How to choose a condensing boiler with the option to connect a storage tank

We are still preparing products.

But you can look at other categories.

How to choose a condensing boiler with cylinder connection option

A condensing boiler with the option to connect an external domestic hot water cylinder is today one of the most widespread solutions for heating and hot water preparation in Slovak households. Despite this, many mistakes are made when choosing one – people confuse a combi boiler with a boiler prepared for a cylinder, underestimate the boiler's output when heating the cylinder, or choose a cylinder of unsuitable volume. The result? Insufficient hot water in the morning, unnecessarily high gas bills, or a boiler overloaded during peak demand.

In this article I'll cover the whole topic in real depth – from the operating principle, through choosing the right output, to specific models and their features. If you're considering buying such a boiler, read the whole thing before you decide.

What a boiler with cylinder connection option actually is – and what it isn't

The first thing to understand: a boiler with cylinder connection option is not a combi boiler. This is the most common mistake I see among customers. A combi boiler has a built-in instantaneous hot water heater – meaning it heats water directly on demand, without any cylinder. A boiler prepared for a cylinder is purely a heating boiler that additionally has a control output (usually an electrical signal or terminal) and hydraulic connection so it can control charging of the external cylinder via a three-way valve or charging pump.

Simply put: combi boiler = water flows straight through the boiler to the tap. Boiler with cylinder = water is first heated in the cylinder, from where it goes to the outlets. These are two different philosophies of hot water preparation, each suited to different situations. If you want this comparison explained in more detail, see the article Difference between a boiler prepared for a cylinder and a combi boiler in our Knowledge Center.

Why choose exactly this solution

The boiler + cylinder system has several major advantages over a combi boiler:

Comfort during simultaneous use: the cylinder can cover simultaneous use of a shower and a sink without a drop in temperature or pressure – a 24 kW combi boiler cannot handle this as comfortably at full flow
Energy efficiency: the boiler heats the cylinder at night or during a cheap tariff period, and during the day you draw the stored hot water without the burner needing to switch on for every draw-off – this is covered in more detail in the article Why cylinder heating saves energy compared to instantaneous heating
Longer boiler lifespan: fewer burner starts = fewer thermal cycles = slower wear of the heat exchanger
Flexibility: the cylinder can later be supplemented with a solar collector, a heat pump, or combined heating
Stable water temperature: no temperature spikes when flow changes, which you'll especially appreciate in the shower

The disadvantage? Greater space requirement (the cylinder takes up room), higher initial investment, and the need to correctly size the entire system.

How condensation works and why it matters especially for a cylinder system

A condensing boiler achieves high efficiency (up to 109% based on calorific value) by cooling the flue gases below the dew point and recovering latent heat energy from condensation of water vapor in the flue gases. For condensation to actually occur, the return water temperature from the heating circuit must be sufficiently low – ideally below 55 °C, optimally 35–45 °C.

In a cylinder system, this has an interesting consequence: when the boiler charges the cylinder, it switches to a higher temperature mode (flow temperature typically 70–80 °C), where condensation occurs less intensively. But for most of the year the boiler operates in low-temperature heating mode, where condensation runs at full effect. The result is high efficiency year-round – provided the system is properly set up and the cylinder is charged only when necessary, not continuously.

Boiler output: how to correctly calculate it for a system with a cylinder

This is the technically most important part of the article. You must assess the boiler's output in two ways: separately for heating and separately for heating the cylinder. The boiler must handle both – and during combined operation (charging the cylinder + heating) the output must not be insufficient.

Output for heating

Basic rule: the boiler's output should match the building's heat loss, not be significantly oversized. For a well-insulated new build with underfloor heating, plan roughly 40–60 W/m² of heated area. For an older, less insulated building with radiators, it can be 80–120 W/m². So for a 150 m² family house you need 6–9 kW for a new build, and 12–18 kW for an old house.

In practice, on custom orders we most often encounter houses of 120–200 m², where the required heating output comes out at 8–15 kW. Condensing boilers in the cylinder-solution category are mostly in the range of 12–25 kW, which covers the vast majority of cases.

Output for cylinder heating

A 150-liter cylinder with a heat exchanger area of about 0.8–1.0 m² can be charged by a 20–25 kW boiler in 20–30 minutes. A 200-liter cylinder in 30–45 minutes. If the boiler's output is too low (e.g. 12 kW), charging takes longer and the cylinder may not be sufficient during the morning peak. A practical minimum for a 150-liter cylinder is a boiler output of at least 15–18 kW when heating hot water.

Some boilers have a specific priority mode for hot water preparation (charging the cylinder) – the boiler temporarily interrupts heating and concentrates its entire output on quickly charging the cylinder. This is advantageous: the cylinder charges quickly, then the boiler switches back to heating. Modern boilers such as the Protherm Gepard Condens 25 MKO naturally have this mode implemented.

Difference between boilers with one and two pumps – system hydraulics

When choosing a boiler with a cylinder, you'll come across an important hydraulic detail: how is the switching system between heating and cylinder heating handled?

There are two basic schemes:

Three-way switching valve: the boiler has a built-in or external three-way valve that switches the flow of hot water either to the heating circuit or to the cylinder's heat exchanger. Simpler solution, fewer components, lower price. Disadvantage: heating is interrupted while the cylinder is charging.
Charging pump with mixer: the boiler has its own heating pump, and the cylinder has a separate charging pump. Both circuits can operate simultaneously. More expensive solution, but more comfortable – heating doesn't stop while the cylinder is charging.

For family houses, a solution with a three-way valve and cylinder priority is fully sufficient in most cases – charging the cylinder takes only 20–30 minutes, and during this time heating doesn't "cool down" thanks to the building's thermal inertia. Only for large buildings or apartment blocks with a bigger cylinder does a two-pump solution make sense.

Overview of specific models – what they offer in practice

Let's look at specific boilers that we encounter most often in custom project practice and that have proven reliability.

Protherm Gepard Condens MKO – a proven basis

The Protherm Gepard Condens 12 MKO is a compact condensing boiler with a nominal output of 12 kW, intended solely for heating with readiness for an external cylinder. The designation MKO (a boiler module prepared for a cylinder) indicates that the boiler contains a terminal for controlling the cylinder's charging pump or a three-way valve. Its advantages are small dimensions, modern Protherm electronics, and good availability of service technicians in Slovakia.

For larger houses or more demanding cylinder-heating requirements, the Protherm Gepard Condens 25 MKO with a 25 kW output is more suitable. This model can even charge a 200-liter cylinder within a reasonable time while heating the building without issues. From experience, I know the Gepard Condens is a reliable boiler with good burner modulation – at low load it modulates output down to 3–4 kW, which is ideal for condensing operation.

Protherm Panther Condens KKO – higher output and robustness

The Protherm Panther Condens 15 KKO is another model from the Protherm range, this time designated KKO – meaning a condensing boiler with cylinder preparation in a more compact casing. The Panther Condens is suitable for slightly larger buildings or where the customer prefers a more robust construction. Compared to the Gepard range, the Panther traditionally has a somewhat more massive heat exchanger, which shows at high temperature demands. You'll find a detailed comparison of both ranges in the article How to connect a cylinder to the Protherm Panther Condens or Gepard Condens.

Vaillant ecoTEC – German precision and intelligent control

The Vaillant VU 246/5-3 ecoTEC pro is a condensing heating boiler with a 24 kW output from the ecoTEC pro range. Vaillant as a manufacturer is synonymous with quality and long service life – their boilers have excellent service support and spare parts availability. The ecoTEC pro model is a mid-range option offering an integrated hydraulic block, a built-in bypass, and a built-in automatic air vent, which makes installation easier.

At a higher level is the Vaillant VU 25CS/1-5 ecoTEC plus IoniDetect – this boiler features innovative IoniDetect technology, an ionization flame sensing system with adaptive control. The system continuously analyzes the combustion process and optimizes the air/gas ratio for maximum efficiency and minimal emissions. For customers who want absolute maximum condensing performance and are interested in the lowest possible operating costs, this is the top choice. Its 25 kW output is sufficient for a cylinder as well as for above-standard large houses.

You'll find a detailed comparison of the Protherm and Vaillant brands in a separate article, Protherm vs Vaillant: comparison of boilers with cylinder connection.

Which cylinder for which boiler – a brief guide

Choosing the cylinder is just as important as choosing the boiler. A few practical rules:

1–2 people, low consumption: 80–100 liter cylinder, 12–15 kW boiler
3–4 people, standard consumption: 120–150 liter cylinder, 15–20 kW boiler
5+ people or a large bathtub/whirlpool: 200+ liter cylinder, 20–25 kW boiler
Two-generation household: 200–300 liter cylinder, 25 kW boiler output

The cylinder should have a heat exchanger volume (coil heat exchanger area) large enough for the boiler to transfer heat efficiently – for a 20–25 kW boiler, the minimum heat exchanger area is 1.0–1.3 m². Cylinders with a small heat exchanger (0.5–0.7 m²) are intended more for solar collectors, and a boiler will charge them slowly and inefficiently.

A more detailed calculation procedure can be found in the article What cylinder volume do I need for my boiler.

Technical installation requirements – what you need to have ready

Before installing a boiler with a cylinder, it's worth checking a few things that customers often underestimate in practice:

Space for the cylinder

A 150-liter cylinder has a diameter of about 55–60 cm and a height of 120–130 cm. A 200-liter cylinder has a height of around 140–150 cm. You must account not only for the cylinder's dimensions, but also for space for piping and possible service access. The cylinder must be accessible for checking the anode protection (magnesium anode) at least once a year.

Hydraulic connection

The cylinder must be hydraulically connected to the boiler via the primary (boiler) circuit. You'll need: a three-way switching valve or charging pump, a check valve, a safety valve, an expansion vessel on the sanitary side (if you don't have one in the house after the water meter), and a control valve on the cylinder branch. The exact procedure is described in the article Installing a cylinder to a condensing boiler – procedure and requirements.

Electrical connection

The boiler must be connected to the cylinder's thermostat or electronics. Most modern boilers handle this via their own control electronics – the cylinder is connected to a dedicated terminal (e.g. "BUS" or "cylinder") and the boiler automatically controls charging. Some older cylinders work only with a classic thermostat and relay – in that case the wiring is simple but less sophisticated.

Chimney and condensate drainage

A condensing boiler produces condensate (acidic water, pH 3–5), which must be drained into the sewer via a condensate neutralizer. Without a neutralizer, you may be fined by the sewer administrator. The chimney must be suitable for a condensing boiler – either plastic coaxial piping (60/100 or 80/125 mm) or a stainless steel liner inserted into an existing chimney.

Output modulation – what it is and why it matters

Modulation is the boiler's ability to vary the burner output between minimum and maximum. For example, a 25 kW boiler with 1:5 modulation can operate from 5 kW to 25 kW. This is a key feature for condensing operation.

Why? Because during transitional seasons (spring, autumn) you only need 4–6 kW for heating. A boiler without modulation would have to keep switching on and off constantly (so-called cycling), which damages the boiler and reduces efficiency. A boiler with deep modulation covers these low demands smoothly – the burner runs for a long time on a small flame, condensation is intensive, and consumption is minimal.

Modern condensing boilers such as the Vaillant ecoTEC plus or Protherm Gepard Condens typically have a modulation range of 1:5 to 1:7, which is excellent for family houses. Older or cheaper boilers have 1:3 modulation, which can lead to cycling in small houses.

Control and intelligent management – weather-compensated control and internet

Modern condensing boilers prepared for a cylinder can be equipped with weather-compensated control – i.e. the boiler adjusts the heating water temperature to the outdoor temperature. At –15 °C outside, water goes to the radiators at 70 °C; at +5 °C outside, only at 45–50 °C. Result: the boiler condenses for a much longer time throughout the year, since it works with lower temperatures.

Weather-compensated control requires an outdoor temperature sensor and a controller capable of weather compensation. Vaillant offers controllers from the sensoTRENS range, while Protherm uses controllers from the Thermolink or MiGo range. All these controllers can nowadays be connected via Wi-Fi and controlled via a smartphone – something you'll especially appreciate when returning from vacation and wanting to pre-heat the house.

For a cylinder system, it's important that the controller can also manage cylinder charging – i.e. set when the cylinder is charged (for example once in the morning at 5:30 and once in the evening at 22:00) so you always have hot water when needed, while the boiler doesn't burn fuel unnecessarily all day.

Operating costs and return on investment

Customers very often ask me: "Is it worth paying more for a condensing boiler when cheaper non-condensing alternatives exist?" The answer in 2024 is clear: yes, it's worth it, and significantly so.

A condensing boiler achieves a seasonal efficiency of around 95–109% (based on the calorific value of gas), while a common atmospheric non-condensing boiler achieves 80–88%. The difference in gas consumption for a house needing 10,000 kWh of heat annually:

Condensing boiler (98% efficiency): gas consumption ≈ 10,200 kWh ≈ 1,020 m³/year
Non-condensing boiler (85% efficiency): gas consumption ≈ 11,760 kWh ≈ 1,176 m³/year
Difference: ~156 m³/year × €0.80–0.90/m³ = €125–140 savings per year

A condensing boiler costs about €300–600 more than an atmospheric one. With savings of €130 a year, the payback period comes to 2–5 years. With proper maintenance, a condensing boiler has a lifespan of 15–20 years. The economic benefit is therefore clear.

A cylinder also allows you to take advantage of a cheaper night-time electricity tariff (if you have a cylinder with electric backup heating) or heating via a solar collector. A more detailed breakdown of savings can be found in the article Why cylinder heating saves energy compared to instantaneous heating.

Service and maintenance – what not to forget

A condensing boiler with a cylinder requires regular maintenance that differs from that of a classic boiler. The main points:

Annual boiler service inspection: checking and cleaning the burner, heat exchanger, ionization sensor, condensate container, checking gas pressure and combustion parameters
Checking the cylinder's anode protection: the magnesium anode in the cylinder gradually dissolves (protecting the cylinder shell from corrosion). Check every 1–2 years, replace as needed
Descaling the cylinder: in areas with hard water (e.g. Bratislava, Trenčín), limescale builds up in the cylinder. Descaling every 2–4 years extends the cylinder's lifespan
Checking pressure in the piping: pressure in the primary circuit should be 1.0–1.5 bar when cold. Check the pressure gauge regularly
Checking the condensate system: cleaning the condensate siphon, checking the neutralizer

Neglecting service has a direct impact on the boiler's efficiency and lifespan. A boiler with a heat exchanger fouled by limescale can lose as much as 10–15% efficiency, and in the case of severe fouling, overheating and irreversible failure can occur. More on this topic can be found in the article Maintenance and service of a condensing boiler with a domestic hot water cylinder.

Most common mistakes when choosing and installing

Over years of practice, I've repeatedly seen the same mistakes. Here are the most important ones:

Oversizing the boiler's output: the customer thinks "more is always better". A 35 kW boiler in a house needing 8 kW will cycle, condense minimally, and will fail soon
Cylinder with a small heat exchanger: the customer buys a cheap cylinder with a 0.5 m² heat exchanger to pair with a powerful 25 kW boiler – the result is slow charging and the boiler working inefficiently
Missing condensate neutralizer: condensate goes straight into the sewer without neutralization – a legal violation and damage to the piping
Incorrectly set cylinder priority: the cylinder charges continuously (thermostat set too high), and the boiler spends 60% of its time heating the cylinder instead of heating the house
Installation without venting the system: air in the piping causes noise, uneven heating, and corrosion
Chimney unsuitable for a condensing boiler: a classic masonry chimney without a liner for wet flue gases – rapid destruction of the chimney structure

Frequently Asked Questions (FAQ)

Can I connect a cylinder to an existing condensing boiler that doesn't have cylinder preparation?

It depends on the specific boiler model. Some older condensing boilers (typically classic combi boilers or pure heating boilers without a cylinder terminal) don't have control electronics for a cylinder. In that case, you would have to connect the cylinder via an external thermostat and relay, or via an external controller. This is technically possible but less elegant. If the boiler has a terminal for a cylinder (e.g. labeled "BUS" or "cylinder"), the connection is direct and simple. Always verify compatibility with the specific boiler model before buying a cylinder.

What's the optimal cylinder temperature – isn't 60 °C unnecessarily high?

A temperature of 60 °C in the cylinder is a hygienic minimum – at this temperature, Legionella pneumophila bacteria, which can multiply in cylinders, are killed. It's recommended to heat the cylinder to 65–70 °C at least once a week (most modern boilers do this automatically in a "legionella cycle"). On regular days, the cylinder temperature can be 50–55 °C if you don't expect water to stagnate for a long time.

What happens when the cylinder isn't enough to cover morning consumption?

If the cylinder runs out of hot water (cools down), the boiler starts a new charging cycle – but this takes 20–40 minutes. During this interim period, you either have no hot water, or it's too cold. Solution: a bigger cylinder, higher boiler output (faster recharging), or setting an earlier morning charging time (the cylinder charges at 5:00, you get up at 6:00). If the problem recurs, it's usually due to an incorrect estimate of demand when choosing the cylinder volume.

Is an expansion vessel also necessary on the sanitary side in a cylinder system?

Yes – the hot water cylinder is a closed vessel in which water expands when heated. If there's no expansion valve or check valve installed after the water meter, the pressure in the cylinder can rise above safe levels. Most cylinders have a built-in safety valve (typically 6 bar), but an expansion vessel on the sanitary side (5–8 liters) prevents unnecessary dripping from the safety valve and extends its lifespan.

Can I add solar collectors to a boiler with a cylinder later on?

Yes, and that's one of the main advantages of a cylinder system. You'll need a cylinder with two heat exchangers – one for the boiler, another for solar – or a cylinder with a directly heated solar circuit. Such a cylinder is 20–40% more expensive from the start, but if you're planning solar within a 5–10 year horizon, it's worth buying it right away. Interconnecting the solar circuit with the boiler circuit and correctly setting priorities is handled by the solar controller.

What's the difference between boilers labeled "MKO" and "KKO" at Protherm?

The designation MKO stands for "Boiler Module with Heating" – i.e. a boiler prepared for connecting a cylinder, without a built-in cylinder. KKO stands for "Condensing Boiler with Heating" – also a heating boiler prepared for a cylinder, but in a different shape range (Panther). Both designations indicate that the boiler has control for an external cylinder. Functionally they're similar but differ in construction, output range, and casing shape. You'll find a more detailed comparison in the article Protherm vs Vaillant: comparison of boilers with cylinder connection.

Conclusion: What to take away

Choosing a condensing boiler prepared for a cylinder isn't complicated once you know the basic principles. Summary of key points:

Size the output correctly – according to the building's heat loss, not floor area. Use a professional calculation, not just rough tables.
Choose a cylinder with an appropriate volume and sufficient heat exchanger area for your boiler.
Choose a boiler with deep modulation – at least 1:5, ideally 1:7.
Don't forget the hydraulics: three-way valve or charging pump, safety valve, expansion vessel on the sanitary side, condensate neutralizer.
Set up the controls correctly: weather compensation curve, cylinder charging times, legionella cycle.
Plan regular maintenance – annual boiler service, checking the cylinder's anode every 1–2 years.

For specific models we have good experience with, visit the category condensing boilers with cylinder connection option directly on Atria.sk. If you're working on a specific project and can't decide, also read the article Frequently asked questions about condensing boilers with cylinder connection option – you might find a direct answer to your case there.

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.