Condensing boiler with a built-in cylinder: a complete guide for 2026
When you start comparing boilers and come across the term condensing boiler with a built-in cylinder, a whole string of questions opens up: What does the cylinder actually do? Is it worth paying extra for a combination in a single casing? What cylinder volume is enough? And why do manufacturers such as Protherm or Vaillant offer several different versions? This article answers all of these questions – and a few more that might never have occurred to you.
We will start with the basics, then move on to the technical details, the advantages, the disadvantages and specific recommendations for different types of household. The article is long, but the subject deserves it – installing a boiler is an investment for 15 to 20 years, and a wrong decision can cost you thousands of euros in inefficient running or an expensive replacement.
What a condensing boiler is and why the word “condensing” matters
A conventional gas boiler burns gas and sends the hot flue gases up the chimney. Those flue gases leave at a temperature of 120 to 200 °C – and with them goes an enormous amount of energy that simply vanishes into the atmosphere. A condensing boiler works differently: it cools the flue gases so far (below 57 °C) that the water vapour in them condenses and releases its latent heat of condensation. That is the essence of the technology – recovering the “hidden heat” from water vapour.
In practice this means that a condensing boiler achieves an efficiency of 97 to 109 % (yes, it can exceed 100 % – it is merely a question of the reference value used in the calculation). Compared with an old atmospheric boiler you will typically save 20 to 35 % on gas each year. At today's gas prices of around 0,08 to 0,10 €/kWh (as of 2026) and a detached house consuming 15 000 to 25 000 kWh a year, that can mean a saving of 300 to 700 € a year.
The condition for the full condensing effect is a low return temperature in the heating system – ideally 30 to 45 °C. That is why condensing boilers work best with underfloor heating or with more modern, larger radiators (so-called low-temperature heating systems). They also work with traditional column radiators at 70/55 °C and are still more economical than old boilers, but the condensing effect is smaller.
Built-in cylinder: what exactly it means and which versions exist
A condensing boiler with a built-in cylinder is an appliance in which a single compact casing houses not only the boiler itself but also a domestic hot water (DHW) cylinder. The result is one appliance that takes care of both heating and hot water – with no need for an external floor-standing cylinder.
There are two basic principles on which the cylinder works:
- Indirect heating via a heat exchanger – the stored volume of water (e.g. 21, 35 or 42 litres) sits in an insulated vessel and is heated by the boiler's primary circuit. The water in the cylinder and the heating water do not mix. This is the more hygienic solution, because the water in the cylinder is not in contact with the heat-transfer surface of the central heating circuit.
- Instantaneous heating with storage – some models combine a small cylinder with an instantaneous heat exchanger. The cylinder serves as a “buffer” and the hot water flows through a plate heat exchanger, where it is brought up to its final temperature.
In terms of cylinder volume, the figures you will most often meet on the market are:
- 21 litres – a compact solution for smaller households or flats
- 35 litres – a medium volume, a compromise
- 42 litres – the most common in the detached-house category, sufficient for 3–4 people
- 50–60 litres – less common for built-in cylinders, more usual for optional external ones
Built-in cylinder vs. external cylinder: what really suits your home
This is a question with no single answer that fits everyone. It depends on the particular circumstances of the household. Let us look at both options objectively.
Advantages of a built-in cylinder
- Space saving – boiler and cylinder in a single casing. For flats, boiler rooms in a hallway or utility rooms with limited space, this is a major advantage. An external floor-standing cylinder (e.g. 100–200 litres) is one more piece of furniture in the room.
- Lower installation costs – fitting one appliance is quicker. There is no need to run connecting pipework between the boiler and a separate cylinder, which eliminates both the heat losses from the pipes and the installer's labour.
- Appearance and compactness – a single white panel on the wall looks far tidier than a boiler plus a large cylinder beside it.
- Faster hot water delivery – the cylinder is already pre-heated, so hot water reaches the outlet sooner than with instantaneous heating.
- Anti-Legionella function – most modern boilers with a cylinder have an automatic heating cycle to 65–70 °C, which prevents the bacterium Legionella pneumophila from multiplying.
Disadvantages of a built-in cylinder
- Limited volume – 42 litres is enough for 3–4 people at ordinary comfort levels, but if you want a long soak in the bath and several people follow one another, the cylinder can run out and you have to wait for it to reheat.
- Higher initial cost – a combi boiler with a cylinder is more expensive than a stand-alone boiler without one, but cheaper than a boiler plus an external cylinder together.
- Serviceability – if the cylinder or the DHW heat exchanger fails, the whole appliance is out of action. With an external cylinder the boiler can at least carry on heating.
- Cylinder energy losses – a pre-heated cylinder gradually cools down (so-called standby losses) even when no water is being drawn off. With well-insulated cylinders these losses are minimal, but not zero.
Specific models: what you will find on the market in 2026
Let us look at the specific products that are available and at what to watch out for when choosing among them.
Protherm Tiger Condens – the flagship for the Slovak market
The Protherm Tiger Condens range is among the best-selling condensing boilers with a built-in cylinder on the Slovak and Czech markets. Protherm has long been popular for its good value for money, readily available servicing and reliability. Let us look at the individual models:
Protherm Tiger Condens 20/26 KKZ 21 – a compact model with a 21-litre cylinder, suitable for flats and smaller households
Protherm Tiger Condens 20/26 KKZ 21 is the version with an output of 20 to 26 kW and a built-in 21-litre cylinder. What do the numbers in the name mean? “20/26” denotes the minimum and maximum heating output (modulation), “KKZ” stands for a condensing boiler with a cylinder (the cylinder version) and “21” is the cylinder volume in litres. This model is suitable for flats or smaller detached houses of up to approx. 120 m² where there are no more than 2–3 occupants. A 21-litre cylinder is sufficient for normal use (showering, washing hands), but if you plan to run baths it may be tight.
Protherm Tiger Condens 20/26 KKZ 42 – the most popular model, with a 42-litre cylinder for detached houses
Protherm Tiger Condens 20/26 KKZ 42 is essentially the same boiler, but with a 42-litre cylinder. This is in practice the most popular version for households of 3–4 in detached houses of up to 150 m². The larger cylinder means the first person can shower at leisure and the cylinder has time to reheat before the second one arrives. An output of 26 kW at full load is sufficient for the great majority of Slovak detached houses from the 1970s to the 1990s (mostly inadequately insulated, with a heat loss of 10–18 kW).
Protherm Tiger Condens 30/35 KKZ 42 – for larger and less well insulated detached houses with a higher output demand
Protherm Tiger Condens 30/35 KKZ 42 is the more powerful sibling, with an output of 30–35 kW and likewise a 42-litre cylinder. This model suits larger detached houses (180–250 m²), older and less well insulated buildings with a high heat loss, or cases where the boiler is linked to a solar system and needs more output to heat the cylinder. An output of 35 kW is powerful indeed – for an ordinary modern low-energy house it is oversized, which does efficiency no favours (the boiler will switch off and on needlessly in short cycles).
A practical tip for choosing the output: The boiler output should match the heat loss of the building. As a rough estimate, that is about 50–80 W per m² for old uninsulated houses and 30–50 W/m² for insulated ones. For an old 120 m² house this means a heat loss of 6–9,6 kW. A 20–26 kW boiler will therefore cover it with a generous margin for heating DHW.
Protherm Tiger Condens with a smart controller – intelligent heating
The Protherm Tiger Condens 20/26 KKZ 42 with a smart controller set – a complete solution for intelligent heating
This set combines the Protherm Tiger Condens 20/26 KKZ 42 with an intelligent controller that can be operated from a smartphone. In practice this means you can switch the heating on before you get home from work, set a weekly programme, monitor gas consumption and receive service reminders. At today's energy prices an intelligent controller can save a further 10–15 % compared with standard thermostat control – simply because the heating runs only when it is really needed.
Vaillant ecoTEC plus – the premium choice
Vaillant VUI 26CS/1-5 ecoTEC plus IoniDetect + sensoCOMFORT 720 is a complete set comprising a premium condensing boiler with ion-based burner diagnostics and the sensoCOMFORT controller. What is IonoDetect? It is a system that monitors combustion by means of an ionisation sensor and automatically optimises the gas/air mixture. The result is maximum efficiency with minimum emissions, and the boiler itself signals when it detects a deviation from the ideal. For customers who want a premium appliance with maximum diagnostics and a long service life, the Vaillant ecoTEC plus is a very good choice – at a higher price.
Thermona THERM 28 KDZ 5 – a Slovak manufacturer
Thermona THERM 28 KDZ 5 is a condensing boiler with a built-in cylinder from the Czech manufacturer Thermona. An output of 28 kW and a 48-litre cylinder make this model a suitable solution for larger detached houses. Thermona is popular for its reasonable servicing prices and the availability of spare parts on the Slovak market, which is no small consideration over the appliance's 15-year service life.
How to work out the right cylinder volume for your household
This is one of the most important things to focus on when choosing. Too small a cylinder will frustrate you with cold water in the shower; too large a one wastes energy keeping unused hot water warm.
A simplified rule recommended by manufacturers and installers alike:
- 1–2 people, shower only: 21–30 litres
- 3–4 people, shower + ordinary use: 35–42 litres
- 4–5 people or with a bath: 50–80 litres (here, however, a built-in cylinder is not enough and an external cylinder or a combined approach is recommended)
- 5 or more people or greater comfort: 100 litres or more – an external cylinder
Bear in mind that the boiler's cylinder is not a vacuum flask – the water in it is heated to 55–65 °C, but when drawn off it is blended with cold mains water down to approx. 38–42 °C. With a 42-litre cylinder at 65 °C and blending to 40 °C you actually get about 60–70 litres of usable hot water, which is a respectable figure for one shower (10–12 litres/minute, 6–8 minutes = about 70–90 litres).at a temperature of 40°C. However, if two people draw water one after the other, the cylinder has to reheat in between.
Installing a condensing boiler with a cylinder: what you need to know beforehand
Installing a condensing boiler is not a job for a “handy neighbour”. It calls for a professional with a valid licence. In 2026 the rule in Slovakia is that a gas appliance may be installed only by a company or person holding the relevant authorisation from the Technical Inspectorate (TIČR/ITI) or from the gas distributor, in accordance with Decree No. 508/2009 Z.z. and the Labour Code in force. After every installation a pressure test, a leak check and commissioning are mandatory.
Condensate drainage
A condensing boiler produces acidic condensate (pH 3–4) at a rate of 1–3 litres per hour at full output. This condensate must be discharged to the drains. Discharging it straight into the waste pipe is permitted in most cases (the volume is small and the sewer neutralises it), but in some situations (e.g. separate surface-water and foul sewers) a neutralisation unit may be required. Check this with your local sewerage operator.
Flue gas discharge – room-sealed vs. open-flued system
Condensing boilers are exclusively of the so-called room-sealed type (referred to as type C or turbo). This means they draw combustion air from outside through a concentric pipe and also discharge the flue gases to the outside – not through a traditional chimney. A twin-wall concentric flue (diameter 80/125 mm or 60/100 mm) is run through the wall or the roof. The advantage: the boiler is independent of the air in the room and needs no air supply to the boiler room. The disadvantage: if you have an existing chimney and want to use it, this is possible only with a special liner – but in most cases it is simpler to take the concentric flue out through an external wall.
Gas supply pressure and pipe sizing
A boiler with an output of 26–35 kW uses 2,5–3,5 m³ of gas per hour at full output. Make sure that the gas service pipe and the gas meter have sufficient capacity. Most households with a G4 connection (4 m³/h) will run a boiler and a cooker without any trouble. When replacing a boiler with a more powerful one, check the capacity of your connection with the gas distributor.
Hydraulic connection and expansion vessel
A condensing boiler with a built-in cylinder has a separate primary central heating circuit and a secondary DHW circuit. Most modern boilers have a built-in circulation pump and expansion vessel. The expansion vessel normally has a volume of 8–12 litres – in larger systems (underfloor heating throughout the house, a greater volume of water in the system) an additional external expansion vessel is needed. The installer will calculate this from the volume of the whole system.
Control and smart technology: why intelligent control is worth the investment
A boiler without good control is like a powerful car without an accelerator pedal – it works, but you are not using its potential. In 2026 the standard is not only that the boiler has a modulating burner (from 20 to 100 % of output), but also that it can communicate with smart thermostats and smart-home systems.
Intelligent control brings these tangible benefits:
- Weather-compensated control – the boiler takes the outdoor temperature into account and proactively adjusts the temperature of the heating water. At -10 °C outside the boiler heats the water to 75 °C, at +5 °C only to 45 °C. The result is comfort without overheating.
- Room controller with a weekly programme – for the night and for times when nobody is at home you can set a lower temperature (e.g. 18 °C instead of 21 °C). Each degree less = approx. 6–8 % gas saving.
- App control – now standard on most premium boilers. You can switch the boiler on before you get home, monitor consumption and receive fault notifications.
- Integration with solar thermal or photovoltaics – some smart controllers can give priority to heating the cylinder when there is plenty of cheap (self-generated) energy.
The Protherm Tiger Condens 20/26 KKZ 42 with a smart controller set is a practical example of “all in one” – boiler and intelligent control in a single package at a price lower than if you bought them separately.
Running, servicing and service life: the real costs over 15 years
A boiler is not an appliance you buy and then forget about. It has prescribed regular service inspections. Let us look at the real costs over a 15-year horizon, which is the average service life of a condensing boiler:
Mandatory annual inspection
Under Slovak regulations (Decree of the Ministry of Transport, Construction and Regional Development of the Slovak Republic No. 282/2012 Z.z.) every gas appliance must be checked by a qualified person at least once a year. In 2026 the price of a service inspection of a condensing boiler is around 60–120 € including VAT (depending on the region and the service company). Over 15 years that comes to 900–1 800 €. Include it in your overall costs.
Spare parts and repairs
A condensing boiler has several components that are subject to wear:
- Circulation pump – service life 8–12 years, replacement 150–300 €
- Expansion vessel – the diaphragm wears through after 8–10 years, replacement 100–200 €
- DHW heat exchanger – in hard water it can become clogged with limescale; chemical cleaning or replacement 200–500 €
- Burner and ionisation electrode – checked and cleaned at every inspection
- Cylinder (internal anode) – cylinders with a sacrificial magnesium anode need the anode replaced every 2–5 years (50–100 €); otherwise the cylinder is at risk of corrosion
The effect of water hardness on the cylinder
This is a subject that gets little attention, yet in Slovakia it is relevant. In most regions of Slovakia the water is moderately hard to hard (8–25 °dH). When heated above 60 °C, hard water forms limescale, which settles in the cylinder and on the heat exchanger surfaces. The effect: reduced heat output (a 1 mm layer of scale = 10 % more energy used for heating) and a shorter service life for the heat exchanger and the cylinder.
The solution: fit a water treatment unit (a softener or a polyphosphate dosing unit) upstream of the boiler inlet. An investment of 100–300 € pays for itself through a longer boiler life and lower energy consumption. Manufacturers such as Protherm and Vaillant recommend this explicitly in their manuals.
Condensing boilers and grants: the position in 2026
In Slovakia there are several support schemes for installing energy-saving heating appliances. The position as of 2026:
- The Zelená domácnostiam II programme – aimed primarily at renewable energy sources (heat pumps, solar, biomass), but a combination with a condensing boiler as a backup source may be accepted
- EU Structural Funds via the ŠFRB – the State Housing Development Fund provides subsidised loans for insulation and modern heating, including condensing boilers
- Tax relief – since 2024 it has been possible to deduct from tax the expenditure on energy upgrades to a detached house, including modern heating. Check the current conditions with a tax adviser or on the website of the Financial Administration of the Slovak Republic
Before buying, we recommend checking the current conditions, as the support schemes change. A good installer will be able to advise you or at least point you to the right institution.
The most common mistakes in selection and installation
After years of hands-on experience we can identify several mistakes that keep recurring:
- An oversized boiler – “if I buy a more powerful one, I'll be on the safe side.” The opposite is true. A boiler that is too powerful switches off and on frequently (cycling), which increases wear and reduces efficiency. A boiler should run at 60–80 % of its output, not at 20 %.
- A cylinder without a water hardness check – see the previous section. Without water treatment in hard-water regions you can ruin a cylinder within 5 years.
- Incorrect flue gas discharge – a shortened concentric flue positioned too close to a window can cause flue gases to be drawn back in. Observe the minimum distances laid down in the STN EN 13384 standard.
- Skipping the pressure gauge test – a pressure test is mandatory after installation. If the installer leaves it out, you will not receive the manufacturer's warranty certificate.
- No control at all – buying an expensive condensing boiler and connecting a simple bimetallic thermostat to it is a waste of the appliance's potential. Invest in a programmable controller at the very least.
Condensing boiler and heat pump: the combination of the future?
In 2026 interest is growing in so-called hybrid systems – a combination of a condensing boiler and an air-to-water heat pump. The principle is simple: the heat pump works efficiently at outdoor temperatures above 0 °C (most of the year), and in severe frost (below -5 to -10 °C) the boiler takes over. The result: low running costs for most of the year and reliable heating even at -20 °C.
In such a hybrid arrangement a condensing boiler with a built-in cylinder is the ideal backup source – the DHW cylinder is part of the boiler, so the whole system stays compact. Some manufacturers (Vaillant, Buderus) offer ready-made “hybrid” boiler + heat pump sets with a single control unit.
What to compare when choosing a specific model: a practical checklist
When comparing models, keep an eye on these parameters:
- Output (kW) – must match the heat loss of the building, calculated in accordance with the STN EN 12831 standard
- Cylinder volume (l) – 21/35/42 litres, according to the number of occupants
- Energy class – at least A, ideally A+
- Burner modulation (%) – the wider the modulation range (e.g. 20–100 %), the smoother the operation
- Cylinder type – enamelled, stainless steel or with a protective anode
- Concentric flue diameter – 60/100 or 80/125 mm (depending on the length of the flue)
- Compatibility with controllers and smart systems
- Warranty period – manufacturers offer 2 years as standard, 5 years with online registration
- Service availability – check how many authorised service partners there are in your region
A real-life example: replacing an old boiler in a 1985 terraced house
Mr Miroslav from Trenčín had an old atmospheric boiler in his house, with a separate instantaneous water heater. The house has 135 m², a family of 4 and radiators at 70/55 °C. The old system used about 2 800 m³ of gas a year = approx. 2 240 € at a price of 0,08 €/kWh (conversion to kWh: 2 800 × 10,5 kWh/m³ = 29 400 kWh × 0,08 = 2 352 €).
After replacing it with a Protherm Tiger Condens 20/26 KKZ 42 with weather-compensated control and a programmable controller, consumption fell to 1 950 m³ of gas = approx. 1 637 €. Saving: approx. 715 € a year. Investment in the boiler including installation: approx. 2 800 €. Payback: less than 4 years. And the 42-litre cylinder met the needs of all 4 family members without any trouble.
Frequently asked questions (FAQ)
Is a condensing boiler with a built-in cylinder also suitable for a flat in a prefabricated block?
Yes, it is an ideal solution for flats where there is no room for a separate floor-standing cylinder. Models with an output of 20–26 kW and a 21–42 litre cylinder will fit into a typical run of kitchen units or a hallway. You do, however, need to check whether a concentric flue can be routed out of the flat (through an external wall or an existing shaft). Some blocks of flats require joint approval for changes to the system – check this with the building manager.
What cylinder volume do I need for a family with 3 children?
For a family of 5 with 3 children, a built-in 42-litre cylinder is at the limit of comfort. Once the children grow up and start showering every day, the cylinder will not be enough. Consider either a model with a larger built-in cylinder (50+ l) or a combination of a boiler with an external 80–100 litre cylinder. Alternatively, a solar cylinder can be added for DHW, leaving the boiler solely as the backup and heating source.
How long does it take to heat an empty 42-litre cylinder?
It depends on the boiler output and the inlet water temperature. With an output of 26 kW and inlet water at 10 °C, a 42-litre cylinder heats up to 60 °C in approx. 8–12 minutes. In practice you never empty it completely, so reheating after a partial draw-off takes 3–6 minutes. That is considerably more comfortable than an instantaneous water heater, where you have to wait for the temperature to stabilise.
Do I have to replace a condensing boiler with a built-in cylinder if the cylinder starts to leak?
It depends on the model and the extent of the fault. In most modern boilers the cylinder can be separated from the heating section – the service engineer can replace just the cylinder, not the whole boiler. Before buying, ask the retailer about the availability and price of a replacement cylinder for the model in question. For premium brands (Vaillant, Protherm) spare parts remain available for a long time.
Is a condensing boiler with a built-in cylinder suitable for use with solar collectors?
Connecting solar collectors directly to the boiler's built-in cylinder is not standard practice, because the cylinder is small and usually has no inlet for the solar fluid. There are solutions, however: an external 150–200 litre twin-coil cylinder with connections for both the solar collectors and the boiler, or certain boiler models with larger built-in cylinders that have a solar inlet (the specific model needs to be checked). If you are planning solar, we recommend designing it as a complete system from the outset – not retrofitting it later.
How do I find out the heat loss of my house so that I can choose the right boiler output?
The most accurate way is an energy audit or a calculation in accordance with the STN EN 12831 standard, carried out by a heating designer. As a rough guide: an old uninsulated 120 m² house has a heat loss of about 12–16 kW, an insulated house of the same area 6–9 kW and a low-energy house 3–5 kW. The boiler output should be at least 20–30 % greater than the heat loss (a margin for heating DHW and for warming up after frosty spells). For most old houses of 100–180 m² the optimum boiler output works out at 20–26 kW.
Conclusion: a condensing boiler with a built-in cylinder – who it suits and who it does not
A condensing boiler with a built-in cylinder is an excellent solution for most Slovak households – from flats to detached houses of up to 200 m² with 2–4 occupants. It combines economical condensing technology with a practical, compact way of providing hot water, saves space and cuts installation costs compared with a separate boiler + external cylinder.
A built-in cylinder is not enough for very large households (5 or more people), for people with big baths and high expectations of DHW comfort, or for systems with solar thermal. In such cases an external 100–200 litre cylinder is the better choice.
When choosing a specific model, pay attention to correct sizing of the output (neither too powerful nor too weak), a cylinder volume that matches the number of occupants, the quality of servicing in your region and investment in good control. At today's energy prices a boiler with weather-compensated control and a weekly programme will save a further 10–20 % compared with a boiler without control – at a gas price of 0,08–0,10 €/kWh that is a real 150–400 € extra a year.
If you are unsure about your choice, do not hesitate to consult a specialist or contact our team – we will be glad to help you choose the solution that is right for your situation.
Do you have a question about this topic?
Can't make up your mind, or dealing with a particular situation in your home? Write to us - we will be glad to advise.