How to choose a boiler and tank set – what to watch out for

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How to choose a boiler and cylinder set – a complete guide for a confident choice

A boiler and hot water cylinder set is one of the most common solutions for central heating and domestic hot water (DHW) preparation in Slovak households. Nevertheless, customers face a whole range of questions when choosing such a set – what boiler output to choose, what cylinder capacity will be sufficient, whether certain combinations are even compatible, and what needs to be resolved before placing an order. In this article, we'll go through the whole topic systematically and with concrete figures so that after reading it you'll know exactly what you're looking for – and why.

What actually is a boiler and cylinder set, and why does it make sense to buy them together

A boiler and cylinder set is a pre-configured combination of two devices: a low-temperature gas boiler (open-flue/atmospheric or condensing/room-sealed) and a hot water cylinder, designed by the manufacturer as a functional unit. It's not just "packing two things into one box" – it's a verified hydraulic and control compatibility, where the manufacturer guarantees that both components work together correctly. This is important for several reasons:

Hydraulic compatibility – the diameter and location of the connections on the cylinder match the boiler outputs, so there's no need to improvise unnecessarily with fittings.
Control compatibility – the boiler's control unit can directly manage the cylinder's charging, and priority logic works without external solutions.
Warranty and service – in case of a claim or fault, you deal with a single seller and manufacturer, not two different parties passing the buck.
Purchase economics – sets are generally more cost-effective than buying individual units when you add up all the accessories.

From experience, I know that a customer who buys a boiler and cylinder separately from different manufacturers deals with, on average, one to two additional complications during installation. Either the controls don't match, or the cylinder doesn't have the correct inlet for the circulation pump that's built into the boiler. A pre-configured set eliminates these worries.

Basic types of sets – open-flue vs. room-sealed boilers

The first decision you need to make even before any sizing is the type of boiler. In practice, you'll encounter two basic categories – an open-flue (atmospheric) boiler and a room-sealed (turbo, closed combustion chamber, forced flue gas discharge) boiler. Each of them has a slightly different role within the set.

An open-flue (atmospheric) boiler (e.g. Vaillant atmoTEC plus VU CZ/SK 200/3-5 + VIH CK 70) burns air taken directly from the room where it is installed and discharges flue gases through a conventional chimney. This means you need a functioning chimney flue and sufficient air supply to the boiler room. The advantage is simpler installation and servicing, while the disadvantage is greater dependence on the chimney's condition and, in most cases, lower efficiency compared to condensing solutions.

A room-sealed (turbo) boiler (e.g. Vaillant turboTEC plus VU CZ/SK 202/3-5 + VIH CK 70) has a closed combustion chamber, drawing combustion air from outside via a coaxial flue, and also discharges flue gases through the same coaxial flue directly through a wall or roof – so a chimney is not a requirement. This is a huge advantage when renovating flats and houses where a chimney either doesn't exist or is not in good condition.

You can find a detailed comparison in the article Open-flue vs. room-sealed boiler in a set – what's the difference in this Knowledge Center. Here we'll focus more on choosing the complete set.

Boiler output and cylinder capacity – how they relate to each other

This is where most mistakes are made. A customer chooses a boiler based on the old boiler they had at home, and a cylinder based on how much space they have left in the boiler room. The result? Either the cylinder can't supply enough hot water for the household at peak times, or the boiler is oversized while the cylinder is too small, causing the boiler to run in short cycles and wear out faster.

The basic rule that works in 90% of typical family household cases is as follows:

1–2 people: 80-litre cylinder, 18–20 kW boiler
3–4 people: 100–120 l cylinder, 20–25 kW boiler
5–6 people: 150–160 l cylinder, 24–28 kW boiler
7 or more people, or a larger holiday property: 200 l cylinder or more, 28–35 kW boiler

These figures apply assuming the cylinders have only one coil (charged from the boiler), the household has no solar collectors, and water consumption is standard – i.e. not a house with a bathtub in every bathroom and shower panels with massage jets. For a more detailed calculation, we recommend reading the article What cylinder capacity do I need for my household as well as Correct sizing of boiler output to cylinder capacity.

Important relationship: the cylinder charging time depends on the boiler's output and its heat exchange surface. A 20 kW boiler at a flow/return temperature of 80/60 °C can heat a 100-litre cylinder from 15 °C to 60 °C in approximately 25–35 minutes. If the boiler is 25 kW and the cylinder is the same, the time shortens to 18–22 minutes. With an oversized cylinder (e.g. 200 l for a 20 kW boiler), the charging time will be disproportionately long, and during daily peak hours the cylinder may not always be able to supply a sufficient amount of hot water.

Overview of specific sets and their suitable applications

Let's look at several specific sets from the range so it's clear how these principles translate into real products.

Protherm Aqua Complet Panther 25 KOO + FE 120 BM

The set Protherm Aqua Complet Panther 25KOO + FE 120 BM is a combination of the condensing Panther boiler with an output of 25 kW and the FE 120 BM cylinder with a capacity of 120 litres. The designation "KOO" means an open combustion chamber – i.e. an open-flue type that needs a chimney and air supply from the surrounding space. This set is ideal for households of 3–4 people in a house with an existing, functional chimney. The FE 120 BM cylinder has a single coil, is made of enamelled steel, and is equipped with a magnesium anode to protect the tank. The Panther boiler is a proven Slovak product (Protherm is historically a Slovak-Czech brand) with reliable controls and available service throughout Slovakia.

Protherm Aqua Complet Panther 25 KTO + FE 120 BM

The set Protherm Aqua Complet Panther 25KTO + FE 120 BM is almost identical, differing in the designation "KTO" – a closed combustion chamber with room-sealed (turbo) flue gas discharge. The boiler therefore does not require a chimney, as discharge is handled via a coaxial pipe through a wall or roof. The cylinder is the same FE 120 BM. This set is a solution for houses without a chimney or for flat renovations where you don't want to invest in repairing the chimney flue. In terms of output and household suitability, it is identical to the KOO variant – 3–4 people, standard water consumption.

Vaillant turboTEC plus VU CZ/SK 202/3-5 + VIH R 120

The set Vaillant turboTEC plus VU CZ/SK 202/3-5 + VIH R 120 brings the turboTEC plus boiler with an output of up to 20 kW and the VIH R 120 cylinder with a capacity of 120 litres. The VIH R cylinder is Vaillant's premium range, with an enamelled tank, a magnesium anode, and a thermometer. This set is somewhat more modest in output (a 20 kW boiler), but it is fully sufficient for 2–3 people and is attractively priced. Vaillant is a German premium brand with excellent service support.

Cylinder with one or two coils – a key decision when planning

A simple cylinder with a single coil is sufficient when you have only one heat source – your boiler. However, if you are planning, or plan in the future, to install solar collectors, a heat pump, or another alternative source, you must choose a cylinder with two coils. The second (lower) coil is designed for a source with a lower temperature level (e.g. solar, where the temperature may be only 40–55 °C), while the upper one serves the boiler.

In practice, I've encountered a case where a customer bought a simple cylinder and two years later wanted to install solar collectors. He had to replace the entire cylinder – essentially doubling the investment. So if there's a realistic chance of solar within a 5–10 year horizon, invest in a dual-coil cylinder right away. We cover this topic in more detail in the article Cylinder with one or two coils – which to choose.

What to watch out for when choosing the installation location

A boiler and cylinder set does not have small dimensions. A typical "side by side" installation takes up 60–90 cm in width (boiler 40–50 cm + cylinder 50–60 cm depending on capacity). Allow for a depth of 50–70 cm, and the height of a 120 l cylinder is typically 1,100–1,200 mm. The boiler is wall-mounted, while the cylinder stands on the floor. So before ordering, measure the installation location and check the technical data sheets of both units.

Minimum service space – there must be at least 60 cm of free space in front of the boiler for service access, and at least 50 cm in front of the cylinder for anode replacement.
Wall load capacity – a boiler filled with water weighs 40–60 kg, so the wall must be masonry or a sandwich construction with steel anchoring profiles.
Gas supply – check the pressure in the gas network and the supply capacity. A 25 kW boiler needs a flow rate of approx. 2.7 m³/hour at full output when burning natural gas (calorific value ~10 kWh/m³).
Electrical supply – most modern boilers require 230 V / 50 Hz, with a minimum 6 A circuit breaker. The socket must be within reach of the boiler.
Condensate discharge (for condensing boilers) – condensate must be discharged into the sewer via a neutraliser; this is not optional, it is a legal requirement.

Controls and accessories – what's included in the set and what needs to be purchased separately

Most pre-configured sets include the boiler, the cylinder, and basic connection accessories (fittings). What is usually not included and must be sorted separately:

Thermostat or room temperature controller – basic units only have an output for an external thermostat (2-point on/off control); a better solution is an OpenTherm controller, which communicates digitally with the boiler and significantly saves gas.
Cylinder drip tray – always recommended for cylinders placed in boiler rooms near valuable equipment or wooden flooring; minimum 10 l capacity.
Cylinder safety valve + expansion vessel for the DHW circuit – the hot water cylinder is sealed under mains water pressure, and as the water heats up its volume expands; a safety valve and expansion vessel (usually 8–12 l) are a safety requirement of the standard.
Cold water inlet filter – prevents limescale and impurities from settling in the cylinder and boiler. A simple mesh filter is an investment of a few euros with a huge impact on the equipment's lifespan.
Air vent and drain valve – the cylinder should always have a drain valve for servicing and venting the system.

Detailed instructions on connecting the boiler to the cylinder can be found in the article How to connect a boiler to a cylinder – installation procedure.

Comparison of Protherm and Vaillant brands – a practical view

Both brands belong to the Vaillant Group and share many technologies. Nevertheless, there are practical differences we encounter in service practice:

Protherm Panther has a long history on the Slovak and Czech markets, meaning excellent availability of service technicians and spare parts even in smaller towns. It is essentially a domestic brand, which in practice means a technician will also come to more remote regions. The electronics are clear and the controls are simple even for less technically skilled users. The FE 120 BM cylinder is reliable and affordably priced.

Vaillant turboTEC plus is a more premium category. The boiler has a wider output modulation range (e.g. 7–20 kW for the VU 202/3-5 model), meaning it responds better to fluctuating heat demand and runs longer at lower output – which is very important in terms of gas savings. Service is also well covered, but premium spare parts are slightly more expensive. For a more demanding customer with a more modern house, Vaillant is logically the better choice. You can find a comparison of Protherm vs. Vaillant sets in the article Protherm Panther vs. Vaillant turboTEC – comparison of sets.

Common mistakes when choosing a set and how to avoid them

Over the years of practice, I've identified several recurring mistakes that occur when choosing a boiler and cylinder set. They are listed here with a specific description so you can eliminate them in advance:

Mistake #1: Choosing based on the old boiler's output – the customer says "I had 24 kW, I want the same." The old boiler may have been oversized from the 1990s, when sizing was done with a large safety margin. Today, after insulating the house, a 15–18 kW boiler may be sufficient. We recommend doing a simple heat loss calculation – either using the floor-area method (enclosed volume × specific heat loss) or having an energy audit performed.

Mistake #2: Underestimating the output needed for DHW – the customer sizes the boiler only for heating and forgets that the boiler must also charge the cylinder, and cylinder charging has priority. If the boiler has only just enough output for heating (e.g. 15 kW in a house needing 14 kW), there won't be enough output to charge the cylinder quickly. Practical rule: the output for DHW should make up at least 20–30% of the boiler's total output.

Mistake #3: Buying a set without consulting the layout – the customer buys a set and finds out that the cylinder is too tall to fit under a low boiler-room ceiling or under a staircase. A 120 l cylinder typically has a height of 1,100–1,180 mm, while a 160 l cylinder can be up to 1,300–1,400 mm.

Mistake #4: No expansion vessel for the DHW circuit – with a sealed hot water cylinder, an expansion vessel for the sanitary circuit must be installed (not the same one as for heating!). Without it, the cylinder's safety valve opens and drips water after every heating cycle – which customers consider a fault, but it's just a missing accessory.

Mistake #5: Installation without a professional – gas appliances may only be installed by a person with a valid certificate according to STN EN, and manufacturers require the first commissioning to be carried out by an authorised service technician, otherwise the warranty is voided. This is not just a formality – incorrect installation can be a source of gas or carbon monoxide leaks.

Long-term costs – what affects operating expenses

The purchase price of a set is only one parameter. What's more important is the total cost of ownership over 10–15 years of operation. Here are the factors that affect it:

Boiler efficiency – a condensing boiler achieves a seasonal efficiency of 90–98%, an open-flue one 82–88%. With an annual gas consumption cost of €1,500, this means a difference of €90–120 per year – over 10 years, €900–1,200 on fuel alone.
Condition of the cylinder's magnesium anode – the anode protects the cylinder against corrosion; it must be checked every 2–3 years and replaced as needed. Neglecting this means premature corrosion of the cylinder and its replacement for €400–700.
Water quality – hard water (above 30 °F) speeds up limescale build-up on the cylinder's coil and in the boiler's heat exchanger. Literally every millimetre of limescale increases energy consumption by 8–10%. In hard water areas, we recommend a water softener or dosing device.
Regular servicing – the boiler should be serviced once a year (gasket replacement, electrode inspection, combustion chamber cleaning). The cylinder should also be checked at that time. Annual servicing from €80–150 is cheaper than a boiler repair costing €400–800 due to neglected maintenance. More information can be found in the article Servicing and maintenance of boiler and cylinder sets – what and how often.

Ordering procedure – step by step

To avoid unnecessary problems, we recommend the following procedure when choosing and ordering a set:

Determine the flue gas discharge type – do you have a functioning chimney? If so, you can consider an open-flue boiler. If not, or if the chimney is in poor condition, choose the room-sealed (turbo) variant.
Estimate the house's heat loss – rough estimate: floor area × 60–100 W/m² (lower value for a well-insulated house, higher for an older house with single-layer masonry).
Determine the number of people and DHW consumption habits – does the household have a bathtub or just a shower? How many people shower at the same time in the morning?
Measure the installation location – width, depth, height; note the location of existing gas, water, and electricity connections.
Decide on the set and check the accessories – expansion vessel, safety valve, filter, thermostat.
Arrange installation by an authorised installer – request a commissioning record, as you will need it for the warranty.

Frequently asked questions about boiler and cylinder sets

Can I install the boiler and cylinder myself if I'm a handy DIYer?

No, for several reasons. Under Slovak regulations (STN EN 1775, Decree No. 508/2009 Coll.), gas appliances may only be connected and commissioned by a person holding a valid license from the Slovak Chamber of Civil Engineers (SKSI) or the relevant certification for installing gas appliances. Besides legal liability, manufacturers make warranties conditional on first commissioning by an authorised service technician who completes the warranty certificate. Electrical installation (230 V supply to the boiler) is also subject to professional inspection.

How much hot water does a 120-litre cylinder hold, and when is it not enough?

A 120-litre cylinder, heated to 60 °C and mixed down to 40 °C (comfortable shower temperature), provides approximately 180–200 litres of usable hot water. An average shower uses 60–80 litres (at 40 °C). So a 120 l cylinder can supply 2–3 people showering without recharging. If 4 people shower in the morning within a short time span (30 minutes), the cylinder won't be enough without recharging – the boiler will keep recharging it, but the hot water flow rate may be limited. For 4 or more people with simultaneous demand, we recommend a 160 l cylinder.

What is priority cylinder charging and why is it important?

Priority cylinder charging means that when the cylinder needs recharging (its temperature has dropped below the set value, e.g. 55 °C), the boiler temporarily pauses heating and devotes its entire output to charging the cylinder. This allows the cylinder to be filled quickly (15–30 minutes), after which heating resumes. Without priority, the boiler would split its output between the cylinder and the radiators – the cylinder would charge slowly and might not be sufficient. All modern boilers in these sets have priority logic built into the control unit.

Do I have to replace the old cylinder when I replace the boiler?

Not always, but it depends on several factors. If the old cylinder shows no signs of corrosion (rusty water, corroded sediment when drained), has a functioning anode, and has the correct capacity, it can continue to be used. A problem may be hydraulic and control incompatibility with the new boiler – a different type of coil, different connection positions, or unsuitable pump control. In practice, replacing the cylinder together with the boiler is always the cleaner and more reliable solution, especially if the old cylinder is more than 10–12 years old.

Is there a difference in operating cost between a set with an open-flue and a condensing boiler?

Yes, the difference is real and measurable. A condensing boiler uses the latent heat in the flue gases (condensation of water vapour from the flue gases returns heat to the heat exchanger), achieving an efficiency above 100% relative to the fuel's calorific value. In practical terms, this means a 10–15% gas saving compared to an open-flue boiler. With an annual gas bill of €1,200, that's €120–180 in savings per year. A condensing boiler is €150–300 more expensive to buy, but the investment pays back within 1–2 years at average consumption.

What to do if the cylinder drips from the safety valve?

Dripping from the cylinder's safety valve is most often caused by a missing or undersized expansion vessel for the sanitary circuit. When water is heated, its volume increases (water expands by about 1.7% of its volume at 60 °C) – if there's nowhere for it to expand, the pressure rises and opens the safety valve. The solution is to install an expansion vessel for the DHW circuit (8–18 l depending on the cylinder capacity). If the expansion vessel is installed and the valve still drips, the safety valve itself may be undersized or damaged. This topic is covered in more detail in the article Common faults in boiler and cylinder sets and how to recognise them.

Conclusion – how to simplify your choice

Choosing a boiler and cylinder set isn't rocket science, but it does require a systematic approach. Three things matter most: the correct boiler output for your specific house (not based on your neighbour's or what you had before), the correct cylinder capacity for your number of people and hot water usage patterns, and the correct type of flue gas discharge (chimney or coaxial) depending on the condition of your property.

If you have a house with 4 people, a functioning chimney, and want a proven Slovak solution, a set like Protherm Aqua Complet Panther 25KOO + FE 120 BM is a solid choice with a long tradition and easily available service. For a house without a chimney or when renovating a flat, we recommend the room-sealed (turbo) variant, for example Protherm Aqua Complet Panther 25KTO + FE 120 BM or Vaillant turboTEC plus VU CZ/SK 202/3-5 + VIH R 120. For more demanding customers with a larger household and an appetite for premium solutions, we recommend browsing the entire range of sets in the category and, if needed, consulting the choice with our experts.

Don't forget about accessories (expansion vessel, filter, thermostat), verified installation, and regular servicing – these are investments that will keep your set running reliably for 15–20 years without major surprises.

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'll be happy to help.