Frequently Asked Questions about Condensing Boilers with a Built-in Tank
Condensing Boilers with Built-in Tank – Answers to Everything You Want to Know
Condensing boilers with a built-in tank are among the most common solutions for combined heating and domestic hot water preparation in Slovak households. Despite this – or perhaps precisely because of it – there is a lot of confusion, half-truths and questions surrounding them that customers keep asking over and over again. If you are deciding on such a device, or already have one and want to understand it better, this article will give you a comprehensive picture based on real-world practice.
From years of experience with customers, we know that most questions revolve around a few key areas: tank volume and hot water comfort, boiler output, the difference compared to external tanks, installation, operating costs and reliability. We will go through all these topics thoroughly, with concrete figures and examples.
What exactly is a condensing boiler with a built-in tank?
Let's start with the basics, because misunderstandings occur here too. A condensing boiler with a built-in tank (sometimes also called a tank boiler, or combi boiler with a tank) is a device that combines three functions in a single cabinet: a condensing heat exchanger for heating, a primary water circulation circuit, and a built-in domestic hot water (DHW) tank with a volume typically ranging from 21 to 60 litres, occasionally more.
Unlike a classic instantaneous combi boiler, here the water for drinking/washing is not heated directly at the moment of draw "on the fly", but is pre-heated and stored in the tank. This fundamentally affects comfort – but also what you can realistically expect from the boiler.
How large are the tanks and which volume suits whom?
This is probably the most frequently asked question. The tanks of built-in boilers most commonly range from 21 to 60 litres. From practical experience, we know that:
- 21 litres – suitable for 1–2 people, an apartment, or a household with very even hot water consumption. A tank of this size can easily handle one person's morning wash and a quick shower. During a bath or simultaneous draw at multiple points, the boiler will "recharge" the tank continuously, which may cause a temporary drop in water temperature.
- 42 litres – the most popular volume for a 2–4 person household in a family house. It covers a shower for 1–2 people in succession, normal dishwashing, and if there is no simultaneous draw in multiple locations, comfort is very good.
- 55–60 litres – for larger families (4–5+ people), houses with higher consumption, or where a bathroom and shower cubicle are expected to be used simultaneously.
A practical example: a customer with a family house, 4 people, 2 bathrooms. They originally had an instantaneous combi boiler and complained about insufficient pressure during simultaneous draw. After installing a boiler with a 42-litre tank, they were satisfied – in the morning they could take turns without waiting, although not simultaneously in both bathrooms at once. For such a situation, we would recommend a 60-litre tank or an external tank with a larger volume.
The topic of choosing the right volume is covered in more detail in the article What tank volume do I need for my house, where you will also find specific calculation tables based on the number of people and habits.
What is the difference between a built-in and an external tank?
This is the second most common dilemma. A condensing boiler with a built-in tank has the tank physically inside the boiler body – it therefore takes up only one spot on the wall, making it a compact solution. A boiler with an external tank consists of the boiler itself (usually an instantaneous combi or classic boiler) and a separate standing tank connected via piping.
A more detailed comparison in terms of costs, space requirements and long-term economics can be found in the article Condensing boiler with tank vs. boiler with external tank: which is more worthwhile.
From a practical point of view, the following applies: if you have an apartment or smaller house with a boiler room/utility room where there is no space for a standing tank, a built-in tank is the logical choice. If you have a large family or high DHW consumption (e.g. a family of 5+, houses with a sauna, larger buildings), an external tank of 150 litres or more will give you significantly higher comfort.
What is the correct temperature to set for water in the tank?
Here we encounter two conflicting requirements: hygienic safety vs. energy savings. The tank should be heated to at least 55 °C to prevent the growth of Legionella bacteria – this bacterium multiplies at temperatures of 20–45 °C and dies at temperatures above 55 °C. Many boilers have an automatic disinfection function (the so-called "anti-legionella" program), during which the tank is heated to 70 °C once a week.
In practice, tanks are usually set to 55–60 °C. The temperature at the tap outlet should be lower (a maximum of 50–55 °C for comfort without risk of scalding), so it is recommended to use a thermostatic valve (mixer) at the tank outlet, which mixes the hotter water with cold water to a safe level.
Setting the tank to 45 °C for the purpose of saving gas is risky from a hygienic point of view – especially in apartments or houses with longer hot water pipe runs, where the water in the pipes cools down and the "Legionella zone" is present for longer.
How quickly does the tank reheat after being depleted?
The reheating time depends on several factors: the boiler output allocated to DHW heating, tank volume, the temperature of the incoming cold water, and the required outlet temperature. Under typical conditions (cold water 10–15 °C, target temperature 55 °C), the following applies as a guideline:
- 21-litre tank: heating takes 8–12 minutes at an output of 20–26 kW
- 42-litre tank: heating takes 18–25 minutes at an output of 20–26 kW
- 42-litre tank at an output of 30–35 kW: 12–18 minutes
This is an important figure for families where one shower follows another in quick succession. If the tank cannot keep up, there are two solutions: a larger tank, or a boiler with a higher output dedicated to DHW. More on this can be found in the article How long does it take to heat water in the tank and what does DHW comfort depend on.
What output boiler with a built-in tank do I need?
The output of a condensing boiler with a built-in tank has two components: heating output and DHW preparation output. These can be the same, or the boiler switches to a higher output when preparing DHW (DHW priority). Both aspects need to be taken into account when choosing the output.
For heating, the basic rule is: a well-insulated modern family house needs about 40–60 W/m² of usable area, while older buildings without insulation need 80–120 W/m². So for a 150 m² house (well insulated), a heating output of 6–9 kW is sufficient, while an equally large uninsulated house needs 12–18 kW.
For DHW, the flow rate and the number of simultaneous draw-off points are decisive. A 20/26 kW boiler has a heating output of 20 kW and a DHW heating output of 26 kW – this is a typical combination. A 30/35 kW boiler is suitable for larger houses and larger tanks.
A more detailed calculation and scenarios can be found in the article What output of a condensing boiler with a tank is sufficient for my house.
Models from practice – what do we install most often?
From everyday project experience, we can say that the most common solution for a family house of 3–4 people with an area of 100–160 m² is a boiler with an output of 20/26 kW and a 42-litre tank. This is precisely the class represented, for example, by the Protherm Tiger Condens 20/26 KKZ 42 – reliable, well supported in terms of service, with good track record in practice.
For smaller apartments and lower DHW consumption, the Protherm Tiger Condens 20/26 KKZ 21 with a 21-litre tank has proven successful – more compact, lighter, and fully sufficient for a 1–2 person household.
For larger houses with higher DHW consumption, where the larger 42-litre tank needs to be filled more quickly, the Protherm Tiger Condens 30/35 KKZ 42 with an output of 30/35 kW is suitable – this boiler's tank heats up significantly faster, which is appreciated by families with higher DHW demands.
For customers who want smart control, direct control via smartphone and top-class comfort, the Vaillant VUI 26CS/1-5 ecoTEC plus IoniDetect + sensoCOMFORT 720 is interesting. This boiler is equipped with IoniDetect technology for optimal combustion and comes with a smart controller, which makes it ideal for modern homes with higher demands on energy efficiency and comfort.
If you want a Protherm system with modern control, a suitable choice is the Protherm Tiger Condens 20/26 KKZ 42 + smart controller – a boiler in a proven class with the added value of smart control right out of the box.
A comparison of individual manufacturers and models in this class can be found in the article Protherm Tiger Condens vs. Thermona THERM vs. Vaillant ecoTEC: comparison of models with a tank.
What are the installation requirements?
Condensing boilers with a built-in tank are heavier than standard instantaneous combi boilers – a tank filled with water adds 20–50 kg to the weight of the appliance. The total weight with water ranges from 60 to 100+ kg depending on the model and tank volume. This places demands on the load-bearing capacity of the wall (typically a solid masonry wall, not a plasterboard partition) and on the type of anchoring elements.
Other installation requirements:
- Condensate drainage: a condensing boiler produces condensate (acidic water, pH 3–4), which must be drained into the sewage system, ideally via a neutralization box.
- Air supply and flue gas exhaust: a sealed system (turbo) with a plastic coaxial duct, or separate ducts. The maximum duct length depends on the model (typically 10–15 m equivalent).
- Water connections: cold water inlet, DHW outlet, heating circuit connection. Shut-off valves are recommended on each connection to facilitate servicing.
- Gas connection: must be sized for the maximum boiler output. For 20–35 kW boilers, a standard house connection with a diameter of DN 20 (3/4") is sufficient.
- Electrical connection: 230 V / 50 Hz, 10 A circuit breaker, grounding.
Installation must be carried out by an authorized person – a gas fitter with the appropriate qualification. The first commissioning (warranty inspection) must be performed by an authorized service technician of the manufacturer, otherwise the warranty may be voided. More about the installation procedure can be found in the article Installing a condensing boiler with a built-in tank: procedure and requirements.
What is the efficiency and how much will I save compared to an old boiler?
Modern condensing boilers achieve a seasonal energy efficiency (SCOP/seasonal efficiency) rating of class A or A+. In numbers, this means utilizing 96–109% of the calorific value of natural gas (above 100% is stated because condensation of flue gases also recovers latent heat that would otherwise escape through the chimney). An old, non-modulating boiler from the nineties has an efficiency of around 70–80%.
Actual savings when replacing an old boiler with a modern condensing one with a tank range from 15% to 35% of annual gas costs, depending on the condition of the old system, the setting of heating curves and the quality of control. For a family house project with an annual consumption of 2,000 m³ of gas, this can mean savings of €300–700 per year at current gas prices.
Smart control also contributes to these savings – boilers such as the Protherm Tiger Condens 20/26 KKZ 42 + smart controller or the Vaillant VUI 26CS/1-5 ecoTEC plus with sensoCOMFORT 720 controller allow setback settings during absence, time programs and remote control, which further reduces consumption.
Is a built-in tank more prone to faults than an external one?
In the long term, built-in tanks have one potential disadvantage: if the tank starts to corrode or the seal is damaged, the repair is more complicated than with a standing external tank. In practice, however, modern tanks are made of steel with an enamel coating, or of stainless steel, and usually last up to 12–15 years without problems if properly maintained.
Proper maintenance is crucial: annual inspection of the anode rod (for enamelled steel), venting, and checking the safety valve and system pressure. Details can be found in the article Maintenance and servicing of a condensing boiler with a tank: what and how often to check.
For typical faults – tank temperature sensor error, exchanger fouling, pressure fluctuations – these can be resolved by a service technician on-site without dismantling the boiler in the case of a built-in tank. More about common problems can be found in the article Common faults of condensing boilers with a built-in tank and their solutions.
How long is the warranty and what is needed to maintain it?
The standard statutory warranty is 24 months. Manufacturers such as Protherm and Vaillant offer an extended warranty (5 years or more) if certain conditions are met: first commissioning by an authorized service technician, annual service inspections carried out by an authorized company, and registration of the product on the manufacturer's website.
From practice, we see that customers who neglect these conditions (e.g. skip the annual inspection) risk not only losing the extended warranty, but also higher service costs and a shortened lifespan of the appliance. A boiler that is regularly serviced can reliably operate for 15–20 years.
Can I connect a boiler with a built-in tank to solar collectors?
Yes, some models directly allow this or are prepared for it. The solar system can supply pre-heated water into the tank, and the boiler only needs to heat the difference up to the required temperature. This can significantly reduce gas consumption for DHW preparation during the summer months.
However, it is necessary to check the specific model of the boiler – not every built-in tank has a connection to a solar circuit. Some models have a special solar connection (dual-circuit tank), others must be combined with an external tank with a coil for the solar circuit, which effectively means switching to a configuration with an external tank. Always consult an installer familiar with the specific model before making a decision.
How do I choose a boiler with a built-in tank – what should I pay attention to?
The choice is not just about output and tank volume. From practice, we recommend monitoring these parameters:
- Energy efficiency class – at least A, ideally A+. This affects annual gas costs.
- Service availability – the manufacturer should have a dense service network in Slovakia. This is not an issue for Protherm and Vaillant.
- Tank material – enamelled steel with a magnesium anode is the standard, stainless steel is the premium choice (does not require anode replacement).
- Maximum tank pressure – tanks are typically rated for 8–10 bar; check whether this corresponds to the pressure in your water supply network.
- Control and connectivity – smart control options, OpenTherm protocol, WiFi module, thermostat compatibility.
- Noise level – a built-in tank adds water heating noises (cracking during expansion) to the boiler's noise. Check the guaranteed dB(A) value.
The entire selection process is covered in a separate article, How to choose a condensing boiler with a built-in tank: what to pay attention to.
Frequently Asked Questions (FAQ)
Can I have a condensing boiler with a built-in tank in an upstairs apartment without a utility room?
Yes, a built-in tank is very practical especially for apartments – it does not require a standing tank in a utility room. However, you need to take into account the higher weight (60–100 kg with water) and the fact that the wall must be sufficiently load-bearing. The condensate drain must be connected to the sewage system, which is usually not a problem in an apartment building. The boiler must be of the sealed type (turbo) with a common air/flue gas duct leading through the exterior wall or the roof.
What happens when I deplete the tank? Will I be left without hot water?
Not necessarily – it depends on how quickly the boiler recharges the tank. Modern boilers switch to DHW priority when the tank is depleted and immediately start heating. During the recharging period, the outlet water temperature will drop and will stabilize again after 5–15 minutes (depending on volume and output). Most households will not even notice this brief drop – the problem only occurs with very intensive simultaneous draw (e.g. several people at once). The solution is a larger tank or a boiler with higher output for DHW.
Is a built-in tank suitable for hard water? What about limescale?
Hard water (above 2.5 mmol/l, corresponding to CaCO₃ above 250 mg/l) is a real challenge for the DHW tank. Limescale deposits on the exchanger and reduces heating efficiency as well as tank capacity. In areas with hard water, it is recommended to install a water softener or polyphosphate dosing device before the boiler, and to regularly check and descale the tank. Manufacturers usually state the maximum water hardness the tank can handle without voiding the warranty – typically 3.0–3.5 mmol/l.
Why do I hear noise in the tank (cracking, knocking)?
Cracking and knocking during heating is usually normal thermal expansion of the water and the tank walls – water heated from 15 °C to 55 °C increases in volume by about 1.5%, which causes mechanical stress and sounds. If the sounds are pronounced, it may also indicate limescale deposits (a layer of scale cracks during heating). The solution is to descale the tank and possibly lower the tank temperature by 5 °C. If the sounds are accompanied by a pressure increase or other faults, a service inspection is needed.
Can I connect a boiler with a built-in tank to underfloor heating?
Yes, condensing boilers with a built-in tank are fully compatible with underfloor heating – and are actually very suitable for it, because underfloor heating operates at low temperatures (30–45 °C), at which the condensing boiler achieves its highest efficiency. It is necessary to have a return water temperature that is sufficiently low, which underfloor heating naturally ensures. When combined with radiators (which require higher temperatures), a mixing unit is recommended.
Do I need to drain the tank regularly?
Not regularly, but it is recommended to inspect the tank once a year and possibly flush it to release sediments. Part of the annual inspection is also checking the anode rod (for enamelled tanks) – it gradually dissolves and protects the tank from corrosion, so it needs to be replaced as needed (typically every 2–4 years, depending on water hardness and anode size). Neglecting anode replacement is one of the most common causes of premature tank corrosion.
Conclusion
Condensing boilers with a built-in tank are a well-thought-out, space-saving solution for most Slovak households – both apartments and family houses. The key to satisfaction is the correct choice of tank volume and boiler output relative to the household's actual consumption, professional installation with initial commissioning, and regular annual maintenance. If these conditions are met, such a boiler will provide reliable heating and hot water comfort for 15 years or more.
If you are unsure about your choice, take a look at the overview of available models in the category of condensing boilers with a built-in tank – or get inspired by specific products and additional articles in the Knowledge Center, where each topic brings another layer of practical information for an informed decision.
Do you have a question about this topic?
Can't decide or are you dealing with a specific situation in your household? Write to us - we will be happy to help.