Frequently Asked Questions about Condensing Boilers with Cylinder Connection Option

We are still preparing products.

But you can look at other categories.

Frequently Asked Questions About Condensing Boilers With Tank Connection Capability

When customers are looking for a condensing boiler with the possibility of connecting a hot water tank, they come with a whole range of questions – some basic, others very specific and technical. In this article we have collected the questions we really encounter most often in practice: during sales, during pre-installation consultations, and during service interventions. If you are planning to buy such a boiler or you are struggling with something specific in an existing installation, you will probably find the answer here.

What does "possibility of connecting a tank" actually mean for a condensing boiler?

This is the first and most basic question we encounter. A condensing boiler with the possibility of connecting a tank is, simply put, an instantaneous or central boiler that has a built-in hydraulic circuit and control system ready to work with an external hot water storage tank. The tank itself is not part of the boiler – the boiler is merely "prepared" for it.

In practice, this means the boiler includes:

a three-way valve or other switching element between the heating circuit and the domestic hot water (DHW) circuit,
a pump for charging the tank (either dedicated or shared),
an input for a tank temperature sensor (NTC probe),
a control system that can switch on the tank charging cycle according to demand and set priorities.

What the boiler does not have and what you need to buy separately: the tank itself, possibly the hydraulic connection between the boiler and the tank, pipe insulation, and several other connection parts. We cover this topic in more detail in the article Difference Between a Boiler Prepared for a Tank and a Combi Boiler, where we also explain why some people confuse these terms with a combi boiler.

Do I need a special tank, or can I use any storage tank?

This is one of the most common questions, and also one of the most important in terms of functionality. The answer is: it depends on the boiler and the type of tank.

Most condensing boilers with tank connection capability work on the principle of charging the tank via a primary circuit – hot water from the boiler flows into a coil heat exchanger inside the tank and indirectly heats the potable water. In this case, you can theoretically use any tank with a heat exchanger compatible with the boiler's output. The important tank parameters are:

Heat exchanger output (kW) – must be able to accept the boiler's output. A 25 kW boiler needs a tank with a heat exchanger capable of transferring at least 20–25 kW.
Location of inlets/outlets – the tank's connections must match the boiler's hydraulic layout.
Input for NTC sensor – the tank must have a place for the temperature sensor required by the boiler for control purposes.
Tank volume – depends on the size of the household and usage pattern (more details in the article What Hot Water Tank Volume Do I Need for My Boiler).

Some boilers, for example from the Protherm or Vaillant ranges, have their own recommended tanks that are specifically sized and tested to work with that boiler. If you use a tank from another manufacturer, it will generally work, but you must check whether the tank's heat exchanger matches the boiler's output and whether the connections are hydraulically compatible. In practice, we often see situations where a customer bought a cheap tank with an undersized heat exchanger and the boiler was unable to charge the tank in a reasonable time – in winter this became a major problem.

Protherm Gepard Condens or Panther Condens – which one for a tank?

Customers compare these two options very often, since both Protherm models are among the best-selling condensing boilers prepared for a tank on the Slovak market.

Protherm Gepard Condens 12 MKO and Protherm Gepard Condens 25 MKO are models designed primarily for smaller and medium-sized houses, where a modulating output of 12 kW or 25 kW is sufficient. Gepard Condens is more affordable, compact and easier to install. They are especially suitable where heating and DHW preparation are handled primarily via a tank with a volume of 120–200 litres.

On the other hand, Protherm Panther Condens 15 KKO belongs to a line of boilers with a higher output range and a more robust design. Panther Condens is popular in households with a larger tank (200+ litres) or where a hot water tank is combined with a solar collector for bivalent heating – because these boilers can handle more demanding hydraulic configurations. You can find a detailed comparison of both lines in the article Protherm vs Vaillant: Comparison of Boilers With Tank Connection.

How does DHW priority work compared to heating?

Most condensing boilers prepared for a tank operate in what is known as DHW priority mode. When the water temperature in the tank drops below the set value (usually 55–60 °C), the boiler switches the three-way valve to charge the tank and heating is temporarily interrupted or limited.

In practice this sometimes surprises customers – for example, the tank may heat up for a shower before the radiators come on in the morning. If this is a problem, the solution is either to correctly set the tank charging hysteresis (most boilers allow this to be set in the controller menu), or to choose a boiler with parallel operation of tank charging and heating. Some models, such as the Vaillant VU 25CS/1-5 ecoTEC plus IoniDetect, have more sophisticated controls that better balance these two demands.

Recommended settings from practice:

Tank hysteresis: charging starts when the temperature drops to 45–50 °C, and ends at 60–65 °C.
Maximum heating block time: adjustable to 20–30 minutes (depends on the controller).
Night-time tank pre-heating (legionella protection): heating to 70 °C once a week – most modern boilers do this automatically.

What is the actual gas consumption of boilers prepared for a tank?

This is a question customers phrase in different ways: "Will it cost me more than a combi boiler?", "Is a tank worth it if I live alone and use little hot water?" or "Where do manufacturers get that 109% efficiency figure?"

Let's start with the basics. Condensing boilers achieve efficiency above 100% (typically 98–109%) because they utilise the latent heat contained in the flue gases – water vapour condenses and in doing so releases heat back into the system. This effect is greater the lower the return water temperature from the heating system. That is why condensing boilers are ideal for underfloor heating (return temperature 30–35 °C) and less efficient with old radiators operating at a high temperature (return temperature 60–70 °C).

When heating the tank, a condensing boiler works with a higher water temperature (typically the boiler heats to 70–80 °C, the tank to 55–65 °C), which slightly reduces the condensing effect during charging cycles. Nevertheless, the total annual savings compared to an old conventional boiler are realistically 15–30%, depending on the condition of the house and the way it is operated.

You can find more about how tank heating saves energy compared to instantaneous (flow-through) heating in the article Why Tank Heating Saves Energy Compared to Instantaneous Heating. There you will also find specific calculations for a typical 4-member family.

What is IoniDetect and why does Vaillant present it as an advantage?

Customers looking at Vaillant models come across the term IoniDetect and don't know exactly what it means. It is an ionisation sensor technology in the burner that continuously monitors flame quality. This allows the boiler to detect in real time any deviations from optimal combustion – not just faulty ignition, but also gradual burner degradation, contamination, or fluctuations in gas quality (change in Wobbe index).

Practical benefit: the Vaillant VU 25CS/1-5 ecoTEC plus IoniDetect can automatically adjust combustion parameters, which extends burner life, reduces NOx emissions and ensures maximum efficiency under various operating conditions. Combined with tank connection capability, this is a solution for demanding households where long-term trouble-free operation with minimal service interventions is expected of the boiler.

Does the tank have to stand right next to the boiler?

Another very practical question that arises from real space constraints. The professional answer: the tank does not have to stand right next to the boiler, but the greater the distance between the boiler and the tank, the greater the heat losses in the piping and the longer the charging cycle takes.

From practical experience: a distance of up to 3–5 metres between the boiler and the tank is standard and causes no complications. At distances of 5–10 metres, quality insulation of the connecting pipe is needed (at least 19 mm insulation on DN 22 pipe). At distances over 10 metres, it may be necessary to consider a more powerful boiler or a more powerful charging circuit, since long runs increase hydraulic resistance and heat losses.

If the tank has to be in a different room than the boiler (for example, the boiler in the boiler room, the tank in the bathroom above it), this is technically feasible, but the designer or installer should include it in the hydraulic calculation. We cover installation requirements in more detail in the article Installing a Tank With a Condensing Boiler – Procedure and Requirements.

What is a magnesium anode and when should it be replaced?

A magnesium anode is a protective element inside the tank that prevents corrosive attack on the steel shell. It works on the sacrificial anode principle – the Mg oxidises preferentially instead of the steel. Without the anode, the tank would start to corrode from the inside, which leads to water contamination with rust and shortens the tank's lifespan.

Manufacturers recommend checking the anode once a year (ideally at every service visit) and replacing it when it wears down to less than 50% of its original diameter. This depends significantly on water quality – in areas with softer water (low mineral content), the anode wears out faster because it is more electrochemically active. In hard water, the anode's lifespan is usually longer, but there is a greater risk of limescale build-up in the tank.

Typical anode lifespan: 1–3 years. Replacement is relatively cheap (an anode costs a few dozen euros), but neglecting it can significantly shorten the tank's lifespan. You can find more about regular maintenance of the whole system in the article Maintenance and Servicing of a Condensing Boiler With a Hot Water Tank.

Vaillant VU 246/5-3 ecoTEC pro – who is this model suitable for?

Customers ask us what type of house the Vaillant VU 246/5-3 ecoTEC pro is ideal for. It is a condensing boiler with an output of 24 kW, designed for medium-sized family houses (typically 120–200 m², depending on the building's thermal insulation condition). When paired with a 150–200 litre tank, it comfortably covers the hot water needs of a 3–5 member household.

How does the Vaillant ecoTEC pro differ from the ecoTEC plus (or IoniDetect)? The Pro line is more affordable and structurally simpler, while the Plus line offers extended diagnostics and more advanced control functions. For an average family without special requirements, the Pro line is an excellent choice with a great price-to-performance ratio.

Can a boiler prepared for a tank be combined with a solar system?

Yes, and this is a combination we install more and more often. In such an installation, the tank must be bivalent – meaning it must have two heat exchangers: one for the solar circuit (lower, primary) and one for the boiler circuit (upper, secondary). Solar collectors cover tank heating on sunny days, and the boiler tops up the temperature to the required level when needed.

Important note: not every boiler prepared for a tank is automatically suitable for bivalent (solar) schemes without modifications. You need to check whether the boiler's controller can work with solar control or an external control unit. Manufacturers such as Protherm and Vaillant have their own control modules and instructions for these configurations. The full connection procedure for specific Protherm models is described in detail in the article How to Connect a Tank to a Protherm Panther Condens or Gepard Condens.

What are the most common mistakes when installing a tank with a condensing boiler?

From practical experience, we know that a large part of the problems customers deal with after installation come from a few recurring mistakes:

Incorrectly sized tank – a tank with a small heat exchanger cannot quickly absorb the boiler's output, the charging cycle is too long, and the customer complains about a lack of hot water.
Missing air vent on the charging circuit – air in the circuit causes noise and inefficient circulation.
Incorrectly connected tank NTC sensor – the sensor is not at the correct depth or in the correct position in the tank, so the boiler evaluates the temperature incorrectly and the tank either does not charge or overheats.
Undersized charging circuit piping – piping that is too thin (DN 15 instead of DN 22) with more powerful boilers causes high hydraulic resistance.
Missing insulation on the connecting pipe – significantly increases heat losses and extends charging time.
Forgotten expansion volume or safety valve on the cold water side of the tank – a legal requirement, and also a safety necessity.

We also address these mistakes in the context of faults in the article Common Faults of Condensing Boilers Operating With a Tank.

How do I choose a boiler prepared for a tank correctly?

The choice is not just about the boiler's output. Here is an overview of the factors to consider:

Building's heat loss – the basic input for sizing the boiler's output (calculation according to STN EN 12831).
Number of people in the household – directly affects the required tank volume.
Hot water comfort requirements – if the household has multiple bathrooms or a high-flow shower, a larger tank and a more powerful boiler are needed.
Heating system – underfloor heating vs. radiators significantly affects the boiler's operating point and condensing efficiency.
Planned combination with a solar system – requires a bivalent tank and compatible boiler control.
Space for the tank – physical dimensions of the boiler room or the room for the tank.
Service availability – both Protherm and Vaillant have a wide service network in Slovakia, which is a practical advantage.

You can find the whole selection process, including specific recommendations and sizing tables, in the article How to Choose a Condensing Boiler With Tank Connection Capability.

Frequently Asked Questions (FAQ)

Can I connect an old tank I already have at home to a new condensing boiler?

Yes, if the old tank meets the technical requirements of the new boiler: sufficient heat exchanger output (in kW), connections of the correct sizes, and an input for the NTC temperature sensor. If the tank does not have an input for the sensor, this can usually be solved with an external NTC sensor mounted on the tank shell, although this is not an ideal solution in terms of measurement accuracy. Also check the old tank for anode condition and possible corrosion – it is not economical to connect an expensive new boiler to a tank that will start leaking within a year.

Why does the boiler keep charging the tank even though the water is hot?

The most common cause is an incorrectly positioned or faulty tank NTC sensor. The sensor reports a lower temperature to the boiler than the actual one, so the boiler keeps seeing a "need to charge". Another cause could be poor electrical contact of the sensor or an incorrectly set reference temperature in the boiler's controller. Solution: check the sensor, its position, and its resistance at the given temperature (typically NTC 10 kΩ at 25 °C). You can find more about faults of this type in the article Common Faults of Condensing Boilers Operating With a Tank.

What pressure should there be in the tank system?

On the primary (heating) side of the boiler and the charging circuit, the standard operating pressure is 1.0–2.0 bar (cold system), with a maximum usually around 3 bar depending on the boiler. On the cold water (secondary) side of the tank, the pressure is determined by the supply pressure of the mains water, typically 2–5 bar. A safety valve (usually 6 bar) and an expansion vessel for hot water must be installed on the cold water side of the tank – this is essential, because when water is heated from 10 to 60 °C, the water in the tank expands, and without an expansion vessel the pressure would rise to dangerous levels.

Is there a difference between a boiler marked MKO and KKO from Protherm?

Yes, this is an important difference. The designation MKO (e.g. Gepard Condens MKO) means that the boiler is designed for heating with the possibility of connecting a hot water tank – in other words, a boiler without a built-in tank, but hydraulically and electronically prepared for one. The designation KKO (e.g. Panther Condens KKO) denotes a combi boiler – that is, a boiler with an integrated plate heat exchanger for instantaneous hot water heating. These two types differ in the way they prepare DHW and are suitable for different types of households. You can find a detailed breakdown in the article Difference Between a Boiler Prepared for a Tank and a Combi Boiler.

How long does it take to charge a 150-litre tank with a 25 kW condensing boiler?

As a rough guide: a boiler with an output of 25 kW (typically using 20–23 kW of net heat output for tank charging during operation) can heat a 150-litre tank from 10 °C to 60 °C in approximately 15–20 minutes, if the tank is correctly sized and the charging circuit is hydraulically sound. If charging takes longer than 30–40 minutes, something is wrong – typically an undersized tank heat exchanger, air in the circuit, or a hydraulic issue.

Is an inspection or protocol required after installing a tank on an existing boiler?

In Slovakia, there is no explicit legal requirement for a separate inspection protocol for a hot water tank as such. However, the following applies: if work is carried out on the gas boiler or gas installation (for example, moving the boiler, changing the connection), this work must be carried out by a person with the appropriate qualification (an authorised gas fitter, or a company authorised to work with gas), and a service inspection report must be issued. In addition, most manufacturers make the boiler's warranty conditional on regular servicing by an authorised service technician. The tank itself (as a pressure vessel) is not subject to regular pressure inspection if its volume does not exceed 100 litres and the operating pressure is not higher than 1 MPa (10 bar) – which is usually the case for domestic DHW tanks.

Conclusion: What to take away from this overview?

Condensing boilers with the possibility of connecting a tank are today the most common choice for both new builds and renovations of family houses. The reason is simple: they combine the high energy efficiency of condensing combustion with the comfort of tank-based hot water heating, while remaining flexible – you can size the tank exactly to your needs, or later replace it with a larger one or add solar pre-heating.

The most important lesson from practice: the boiler and the tank form a system. It is not enough to buy a quality boiler – the tank, its heat exchanger, volume, hydraulic connection and correct control settings must all be matched to each other. Otherwise, even an excellent boiler can deliver poor results.

If you are looking for specific products, take a look at the whole category of boilers with tank connection capability, where you will find the current range including the Protherm and Vaillant models mentioned in this article. For further information on selection, installation and operation, we recommend the other articles in this Knowledge Centre.

Do you have a question on this topic?

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