Condensing Boiler with Built-in Tank vs. Boiler with External Tank: Which Is More Worthwhile
Condensing boiler with built-in cylinder vs. boiler with external hot water cylinder: which one really pays off more in practice?
When a customer decides how to solve domestic hot water (DHW) heating at home, they usually face one of two basic options: either buying a condensing boiler with a built-in cylinder that has everything in one casing, or getting a classic condensing combi boiler and purchasing an external hot water cylinder to go with it – either upright or horizontal. Both approaches are technically well-established and commonly used in practice. The problem is that each has a different profile of advantages and disadvantages, and what works great for one customer may be a completely unnecessary compromise for another.
In this article, I'll break it down in real depth – from technical comparison through calculations and specific examples to practical recommendations from experience. If you've ever dealt with a case where a customer was torn between these two solutions, you might know the feeling that neither is clearly "better" – it really depends on the specific situation.
How does a condensing boiler with a built-in cylinder actually work?
A condensing boiler with a built-in cylinder – known on the Slovak market for example under the commercial name "cylinder boiler" or "combined cylinder boiler" – combines in one compact casing a condensing burner circuit designed for both heating and hot water production, and a thermohydrodynamic domestic hot water cylinder. The cylinder is physically located directly inside the boiler casing, usually in its lower part or behind the heating block.
Typical volumes of built-in cylinders range from 21 to 55 litres, with the most common variants being 21 l, 30 l, and 42 l. The cylinder is heated with priority – meaning that whenever DHW heating is needed, the boiler temporarily prioritises heating the cylinder over space heating. This system guarantees a relatively fast supply of hot water without the waiting time a customer would experience with a combi boiler or a smaller water heater.
It's important to understand one technical detail: the built-in cylinder in a condensing boiler is not a pressurised cylinder in the classic sense (like an enamelled cylinder from Dražice or Regulus). Most built-in cylinders are made of stainless steel or plastic and operate under cold water pressure – these are indirect-heating cylinders, where boiler water passes through a heat exchanger inside the cylinder and heats the drinking water without mixing the circuits.
Technical parameters of built-in cylinders – what the numbers say
Let's compare specific products we come across in practice. Protherm Tiger Condens 20/26 KKZ 21 is an entry-level model with a 21-litre cylinder – suitable for a one- or two-person household with a single bathroom and modest DHW requirements. The boiler output is 4–20 kW for heating and 8–26 kW for DHW heating, which is enough for houses up to roughly 120 m² of floor area in a well-insulated building.
A step up is the Protherm Tiger Condens 20/26 KKZ 42 with a 42-litre cylinder. Same output, but a larger cylinder – more comfort for families with two children, one bathroom and a shower. This model is in practice the most frequently sold variant from the Tiger Condens range, because 42 litres provides enough basic comfort for most 3-4 member Slovak households without unnecessary oversizing.
For more demanding houses and higher output, there's the Protherm Tiger Condens 30/35 KKZ 42 – it offers a heating output of up to 30 kW and DHW heating up to 35 kW, making it ideal for larger houses (150–200 m²) or new builds with underfloor heating and higher demand.
For a premium solution with intelligent control, I'd also highlight the Vaillant VUI 26CS/1-5 ecoTEC plus IoniDetect + sensoCOMFORT 720 – this boiler combines ionisation flame sensor technology, condensing operation, and connected sensoCOMFORT control, making it one of the smartest solutions in this category. For a customer who wants smartphone control comfort and doesn't want to bother with manual adjustments, this is a very attractive choice.
Advantages of a condensing boiler with a built-in cylinder – and where it truly "wins"
1. Space efficiency and simplicity of installation
This is the number one argument customers hear most often – and it's a real one. A boiler with a built-in cylinder takes up only one spot on the wall. Everything is connected internally – no external piping between the boiler and the cylinder, no separate cylinder on the floor or next to the wall. This is a huge advantage in flats or smaller houses, where every square metre of the utility room is precious.
In practice, I've seen jobs where the customer had a niche of only 60 × 60 cm available – in such a case, an external cylinder isn't even physically possible without construction work. A boiler with a built-in cylinder fits into such a space without any problem.
The installer also saves time and material – there's no need to run an extra pair of pipes from the boiler to the cylinder, install a circulation pump for the cylinder's primary circuit, insulate joints, and so on. For the customer, this means a lower installation price – depending on the specific situation, we're talking about savings of 100 to 300 euros on materials and labour alone.
2. No heat losses in piping between the boiler and cylinder
When an external cylinder is placed in the boiler room next to the boiler, heat losses in the piping are minimal. But if the customer wants to place the cylinder in a different room, or further away from the boiler (which is common in family houses), significant heat losses occur. Uninsulated piping, for example 5 metres long, can cool down by several degrees overnight, meaning additional heating in the morning.
With a built-in cylinder, the heat exchanger is directly inside the boiler. Losses are minimal – just the natural heat loss of the cylinder through the boiler casing wall, which is typically under 1–2 kWh per 24 hours.
3. Fast DHW heating thanks to the high burner output
A condensing boiler has the full burner output available when heating DHW – for the 20/26 KKZ model, that's 26 kW. A 42-litre cylinder heats up from 15 °C to 55 °C in less than 8–12 minutes. Customers won't notice any waiting time during morning showers – as long as the cylinder isn't empty, hot water is available immediately.
4. Fewer devices = fewer failures and lower service costs
A system with a built-in cylinder has fewer mechanical components than a combination of boiler + external cylinder. There's no separate pump group for the cylinder's primary circuit, no four-way valves for switching between the cylinder and heating. Fewer moving parts statistically means fewer failures.
This is also reflected in long-term operating costs. The service technician has fewer things to check during the annual inspection. If you look at the topics in this Knowledge Centre – for example Common faults of condensing boilers with a built-in cylinder and their solutions or Maintenance and servicing of a condensing boiler with a cylinder: what and how often to check – these service agendas are simplified precisely because there's no need to deal with external piping and its components.
Where a built-in cylinder isn't enough – limitations and real-world limits
Cylinder volume: this is the core of the problem
A built-in cylinder in the 21–42 litre range is a comfortable solution for 2–4 people with one bathroom and typical DHW usage habits. Problems arise when you have:
- 5 or more people in the household with typical shower habits,
- multiple bathrooms where two people shower at the same time,
- a large bathtub (volume 150–200 litres) that you want to fill without interrupting DHW comfort,
- a household with significant peaks in DHW consumption (for example, everyone going to work in the morning and showering at the same time).
In these cases, a 42-litre built-in cylinder simply isn't enough. Once the hot water supply is used up, a so-called "cold gap" occurs – the cylinder reheats quickly (in 8–12 minutes), but during that time you're under cold water. Unacceptable for some, quite tolerable for others – it depends on habits.
An external cylinder with a volume of 100, 150, or 200 litres effectively solves this problem. For a house with 5 people and two bathrooms, a 150-litre cylinder combined with a condensing boiler is a standard and proven solution. You can find more on this topic in the article What cylinder volume do I need for my house in this Knowledge Centre.
Limitations during renovations and replacement of an old boiler
A common situation in practice is renovation – the customer replaces an old conventional boiler with a new condensing one while an existing large DHW cylinder is already present in the boiler room (for example, a 200-litre Dražice or Regulus). In such a case, it would be economically wasteful to throw away the cylinder and buy a boiler with a built-in cylinder – it's much more sensible to connect the new boiler as a heat source for the existing cylinder.
The opposite also works – if the customer has no cylinder and the boiler room is small, a built-in cylinder is the natural choice without having to deal with layout issues.
Condensing boiler with an external cylinder – when it's clearly the better choice
An external DHW cylinder combined with a condensing boiler makes sense in clearly defined situations:
- Large houses and large families: A house with 5 or more people, where daily DHW consumption exceeds 200 litres, simply requires a 150–200 litre cylinder. A 42-litre built-in cylinder would be a constant source of frustration in such a house.
- Renovations with an existing cylinder: If you have a working 120 or 200 litre cylinder in the boiler room, there's no reason to replace it. You can connect the new condensing boiler to it via the primary circuit and save thousands of euros.
- Houses with multiple bathrooms: Two bathrooms with a bathtub and shower being used simultaneously require capacity that a built-in cylinder cannot cover.
- Holiday cottages with high weekend consumption: A 200-litre external cylinder can be heated overnight during the cheap tariff, giving you a comfortable supply during the day without needing the burner.
- Combination with a solar collector: An external cylinder can be equipped with two heat exchangers – one for the boiler and one for the solar collector. A boiler's built-in cylinder doesn't allow such bivalent operation.
Economic comparison: purchase price vs. operating costs
This is where the decision gets more interesting, as several variables intersect here. Let's compare two typical configurations for a 4-member household in a 130 m² house:
Option A – condensing boiler with a built-in 42 l cylinder: The boiler price ranges from €900–1,600 depending on the manufacturer and equipment. Installation is simpler – no external piping, no additional cylinder. Total installation cost, excluding the heating distribution system, is typically in the range of €2,000–2,800 including materials and labour.
Option B – combi condensing boiler + external 120 l cylinder: The boiler alone, without a cylinder, is €100–200 cheaper. But a 120 l external cylinder costs an additional €350–600 (depending on quality). Add to that materials for connecting them (piping, insulation, cylinder group pump, expansion vessel) at €150–250, plus 2–3 extra hours of labour. Total price typically €2,400–3,200.
This comparison shows that a built-in cylinder is economically more advantageous for the same capacity (40–42 litres). However, if the customer needs 120 litres of capacity, an external solution is necessary – and the price naturally increases, but it's a different comfort level.
In terms of operating costs, the differences aren't dramatic. A condensing boiler achieves the same condensing efficiency regardless of whether it has a built-in or external cylinder – it depends on the return temperature. The only relevant factor is the cylinder's standing loss, where a larger external cylinder logically loses more heat over 24 hours. For a well-insulated 120-litre cylinder, we're talking about 1.5–2.5 kWh per day, which at a gas price of €0.14/kWh represents roughly €8–12 more per year compared to a built-in 42-litre cylinder. A negligible difference.
Intelligent control and DHW comfort with a built-in cylinder
Modern condensing boilers with built-in cylinders are equipped with sophisticated control electronics that prioritise DHW heating over space heating while also allowing time programmes to be set for heating the cylinder. This is important both for comfort and savings.
For example, the Protherm Tiger Condens 20/26 KKZ 42 with smart controller allows not only programming DHW heating via a smartphone app, but also real-time consumption monitoring, detection of unusual conditions, and remote diagnostics. This means the customer can programme the cylinder heating for, say, 5:30 am before waking up, ensuring the cylinder is full of hot water exactly when the family needs it. This function partly compensates for the smaller volume of the built-in cylinder – effective heating management significantly reduces the risk of a "cold gap" even with a 42-litre cylinder.
If you're interested in how long it takes to heat water in the cylinder and what DHW comfort depends on, this topic is covered in detail in a separate article in this Knowledge Centre. In short: a 42-litre cylinder heats up from room temperature to 55 °C at full boiler output (26 kW) in less than 10 minutes, which is an acceptable time in practice.
Space and construction requirements – what not to forget
When installing a condensing boiler with a built-in cylinder, the same space requirements apply as for a standard wall-mounted boiler – the boiler must be mounted on a load-bearing wall, there must be sufficient space in front of it for servicing and handling (at least 60 cm in front of the front panel), and although a boiler with a cylinder is deeper than a regular combi boiler, the dimensions are still reasonable.
Typical depths of cylinder boilers: the Protherm Tiger Condens KKZ 42 has a depth of approximately 47–49 cm, while a standard combi boiler has 30–35 cm. This can be relevant when installing into a shallower niche or cabinet. Always check the actual dimensions of the specific model in the technical documentation before purchasing.
The topic of installation requirements is covered in detail in the article Installing a condensing boiler with a built-in cylinder: procedure and requirements in this Knowledge Centre – we recommend reading it before agreeing on a specific procedure with your installer.
Combination with renewable sources – possibilities and limits
If you plan to add solar collectors or a heat pump for DHW preparation in the future, a condensing boiler's built-in cylinder is a limitation. Most built-in cylinders have only one heat exchanger – for the boiler. Bivalent external cylinders with two heat exchangers (lower one for solar, upper one for the boiler) are a standard part of solar systems.
However, this doesn't mean a boiler with a built-in cylinder isn't suitable for houses with solar pre-heating. The solution is a hydraulic connection where the solar system pre-heats the cold water before it enters the boiler's cylinder – with water pre-heated to 35–40 °C, the boiler finishes heating the cylinder in a much shorter time and with lower gas consumption. However, this solution requires an experienced designer and installer who can design it correctly.
Choosing the right boiler output with a cylinder – a practical calculation
The output of a boiler with a built-in cylinder must cover two needs simultaneously: the heating load of the house and DHW heating. Since DHW heating has priority, the burner output for DHW heating (usually higher than the heating output) must be sufficient to quickly refill the cylinder after use.
For guidance: a house with a floor area of 120 m², well insulated (annual heat consumption approx. 8,000–10,000 kWh), with 3–4 people, needs a heating output of 6–10 kW and DHW heating of 20–26 kW. The 20/26 KKZ 42 model is ideal for such a house. For a larger house of 160–200 m² with worse thermal insulation and a heating output of 15–20 kW, the 30/35 KKZ 42 model is more suitable.
A more detailed calculation guide can be found in the article What output of a condensing boiler with a cylinder is enough for my house in this Knowledge Centre, where the calculation methodology for heat losses is explained step by step.
Frequently Asked Questions (FAQ)
Is a 42-litre built-in cylinder really enough for a 4-member family?
For a typical 4-member family with one bathroom and normal shower habits (not two simultaneous showers, not a large bathtub), 42 litres is usually sufficient – provided the boiler has enough output for a quick reheat after morning use. Smart scheduled heating is also key – heating the cylinder before the expected peak demand significantly increases comfort. However, if you have two teenagers prone to long showers, it's better to consider an external 120-litre cylinder.
Can I add an external cylinder to an existing boiler with a built-in cylinder?
Technically it's possible – some condensing boilers with a built-in cylinder have an output for an external cylinder, with the internal cylinder serving as a flow-through pre-heater. In practice, however, installers don't recommend this, as the hydraulics become unnecessarily complicated. If you anticipate needing a larger DHW volume, choose a combi condensing boiler and an appropriately sized external cylinder from the start.
Which manufacturer has a better built-in cylinder – Protherm or Vaillant?
Protherm (Vaillant Group) and Vaillant are technologically related manufacturers – Protherm is a sister brand of Vaillant within the same corporation. The technical quality of the cylinders is comparable. Vaillant ecoTEC plus offers ioniDetect technology (combustion quality detection) and more sophisticated controls, making it suitable for customers who want premium comfort. Protherm Tiger Condens is a technically solid solution with proven components at a lower price. A more detailed comparison can be found in the article Protherm Tiger Condens vs. Thermona THERM vs. Vaillant ecoTEC: comparison of models with a cylinder in this Knowledge Centre.
What is the lifespan of a built-in cylinder, and can it be replaced?
A stainless steel built-in cylinder, with proper maintenance (regular descaling, checking the magnesium anode if present), has a lifespan of 10–15 years; plastic cylinders are similar. Replacing just the cylinder without replacing the entire boiler is technically possible, but in practice laborious and not always cost-effective – it depends on the design of the specific model. In many cases, if the cylinder fails, replacing the entire unit is considered, especially if the boiler is older than 10 years and approaching the end of the burner's and heat exchanger's lifespan.
Is a condensing boiler with a built-in cylinder suitable for underfloor heating?
Yes – a condensing boiler with a built-in cylinder is suitable for underfloor heating and even advantageous, since underfloor systems operate with low return temperatures (25–35 °C), which maximises condensing efficiency. However, proper hydraulics must be ensured – there should be a mixing valve or three-way valve between the boiler and the underfloor circuit to prevent the low-temperature system from overheating at full boiler output.
What if we have hard water – does it affect the built-in cylinder?
Hard water (over 20 °FH) significantly increases the rate of limescale build-up in the cylinder and on the heat exchanger. A built-in cylinder is more sensitive in this regard than an external enamelled cylinder, as it has a smaller volume and heats up faster (meaning faster scale build-up). For water hardness above 25 °FH, we recommend installing a water softener or at least a polyphosphate dosing device on the cold water inlet to the cylinder. The topic of regular maintenance and limescale removal is covered in a separate article, Maintenance and servicing of a condensing boiler with a cylinder: what and how often to check, in this Knowledge Centre.
Conclusion: how to choose the right solution?
After this whole analysis, the conclusion might be less dramatic than you expected: there's no single "correct" solution. A condensing boiler with a built-in cylinder is an excellent choice for most typical households with 2–4 people, one bathroom, and limited space in the boiler room. It's compact, cost-effective, reliable, and, in the case of premium models, also technologically advanced.
An external cylinder makes clear sense where the built-in one isn't sufficient in capacity – large families, multiple bathrooms, combination with a solar system, or simply an existing cylinder that there's no reason to discard. In such cases, investing in an external cylinder is economically justified and will bring a real improvement in comfort.
If you're facing this decision, go through the decision tree above in this article, or take a closer look at the category of condensing boilers with a built-in cylinder, where you'll find the current offer with technical parameters. If you're still not sure after reading this article, check out related topics in the Knowledge Centre – for example, How to choose a condensing boiler with a built-in cylinder: what to watch out for or Frequently asked questions about condensing boilers with a built-in cylinder, which summarise practical experience from many real-life projects.
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.