How to Choose a Low-Temperature Boiler with a Built-in Tank: Volume, Output and Operating Mode
Low-temperature boiler with built-in tank: comprehensive guide to selection
When someone is deciding between several types of heating systems, a low-temperature boiler with an integrated hot water tank (DHW) appears to be a simple solution at first glance – one compact unit, one device in the boiler room, one installation. The reality, however, is a bit more complex. This is where many projects fail: the customer chooses a boiler based on price or external dimensions, and after half a year, they find out that hot water runs out in the morning, or on the contrary, that the tank is so oversized that the hot water sits in the boiler for hours and loses its temperature before it is used.
In this article, we will look at the selection process systematically – from understanding what a low-temperature boiler with an integrated tank actually is and how it works, through the correct choice of tank volume, boiler power and operating type, all the way to practical scenarios from installations that clearly illustrate where most mistakes are made.
What is a low-temperature boiler with a built-in tank and why it is a different category than a combi boiler
Many customers confuse a low-temperature boiler with an integrated tank with a classic combined condensing boiler. The difference is fundamental. A combi boiler heats hot water on demand – when you turn on the tap, the boiler immediately starts heating and the water is heated while flowing. There is no tank, no reserve. This works well for low consumption, but has physical limits in flow rate.
A low-temperature boiler with an integrated tank (integrated water heater) works differently: the tank – most often 80, 100, 120 or 160 liters – is physically part of the boiler casing. The boiler continuously heats the tank content to a set temperature (usually 55–65 °C), and from this tank you draw hot water. You have a certain volume available immediately, without waiting for heating. This is the key difference in terms of comfort at higher consumption – for example, when three people are taking turns in the bathroom in the morning.
A low-temperature boiler (not condensing) operates at flue gas and return temperatures at which water vapor does not condense. Typical heating water temperature: 60–80 °C (supply) / 40–60 °C (return). This is a technology older than condensation, but still fully functional, reliable and economically sensible in certain situations. For more information on the differences between these two technologies, see our overview article Condensing vs. low-temperature boiler with tank: which one is more cost-effective.
Boiler power: how to calculate it correctly and why it should not be bought "with a reserve"
One of the most common mistakes we see in practice is so-called boiler overdimensioning. The customer thinks that higher power = faster heating = better comfort. In reality, it works exactly the opposite. A boiler that has significantly higher power than the house needs operates in short cycles – it starts up, quickly reaches the desired temperature, turns off, cools down after a while, and restarts. This so-called cycling mode is the enemy of reliability, shortens the life of the burner and heat exchanger, and increases consumption.
The correct calculation of power starts from the building's heat loss. This should be calculated by a designer or energy auditor, but in practice, the following rule is used for an approximate estimate:
- Old family house without insulation (built before 1990): approx. 80–120 W/m² of heated area
- Partially insulated house (windows, roof, but original walls): approx. 50–80 W/m²
- House after comprehensive insulation (façade, roof, windows, floor): approx. 30–50 W/m²
- New house built to standards from 2010+: 20–35 W/m²
Practical example: a family house of 140 m² of heated area, partially insulated (new windows, original façade). Estimated heat loss: 140 × 65 = 9,100 W, i.e. approx. 9 kW. To this we add the power required for DHW heating – for a family of four in a tank, approx. 2–3 kW effectively. The total power should not significantly exceed 12–14 kW. A 14 kW boiler is ideal here – it covers heating and water heating without cycling mode.
A typical representative in this category is the Thermona THERM PRO 14 KX – a low-temperature boiler with a power of 14 kW and an integrated 80-liter tank, suitable precisely for houses with a heated area of 100–160 m² with standard insulation. The manufacturer Thermona has a long tradition in this segment and these boilers are very widespread in practice, which you will also appreciate in terms of the availability of service technicians.
Storage tank volume for DHW: how to choose it correctly
The volume of the storage tank is a parameter where customers most often make mistakes in both directions. A too small tank means that during the morning peak (multiple people showering one after another), hot water runs out and you have to wait for reheating. A too large tank unnecessarily takes up space, costs more, and above all – hot water in the tank stands and loses its temperature, increasing stationary heat losses and thus operating costs.
Basic reference values for typical households:
- 1–2 people, apartment or small family house: 80 liters is usually sufficient if the tank is properly set and heating takes place at night (at a cheaper rate)
- 3–4 people, family house: 100–120 liters is the standard for comfortable use without limitations; for a larger house with a bathtub we recommend 120 l
- 4–6 people or a house with a bathtub + shower + kitchen with high consumption: 160 liters and more
However, it is also important how you fill the tank – that is, the boiler's power and the speed at which the tank can be reheated. If your 80-liter tank lasts only for two 7-minute showers, but the boiler reheats it in 20 minutes for further use – this may be an acceptable solution in practice for a two-person household. For a four-person family, where people alternate in the morning, it is not enough.
A more detailed calculation method for the tank can be found in the article What DHW tank volume do I need for a family house or apartment, where the entire procedure is explained in detail with specific examples for different types of households.
Types of operation: single-fuel vs. dual-fuel boilers with a tank
In the category of low-temperature boilers with an integrated tank, we encounter two basic types in practice according to the mode of operation:
Boilers with one heat source (single-fuel, gas)
A classic configuration – the boiler burns natural gas (or LPG) and exclusively from this source heats both the heating circuit and the DHW tank. Simple installation, simple control. An example is Thermona THERM PRO 14 KX – a compact boiler with a nominal power of 14 kW, an integrated 80-liter tank, suitable for a standard household supplied with natural gas.
This solution is ideal where:
- solar heating or another additional source is not planned
- simplicity of operation and service is a priority
- an adequate supply of natural gas is guaranteed in the long term
- you want to keep investment costs low
Boilers with two inputs (combined with the possibility of connecting solar collectors or another source)
Some models in this category have a tank equipped with two heat exchangers – lower and upper. The lower heat exchanger serves the primary source (boiler), the upper one for the secondary source – typically solar collectors or a heat pump. Such a tank is referred to as bivalent.
An example is Thermona THERM PRO 14 TKX or Thermona THERM PRO 14 TKX (second version) – boilers with a power of 14 kW and an integrated 120-liter tank with two heat exchangers. This solution allows connecting solar collectors, which in the summer period ensure DHW heating almost for free (only the cost of a circulation pump), while the boiler only reheats in case the solar gain is insufficient.
A bivalent tank is worth it if:
- you plan to install solar panels now or in the near future
- you have a suitably oriented roof (south, southwest, slope 30–45°)
- your house is located in an area with good solar radiation (central Slovakia and lower regions)
- you want to diversify energy inputs and reduce dependence on gas
More on this topic can be found in the article How to connect solar collectors to low-temperature Thermona boilers with a tank.
Large model for more demanding requirements: Thermona THERM 28 LXZ.A 5
If you have a larger family house, a larger family, or a house with a higher standard of bathroom (larger bathtub, hydromassage shower, multiple bathrooms), the question arises whether 14 kW and a 80–120 liter tank will be sufficient. In such cases, a more powerful boiler with a larger tank is the right choice.
Thermona THERM 28 LXZ.A 5 is a low-temperature boiler with a power of 28 kW and an integrated 160-liter tank. This model is intended for houses with a heated area from 180 to 280 m² (depending on the level of insulation), or for houses with higher DHW consumption. A five-person family with two bathrooms and a large bathtub will have enough hot water even during the morning peak. A 160-liter tank at 60 °C and mixing to 42 °C (shower) provides effectively about 230–250 liters of usable hot water – enough for 4–5 showers or one bathtub and two showers.
In practice, we have encountered cases where a customer chose a 14 kW model for a 220 m² house with a five-person family, and after the first winter, they were dissatisfied – the boiler could not keep up with heating on frosty days, and in the morning, one had to wait for hot water. Replacing it with a 28 kW model immediately solved the problem.
Domestic Hot Water (DHW) priority versus heating – setting up the regulation
Most low-temperature boilers with an integrated tank operate in the so-called DHW priority mode. This means that when the temperature in the tank drops below a set value (typically 55 °C), the boiler interrupts heating and dedicates full power to heating the tank. As soon as the tank reaches the desired temperature, the boiler switches back to heating.
This solution is ideal for most households – it ensures that DHW is always available, and the interruption of heating for 20–30 minutes is practically imperceptible in a standard house. Radiators store heat and the room does not cool down by more than tenths of a degree.
There is also a parallel mode (some boilers support it), where the boiler heats the tank and simultaneously supplies heating at reduced capacity. This is suitable for extremely cold days or houses with very low thermal mass (for example, lightweight wooden structures). In practice, however, this mode is less used, as most masonry houses do not need it.
Another important setting is the so-called night tank suppression. If you set the tank not to heat between 22:00 and 5:30, you save energy and at the same time eliminate the growth of legionella (this is only temporary – the tank must be heated to at least 70 °C at least once a week for thermal disinfection). If you have a dual-rate electricity meter and the boiler has an electric heating element, night heating can be combined with cheaper night-time electricity – but this is mainly a topic for electric tanks, not gas boilers.
Dimensions, space in the boiler room, and installation requirements
Boilers with an integrated tank are larger than standard through-flow boilers. This is a physical necessity – the tank has to be somewhere. Typical dimensions for the category with an 80–120 liter tank: height 1 400–1 600 mm, width 550–650 mm, depth 600–700 mm. A boiler with a 160-liter tank can be as high as 1 700–1 900 mm.
Before choosing, always check:
- the ceiling height in the boiler room (including flue gas exhaust through the chimney – there must be space for connection)
- the accessible width of the boiler room door (the boiler must fit inside during installation)
- the distance from flammable materials (walls, shelves) – at least 20–30 cm on each side of the service access
- sufficient fresh air supply for combustion (for turbo boilers with air intake from the flue jacket, the situation is simpler)
Detailed requirements for the boiler room, including regulatory standards, can be found in the article Low-temperature boiler with a tank – requirements for the boiler room, dimensions, and space.
Practical scenarios: which type of boiler for whom
Scenario 1: Older building, two-generation family, natural gas
A house built in 1975, masonry construction, windows replaced, no insulation on the façade, heated area 160 m², 5 people including seniors. The boiler runs on natural gas, exhaust air through the chimney. In this case, we recommend the Thermona THERM 28 LXZ.A 5 – a 28 kW output covers the heat loss of the unrenovated building even in severe frost (calculated loss about 18–20 kW), a 160-liter tank ensures sufficient DHW for a five-person family. Alternatively, insulating the building would allow a 14 kW boiler – but that is a separate investment decision.
Scenario 2: New family house, 3 people, solar panels are being considered
A new house, low-energy standard, 130 m², three adults, planning to install solar thermal collectors in two years. A suitable choice is a bivalent boiler, for example, the Thermona THERM PRO 14 TKX – a 14 kW output is sufficient for a low-energy house, a 120-liter tank with two heat exchangers is ready for solar collectors. Installing a bivalent tank now and connecting the collectors in two years is much cheaper than later replacing the entire boiler.
Scenario 3: Small family house, retirees, minimal DHW consumption
A house of 80 m², 2 people, low DHW consumption, no interest in solar, natural gas. Here, a basic model Thermona THERM PRO 14 KX with an 80-liter tank is sufficient. A 14 kW output is slightly oversized for an 80 m² new house, but realistic for an older building without insulation – and a small tank is enough for two people.
What else influences the choice: connection to an existing chimney or turbo
Low-temperature boilers can operate in two flue gas exhaust configurations:
- Boiler with natural draft (atmospheric): combustion air is drawn from the boiler room, exhaust gases are vented through the chimney. Requires a functional chimney opening (inspection!) and sufficient ventilation of the boiler room. An older and cheaper principle.
- Boiler with forced flue gas exhaust (turbo, closed combustion chamber): air is drawn from outside through a double-walled pipe (coaxial), exhaust gases are vented outside. Safer, does not require boiler room ventilation, but must be led directly outside through a wall or through the chimney with a special insert. More flexible for installation in modern houses.
If you have an older chimney, have it inspected by a chimney sweep before selecting the boiler. We have often encountered situations where a customer bought an atmospheric boiler, but the chimney was in poor condition and required relining – which increased the installation cost by 600–1 200 €.
Price vs. savings: what really matters
Low-temperature boilers are cheaper in the purchase price than condensing boilers. However, condensing boilers have a higher efficiency (95–109 % vs. approx. 85–92 % for low-temperature boilers). The difference in annual gas consumption for an average family house can be 200–400 m³ of gas, which at a price of 0.80 €/m³ represents 160–320 € per year. If a condensing boiler costs 500–800 € more than a low-temperature boiler, the payback period is 2–4 years. That is not a long time.
On the other hand, there are situations where a low-temperature boiler really pays off: renovations with old systems and radiators designed for 80/60 °C (a condensing boiler would not condense in this mode), a limited budget, or simply a customer who plans to stay in the house for 5–7 years and then sell it.
The choice between these technologies is discussed in more detail in the article Condensing vs. low-temperature boiler with a tank: which one is more cost-effective.
Anode, limescale and the lifespan of the tank
The built-in tank is a steel tank where electrolytic corrosion and limescale deposition occur. In order for the tank to last 15–20 years, it must be protected. Modern tanks have an enamelled (glazed) lining and a sacrificial magnesium anode. The anode is gradually "sacrificed" electrochemically and protects the steel tank from rusting.
Key responsibility of the owner: inspect and replace the anode – typically every 2–3 years (depending on water quality; hard water wears out the anode faster). A neglected anode leads to tank corrosion, which is usually the end of the tank. Replacing the tank is an expensive repair – that is why regular anode inspection is really important.
More on this topic can be found in the article Rusting, limescale and the tank anode – how to extend the life of your boiler.
Most frequently asked questions (FAQ)
What is the difference between Thermona THERM PRO 14 KX and THERM PRO 14 TKX?
Both boilers have the same power of 14 kW, but differ in the tank. The KX model has an 80-liter tank with one heat exchanger – it is designed exclusively for heating with natural gas. The TKX model has a 120-liter tank with two heat exchangers (bivalent) and allows connection of solar collectors or another secondary heat source to the upper heat exchanger. If you plan to install solar collectors in the future, choose the TKX.
Can I connect underfloor heating to a low-temperature boiler with a tank?
Yes, but with limitations. Underfloor heating operates with water temperature of 30–45 °C, while a low-temperature boiler delivers 60–80 °C. A mixing unit (thermostatic valve or three-way valve with a pump) must be installed between the boiler and the underfloor heating loop to reduce the temperature to the desired level. Underfloor heating itself is not a problem, but it requires a proper hydraulic design. The boiler itself is not designed for this without this component.
How long will the built-in tank last?
With regular anode replacement every 2–3 years, descaling (if the water is hard) and annual inspection of the expansion tank pressure, the realistic lifespan of the tank is 15–20 years. Without maintenance, the tank may fail due to corrosion in as little as 5–8 years – and replacing the entire boiler or just the tank is financially burdensome. Prevention is much cheaper. More in the article Maintenance and service of a low-temperature boiler with a tank: what and how often to check.
Do I need a special permit or inspection for a boiler with a tank?
Yes. Before putting the boiler into operation, professional installation by a qualified person, gas appliance inspection and chimney inspection (if used) must be carried out. Without these documents, your insurance company may not cover damage in the case of a breakdown or fire. Always ensure installation is done by a certified installer – in addition, many manufacturers (including Thermona) require professional installation with a service report to guarantee the warranty period.
Is a low-temperature boiler with a tank suitable for a heat pump as a supplementary heat source?
Direct connection of a low-temperature gas boiler with a heat pump is technically complicated and rarely done in practice – a heat pump operates at different temperatures and requires its own tank or a special buffer. A bivalent tank (model TKX) is designed for solar thermal collectors, not for a heat pump. If you plan to install a heat pump, consider a hybrid system with a condensing boiler and a heat pump – that is a different category of solutions.
What happens if the tank runs dry or is left without water?
Modern Thermona boilers have a tank temperature sensor and a temperature limiter (STB). If the tank runs dry (e.g., due to a faulty inlet valve), the temperature rises quickly and the STB shuts off the boiler to prevent damage. Nevertheless, you should never operate the tank without water, even for a short time. When refilling the tank, always open the air vent to avoid air pockets, which can cause noise or uneven heating.
Conclusion: the right choice is not only about power
Selecting a low-temperature boiler with a built-in tank is not just about watts and liters. It is a combination of several factors: the heat loss of the house, the number of people and their habits, the existing installation (chimney, heating system), future plans (solar, insulation) and, of course, the available budget. Each of these parameters influences the final decision and neglecting even one of them can lead to unnecessary discomfort or wasted money.
In the category low-temperature boilers with built-in tanks on atria.sk you will find models covering various power classes and tank configurations – from a compact 14 kW model with an 80-liter tank up to a powerful 28 kW boiler with 160 liters. If you are unsure about the choice, do not hesitate to consult with an expert – the right choice will save you much more than a few minutes of consultation cost.
Do you have a question about this topic?
Not sure how to decide or dealing with a specific situation in your household? Write to us – we are happy to help.