an area of at least 0.4 m², preferably 0.6–0.8 m². A good solar tank of 200–300 l has a lower coil with an area of 1.2–2.0 m².

For heat pumps, this requirement is even stricter. A heat pump operates with an outlet temperature usually of 45–55 °C (monovalent operation) and needs a very large heat exchange surface area to efficiently heat the tank. For heat pumps, a heat exchange area of 1.5–2.5 m² is recommended for the lower coil in tanks of 200–300 l. There are also special heat pump tanks with one huge coil or without a coil (direct heat pump heating), but that's a different category.

Specific models and their coils – a look at practice

Let's look at a few real examples from the product range to apply these principles to specific products.

The Vaillant VIH CK 70 tank (70 l volume) is a single-coil tank intended primarily for smaller households or as an addition to a larger system. The heat exchange surface area is approx. 0.8–0.9 m², which is fully sufficient for a boiler operating at 75 °C. In the set Vaillant atmoTEC plus VU CZ/SK 200/3-5 + VIH CK 70 or Vaillant turboTEC plus VU CZ/SK 202/3-5 + VIH CK 70, this is a compact, affordable solution for a household of 2–3 people with no ambitions to add solar later. The 70-litre tank is on the lower end – for a family of 4 it may be insufficient during morning peak use – but for a smaller apartment or a customer who showers sequentially, it works without problems.

In contrast, the Vaillant VIH R 120 tank in the set Vaillant turboTEC plus VU CZ/SK 202/3-5 + VIH R 120 has 120 litres and a larger coil, which makes it suitable for a larger family. It's still a single-coil tank, but with higher DHW comfort.

The Protherm FE 120 BM tank has one lower coil and a volume of 120 l. It's a solid tank for a set with a single heat source. The heat exchange surface area is sufficient for the Protherm Panther boilers it accompanies. If the customer later wanted to add solar collectors, they would have to replace the tank with a two-coil model – this is exactly the argument for why it pays to think ahead in new builds.

When a two-coil tank is absolutely necessary

In practice, there are several typical scenarios where a two-coil tank is a requirement for correct system operation:

1. Combination of solar collectors and a boiler

This is by far the most common case. An investor is building a family house and installs solar collectors for DHW heating right during construction. A two-coil tank is a technical necessity in this case: the lower (solar) coil with a larger area receives heat from the solar circuit, the upper (boiler) coil tops up the tank when solar isn't enough. These two circuits are physically separated and each operates at a different temperature and pressure.

2. Heat pump as the main source + boiler as backup

In bivalent systems (heat pump + gas/electric boiler), two coils are also needed. The heat pump operates at a low outlet temperature and needs a large-area lower coil. The boiler then serves as a backup source during extreme cold, when the heat pump isn't sufficient, using the upper coil.

3. Combination of a solid fuel boiler (buffer boiler) and a gas boiler

In some households, customers combine a wood or pellet boiler (without adjustable output) with a gas boiler as a backup source. A two-coil tank allows both sources to be connected so they operate independently. The solid fuel boiler heats the tank via the lower coil, the gas boiler is the backup via the upper coil.

4. Future system expandability

This is an argument that customers tend to underestimate at purchase, but appreciate 5 years later. If you install a single-coil tank today and decide to add solar in 3 years, you'll have to replace the tank. Replacing a tank in an existing boiler room is always more expensive and complicated than choosing correctly from the start. A two-coil tank costs 10–15% more at purchase, but can save you a replacement costing several hundred euros later.

Clear-cut scenario: a two-coil tank is NOT needed

Let's be fair – a two-coil tank isn't always better. In many cases it's an unnecessary expense. When a customer installs a set with just a gas boiler in an apartment or smaller house, where there's no terrace or roof for solar, no space or interest in a heat pump, a single-coil tank is a fully-fledged solution with no compromises.

A two-coil tank has several disadvantages compared to a single-coil one:

Higher price – the difference for 120–200 l tanks is usually 80–200 €.
Greater weight and dimensions – a two-coil tank is usually taller or larger in diameter, which can be a problem in a small boiler room.
The second circuit must be properly connected – if you don't want the lower coil to unnecessarily draw heat from the tank, the solar circuit must be correctly regulated and insulated. Incorrect installation (e.g. an unconnected solar circuit full of air) can cause problems.
More complex regulation – with two heat sources, priority must be set correctly so the sources don't "fight" each other.

How to compare tank technical parameters – what to look for

When choosing a tank in combination with a boiler, we recommend looking at these key parameters in the datasheet:

Volume (l) – the basis for covering consumption. For a family of 4, at least 120 l, ideally 150–200 l. The topic of volume is covered in more detail in the article What tank volume do I need for my household.
Coil heat exchange surface area (m²) – 0.8–1.4 m² is sufficient for a boiler, at least 1.2 m² for solar, 1.5 m² or more for a heat pump.
Continuous heating output (l/h or NL) – indicates how many litres of DHW per hour the tank can deliver. Important for households with high peak demand.
Insulation – heat losses (W/24h) – the lower, the better. Modern tanks usually have heat losses below 70 W/24h at 65 °C.
Maximum tank operating pressure – usually 6–10 bar, depending on the system. Compatibility with the connected pressurized network should be checked.
Inner tank material – enamelled steel (most common, affordable, needs a magnesium anode) or stainless steel (more expensive, longer lifespan, no anode needed). More on this in the article Service and maintenance of a boiler and tank set – what and how often.

Practical examples from installation practice – what really happens

Case A: New build, family of 4, a real mistake in the choice

The customer bought a set with a single-coil 120 l tank. A year later, they had 3 flat solar collectors installed. The service technician found that the existing tank had no connections for a solar circuit. The customer had to buy a new two-coil tank (200 l), pay for removing the old one, installing the new one, refilling the circuits, and new wiring. Total additional cost compared to the correct choice from the start: approx. 650 €. Conclusion: in a new build, always ask yourself whether you're considering solar in a 5–10 year horizon.

Case B: Apartment, 2 people, single-coil 100 l tank – the right choice

A couple in a 68 m² apartment is planning a set with a gas condensing boiler + 100 l tank. Neither solar nor a heat pump is an option in the apartment building. A single-coil tank is the ideal choice – simple, reliable, 5–10 cm smaller in diameter than an equivalent two-coil model, and 120 € cheaper. The tank will function without any limitations.

Case C: Family house, pellet boiler + gas boiler as backup

The customer had a pellet boiler, but when the pellet supply was interrupted in winter, their house froze. They installed a small gas condensing boiler as a backup. The tank had one coil – for the boiler. Technically, the pellet boiler and gas boiler could be connected to a single coil via a three-way valve, but it's more efficient and cleaner to solve this with a two-coil tank, where each boiler has its own circuit. In this case, the customer replaced the 120 l tank with a two-coil 160 l one, and the system now works without problems.

Regulation and heating control with two coils

Correct control setup is critical with a two-coil tank. The boiler and the solar system (or heat pump) must be controlled so they don't unnecessarily overlap. Basic regulation principles:

Solar priority – when solar is supplying heat, the boiler should not interfere with heating the tank. This is ensured by a solar controller with sensors on the collector and in the tank.
The boiler starts when the temperature drops below the set value – usually a sensor in the upper part of the tank is set to 55–60 °C. If the temperature drops below this value (solar isn't enough), the boiler switches on and tops up the upper section via the upper coil.
Legionella protection – once a week (or once every two weeks), the system should heat the tank to 70 °C for at least 30 minutes. Modern controllers handle this automatically. It's important that this function doesn't activate at a time when the boiler would have to work unnecessarily – ideally early in the morning, when solar isn't yet sufficient but there's still leftover heat in the tank from the previous day.
Connection to the boiler control unit – modern boilers (Vaillant, Protherm) allow direct connection with solar regulation via a digital bus (eBUS, OpenTherm). This allows for more sophisticated control and better energy efficiency.

The topic of correct connection and installation is covered in detail in the article How to connect a boiler to a tank – installation procedure.

Economics of the choice: return on the price difference

Let's compare the finances. A single-coil 150 l tank typically costs 350–550 €. A two-coil tank of the same volume costs 450–700 €. The price difference is therefore 100–200 €. If you view this difference as an investment in the future possibility of a solar installation, it's an almost negligible amount – the solar system itself (collectors, pump station, regulation, installation) costs at least 2,000–4,000 €.

On the other hand, if you'll never have solar and buy a two-coil tank just "to be safe", you'll pay 100–200 € extra and never use the second coil. This is a decision the customer must make themselves based on a realistic estimate of future plans.

A more detailed comparison of specific sets including prices can be found in the article Protherm Panther vs. Vaillant turboTEC – comparison of sets.

Summary – a decision tree to conclude

If you've read this far, you have enough information for an informed decision. For clarity, let's summarize the key questions you should ask yourself:

Do I have now, or plan within a 5-year horizon, solar collectors? → Yes: a two-coil tank is a necessity. No: a single-coil tank is sufficient.
Am I considering a heat pump? → Yes: a two-coil tank with a large lower coil area (or a special heat pump tank). No: a single-coil tank is sufficient.
Do I have two boilers (e.g. pellet + gas backup)? → A two-coil tank is the right choice for a clean, independent connection of both sources.
Am I installing only a gas boiler, in an apartment or smaller house, with no expansion planned? → A single-coil tank is an economical and fully adequate solution.
Am I in a new build, not planning now but maybe later? → A two-coil tank as an investment in flexibility – the price difference is small, the savings from a later change significant.

Frequently Asked Questions (FAQ)

Can I connect solar to a single-coil tank later?

Technically, this isn't straightforward. A single-coil tank has no connections for a solar circuit. Some technicians solve solar via an external (plate) heat exchanger outside the tank, which is possible but more complex and less efficient. It's much cleaner and cheaper to replace the tank with a two-coil model. That's why we always recommend thinking about this in advance for new builds.

Is a larger heat exchange surface area always better?

Not necessarily. With a high-temperature source (gas boiler at 75 °C), a large coil area isn't necessary – the tank heats up quickly even with a smaller coil. A large area is crucial for low-temperature sources (solar, heat pump), where the temperature gradient is small and heat transfer without a large area isn't sufficient. An oversized coil for boiler heating isn't a mistake, but it brings slightly higher initial costs without practical benefit.

Can I operate a two-coil tank with only one source (just the boiler) and not use the second coil?

Yes, this is a common situation. The solar (lower) coil simply remains unconnected – no medium flows through it. The tank functions completely normally with the boiler via the upper coil. The only recommendation: fill the unconnected circuit with at least corrosion inhibitor or close it off, to prevent corrosion inside the coil. In practice, for short-term non-use (waiting for a solar installation), this is simply solved with closed valves.

How do I know which coil is upper and which is lower when choosing a tank?

In the datasheet or technical drawing of the tank, both coils are shown with their position and heat exchange surface area marked. They're usually labelled as coil 1 (upper, for the boiler) and coil 2 (lower, for solar). Some manufacturers state the number of coils directly in the tank's name (e.g. "BSB 200/2" = 2 coils). If in doubt, always ask for the product's technical data sheet.

Does the number of coils affect the tank's lifespan?

Not directly. The tank's lifespan depends primarily on the quality of the internal enamel coating or stainless steel, the regularity of magnesium anode replacement (for enamelled tanks, every 2–4 years), water quality, and pressure conditions. A two-coil tank with two circuits has slightly more connections and thus more potential leak points, but with correct installation this isn't a relevant factor. More on maintenance in the article Service and maintenance of a boiler and tank set – what and how often.

Is there a difference in energy class between single- and two-coil tanks?

The tank's energy class (according to the ErP directive) relates to the tank's heat losses – i.e. how well it's insulated. This value doesn't directly depend on the number of coils, but on the thickness and quality of insulation, tank size, and seal quality. Tanks of the same volume can have the same energy class regardless of how many coils they have. Always check the heat loss value in kWh/24h – the lower, the better.

Conclusion – choosing wisely means thinking ahead

The decision between a tank with one or two coils isn't just a technical formality – it's a strategic decision about how flexible and efficient you want your hot water preparation system to be over a 10–20 year horizon. A single-coil tank is the right choice for a simple, closed system with a single heat source. A two-coil tank is a necessity for any combination of sources – and a sensible investment for anyone who doesn't rule out adding solar collectors or a heat pump in the future.

We recommend addressing this question right at the start of every order – before the boiler is chosen, before the pipes are ordered. Correctly sizing the boiler's output to the tank's volume and number of coils is the foundation that will affect comfort and operating costs for the entire lifespan of the system. If you're not sure, also check out the article Correct sizing of boiler output to tank volume or How to choose a boiler and tank set – what to watch out for.

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.