Frequently Asked Questions about Low-Temperature Boilers with a Built-in Tank
Frequently Asked Questions about Low-Temperature Boilers with Built-in Tank
Low-temperature boilers with a built-in domestic hot water tank are among the most common solutions for heating and hot water preparation in family houses in Slovakia. Even though this is a technically well-proven technology with decades of tradition, both customers and installation technicians keep running into the same questions – what actually is a low-temperature boiler, why does it have a built-in tank, what tank volume to choose, what's the difference between individual Thermona models, what happens when the tank heats up, and dozens of other practical matters. In this article, I have tried to gather all the essential questions that I regularly answer in customer communication, and I have answered them in a way that gives you real orientation – not just a marketing brochure.
What actually is a low-temperature boiler and why does it have a "built-in tank"?
A low-temperature boiler is a gas boiler operating with a heating water temperature typically in the range of 40 – 75 °C, and its construction is not optimized for flue gas condensation (unlike a condensing boiler). The boiler burns natural gas or propane-butane, and transfers heat to the heating system via a heat exchanger.
The term "built-in tank" means that the boiler has a domestic hot water (DHW) tank directly integrated into one compact body – i.e. not an external standing boiler, but a tank built inside or directly below/above the boiler unit. The tank volume typically ranges from 80 to 160 liters, some combinations reach up to 200 liters. The tank is heated indirectly – via the boiler circuit through a built-in coil heat exchanger – or by a combination of the boiler circuit and solar circuit (in models with two heat exchangers).
From an installation point of view, the advantage is one compact device instead of two separate ones, which saves space in the boiler room, reduces the number of hydraulic connections and simplifies control. Typical products in this category, which you can find on atria.sk in the section with built-in tank, are for example Thermona THERM PRO 14 KX or Thermona THERM 28 LXZ.A 5.
What is the difference between the KX, TKX and LXZ models?
In the Thermona range, we encounter several type designations that at first glance look complicated, but have a simple logic:
- KX – basic low-temperature boiler with a built-in tank and one coil heat exchanger. The boiler is not condensing, the tank is heated exclusively by the gas burner via the boiler circuit. Typical example: Thermona THERM PRO 14 KX. Also available in a second configuration – Thermona THERM PRO 14 KX (second variant) with a different accessory set or type of flue gas exhaust.
- TKX – the same basic platform as KX, but the tank is equipped with two coil heat exchangers. The lower heat exchanger is intended for the solar circuit (collectors), the upper one for boiler heating. The boiler can therefore work together with solar DHW preparation. Examples: Thermona THERM PRO 14 TKX and Thermona THERM PRO 14 TKX (second variant).
- LXZ – higher power class (28 kW), a boiler with a larger tank and significantly higher heating output. Suitable for larger family houses, or buildings with a higher hot water demand. Example: Thermona THERM 28 LXZ.A 5.
From the customer's point of view, the key decision is simple: if you have or plan to have solar collectors, choose the TKX variant; if not, KX is sufficient. The power class (14 vs. 28 kW) depends on the building's heat loss and the number of people in the household.
What should you consider when choosing boiler output?
The boiler's output determines how much heat it can deliver to the heating system per unit of time while also heating the water in the tank. For a family house with a typical heat loss of 7 – 10 kW (a new build of 120 m²), a 14 kW boiler is fully sufficient, while for an older, poorly insulated building requiring 15 – 20 kW, a 28 kW model is appropriate. Important note: oversizing the boiler "for reserve" is not always advantageous – a boiler that is too powerful operates in short ignition cycles (so-called cycling), which reduces efficiency and burner lifespan.
A detailed selection procedure including specific calculations can be found in the article How to Choose a Low-Temperature Boiler with a Built-in Tank: Volume, Output and Type of Operation in the Knowledge Center.
Why a built-in tank and not an external boiler?
This is one of the most frequently discussed questions in customer consultations. The answer depends on the specific situation:
- Space saving: A built-in tank saves boiler room floor space – the device is compact, usually about the size of a classic standing boiler (width approx. 60 – 70 cm, height 180 – 200 cm). An external boiler with a 120-liter tank takes up an extra approx. 0.3 – 0.5 m².
- Simpler installation: Fewer hydraulic connections, one controller handles both functions – heating and DHW, which reduces the risk of faulty installation and service costs.
- Disadvantage – limited volume: A built-in tank has a limited volume (80 – 160 l). For families of 4 or more people with increased DHW consumption (2 bathrooms, a large bathtub, etc.), an external boiler of 200 – 300 liters may be necessary.
- Disadvantage – repairability: If the tank fails (corroded anode, tank leaking), replacement can be more complicated than with a separate external boiler.
From practical experience: for a 3-member family in a new build with one bathroom and one shower, a built-in tank of 100 – 120 l is the ideal solution. For a 5-member family with two bathrooms and a shower plus bathtub, consider either a model with a 160 l tank or a combination of a boiler with an external tank. More on volume calculations can be found in the article What DHW Tank Volume Do I Need for a Family House or Apartment.
How does DHW priority switching work and why is it important?
In low-temperature boilers with a built-in tank, the control system solves a key problem: the boiler has one burner, and it must handle two tasks – heating and DHW heating. The solution is so-called DHW priority switching.
Priority switching means that when the temperature in the DHW tank drops below a set value (e.g. below 50 °C), the controller automatically switches the boiler to tank heating mode. During tank heating, the heating circuit is temporarily interrupted or limited – the boiler works exclusively or preferentially to heat the DHW. Once the set tank temperature is reached (e.g. 60 °C), the system switches back to heating.
Practical consequences: in winter during hot water use (morning shower, cleaning), the heating circuit outlet temperature may be reduced by 2 – 5 °C for 10 – 20 minutes. For most households, this drop is imperceptible – radiators have thermal inertia and the room temperature does not drop. Problems only arise with an undersized boiler output or with extremely high DHW consumption.
What is a magnesium anode and why do I need to check its condition?
The built-in tank is a steel tank lined with enamel or an internal anti-corrosion layer. To prevent rapid rusting of the tank, a sacrificial magnesium anode is fitted inside – a rod made of magnesium alloy that electrochemically "sacrifices" itself for the tank walls. The anode gradually dissolves and protects the steel walls from corrosion.
The problem arises when the owner forgets about the anode and never checks it. Within 2 – 5 years (depending on water quality), the anode can be completely consumed. From that moment on, the tank itself starts to rust, which leads to brown water from the tap, tank leakage and eventually the need to replace the entire boiler with tank – which is a several times more expensive intervention than replacing the anode for 20 – 40 €.
Recommended anode inspection interval: every 2 years during the annual boiler service. In areas with aggressive (soft) water, even once a year. If the anode has lost more than 2/3 of its mass, it needs to be replaced. This topic is covered in detail in the article Rusting, Limescale and Tank Anode – How to Extend the Boiler's Lifespan in the Knowledge Center.
What is the actual lifespan of a low-temperature boiler with a tank?
With normal maintenance and regular servicing (once a year), the lifespan of a Thermona gas boiler ranges between 15 – 25 years. The tank is usually the weaker link – with a neglected anode, it can crack or start corroding even after 10 years, while with careful maintenance it lasts 20 years or more.
From experience, I know that the most common reason for premature replacement of a boiler with a tank is not a burner or heat exchanger failure, but precisely a scaled-up or ruptured tank. The second most common reason is neglected expansion vessel replacement and failures caused by overpressure. Replacement and common failures are covered in the article Common Failures of Low-Temperature Boilers with Tanks and How to Recognize Them.
How long does it take to heat the tank from cold water?
This is a very practical question to which many customers want a specific numerical answer. The tank heating time depends on:
- the tank volume (e.g. 100 l vs. 160 l)
- the incoming cold water temperature (summer approx. 12 – 15 °C, winter approx. 6 – 10 °C)
- the set target temperature (typically 55 – 60 °C)
- the boiler output (14 kW vs. 28 kW) and the output of the tank's coil heat exchanger
Approximate values for a 14 kW boiler and a 100-liter tank: heating from 10 °C to 55 °C takes approx. 25 – 40 minutes. A 160-liter tank with the same boiler: 45 – 60 minutes. A 28 kW boiler with a 160-liter tank: approx. 25 – 35 minutes. These times apply to an empty tank – in normal operation, the tank is only reheated by 5 – 15 °C (after a shower was taken), which typically takes 10 – 20 minutes.
Important note: the tank never fully empties in one use – cold water enters at the bottom, hot water is drawn from the top. With a shower flow of 8 l/min and a 100 l tank, you have approx. 12 minutes of pure hot water available, after which the temperature starts to drop – which is more than enough for a normal shower.
Can a low-temperature boiler with a tank operate without a heating system – just as a water heater?
Yes, this is technically possible and some customers operate the boiler this way, for example during the summer months when heating is not needed. The Thermona controller allows this – the heating circuit is simply switched off (by setting summer mode) and the boiler works exclusively to heat the DHW tank according to the tank thermostat setting.
Note: during a long-term summer operation mode, make sure the heating system is not permanently without circulation. Stagnant water in the pipes can overheat if the circuit is accidentally opened and cause problems. Some Thermona controllers allow automatic filling of the heating system even in summer – I recommend using this feature.
How do the TKX models with a solar circuit work? Is it worth it?
The TKX models (e.g. Thermona THERM PRO 14 TKX) have a tank equipped with two coil heat exchangers. The lower heat exchanger is connected to the solar circuit (flat or vacuum collectors on the roof), the upper heat exchanger to the boiler circuit.
Operating principle: the solar circuit heats the water in the tank during the summer months to 40 – 60 °C without turning on the burner. If solar energy is insufficient (cloudy day, night hours), the boiler automatically reheats the tank to the required temperature. During the summer period, solar collectors (2 – 3 panels for a 4-member family) can cover up to 60 – 80% of the DHW energy consumption.
Economic return on solar collectors combined with a TKX tank: with an installation cost of 2,500 – 4,000 € and savings of 150 – 250 € per year on gas, the payback period is 10 – 18 years, which is economically justified given the system's 25-year lifespan. This topic is discussed in more detail in the article How to Connect Solar Collectors to Low-Temperature Boilers with Thermona Tanks.
What are the installation space requirements?
Low-temperature boilers with a built-in tank are typically of standing design – the height of the unit ranges from 170 cm to 200 cm, width 60 – 70 cm, depth 60 – 70 cm. For comparison, the Thermona THERM 28 LXZ.A 5 boiler has dimensions of approx. 600 × 700 × 1,900 mm. It is therefore a device comparable in size to a refrigerator.
Technical standards apply to the boiler room: the minimum boiler room volume for boilers up to 14 kW is 1.5 m³, with 7.5 m³ recommended for comfortable operation. The boiler must be ventilated (window or vent opening of min. 150 cm²), and there must be a supply of combustion air or a turbo version with a closed flue gas chamber (independent of the space). The floor must be non-combustible, and the boiler must not be installed in an enclosed ventilated space without an approved turbo version.
More on space requirements can be found in the article Low-Temperature Boiler with Tank – Boiler Room Requirements, Dimensions and Space.
Is a low-temperature boiler with a tank worse than a condensing boiler? Is it worth paying more for a condensing one?
This is the most common question when deciding on a purchase. Briefly: a condensing boiler has higher efficiency (95 – 109% according to the measurement methodology), a low-temperature boiler achieves 88 – 92%. The difference in gas consumption at the same output is 8 – 15% in favor of the condensing boiler.
However, be careful about the context: a condensing boiler takes advantage of flue gas condensation only when the return temperature is sufficiently low (below 57 °C). If you have old cast iron radiators set to 70/50 °C, the condensing boiler only condenses partially and the savings are much smaller. With underfloor heating or a modern system with low-temperature settings, the condensing boiler is clearly better.
The price difference between a low-temperature and a condensing boiler with the same tank volume is typically 500 – 1,500 €. With gas consumption of 2,000 m³/year and savings of 12%, that's 80 – 120 € in annual savings – so the payback period is 5 – 12 years. For new houses with underfloor heating, the condensing boiler is clearly worth it. For old houses with non-replaceable radiators, a low-temperature boiler can be more economically advantageous. A detailed comparison is in the article Condensing vs. Low-Temperature Boiler with Tank: Which Is More Worthwhile.
Can the boiler prepare DHW overnight without adjusting settings?
Yes, Thermona controllers allow setting a time program for DHW heating. Recommended practice: set night-time tank heating (e.g. from 22:00 to 6:00) during a cheaper electricity rate – this applies to the boiler as a whole, not to electric heating, but it may be advantageous to switch the DHW heating priority to the night, so that in the morning the tank is full of hot water without burdening the morning peak. This setting is a standard controller function in all Thermona THERM PRO models.
How is the tank temperature set correctly?
The recommended DHW tank temperature is 55 – 60 °C for hygienic reasons – at this temperature, Legionella (Legionella pneumophila), a bacterium that can multiply in stagnant hot water, dies. Setting it below 50 °C is risky from a hygienic point of view.
If you have the tank set to 55 °C and the tap water is too hot, simply mix it with a thermostatic valve on the DHW distribution (I recommend installing an anti-scald valve set to 42 – 45 °C). This keeps the tank hygienically safe, while you get a pleasant temperature at the tap.
Some Thermona controllers offer an automatic "anti-legionella" cycle: once a week (typically at night) the tank is automatically heated to 70 °C for 1 hour and then returns to normal temperature. This is a sensible feature that I recommend activating.
What are the most common installation mistakes for boilers with a built-in tank?
From years of service experience, I have compiled a list of mistakes I see repeatedly:
- Incorrectly sized expansion vessel: The expansion vessel must be sized for the volume of water in the entire system, including the tank volume. Many installers forget to include the tank in the calculation, which causes repeated discharge through the safety valve.
- Missing or incorrectly set DHW tank safety valve: The DHW tank must have its own safety valve (typically set to 6 bar) and this valve must have a discharge to a safe location. If the discharge leads to a closed space without a drain, opening it will cause flooding.
- Absence of a check valve before the cold water inlet to the tank: Without a check valve, tank water is pushed back into the cold water supply during thermal expansion, causing pressure problems and hygiene issues in the system.
- Unconnected or uninsulated tank return pipe: The tank return pipes must be properly thermally insulated to avoid unnecessary heat losses.
- Incorrectly set boiler temperature with solar connection (TKX): With the TKX model, the control must be coordinated with the solar controller, otherwise solar heating and boiler heating interfere with each other.
A detailed installation procedure with a checklist can be found in the article Installation of a Low-Temperature Boiler with a Built-in Tank: Procedure and What to Know.
How much does regular servicing cost and what does it include?
Annual servicing of a low-temperature boiler with a tank usually takes 60 – 90 minutes and includes: burner inspection and cleaning, flue gas measurement (CO, CO₂ analysis, flue gas temperature), heating water pressure check, expansion vessel check, safety valve check, DHW tank inspection including the anode, seal and hydraulics check, and records in the service log.
Price depending on the service company and region: 60 – 120 € excluding VAT for a complete inspection (excluding spare parts). Tank anode replacement: 40 – 80 € including material. Descaling the tank of limescale: 80 – 150 € depending on scope. Total annual maintenance costs: 100 – 200 €, which, in the context of the equipment's lifespan, prevents problems costing thousands of euros.
A regular maintenance plan and a detailed description of individual tasks can be found in the article Maintenance and Servicing of a Low-Temperature Boiler with Tank: What and How Often to Check.
Frequently Asked Questions (FAQ)
Can I connect a low-temperature boiler with a tank to an existing underfloor heating system?
Yes, a low-temperature boiler is, in terms of the working water temperature (40 – 55 °C), compatible with underfloor heating, which typically requires a temperature of 35 – 45 °C. You just need to verify that the boiler controller allows setting low outlet water temperatures and that the circulation pump is properly sized for the underfloor system. Thermona THERM PRO models allow this without any problems.
What happens if the tank's protective anode is depleted and I don't know it?
The tank will start to corrode from the inside – the steel walls react with water and oxygen, brown deposits form in the water, and the tank walls gradually corrode. The first visible symptom is brown water from the hot water tap, or a rusty smell. If you don't act, the tank may start leaking – water penetrates the insulation, damages surrounding equipment, and eventually requires replacement of the entire boiler. That is why checking the anode every 2 years during the annual service is an absolute necessity.
I have a 120-liter tank, but after a morning shower (2 people) my hot water runs out. Is the tank too small?
A 120-liter tank should be sufficient for 2 people – 2 showers of 8 minutes each consume approx. 80 – 100 liters of hot water (mixed with cold). The problem is probably not the tank volume, but the tank temperature setting or the heating speed. Check: (1) What is the set tank temperature? It should be 55 – 60 °C. (2) Is DHW priority switching working? (3) Is the tank scaled with limescale, which reduces the efficiency of the heat exchanger? Call a service technician for diagnostics.
Is there a difference between the first and second variant of the Thermona THERM PRO 14 KX model?
The Thermona THERM PRO 14 KX and THERM PRO 14 KX (second variant) models typically differ either in the type of flue gas exhaust (e.g. B23 vs. C33 turbo version with a coaxial outlet), or in the accessory set in the basic package, or in the controller version. Always verify the exact differences in the current technical data sheets or with a sales advisor – the manufacturer may update these details during the model year.
Can a boiler with a built-in tank operate without a hot water circulation circuit?
Yes, the DHW circulation circuit is an optional add-on, not a necessity. Without circulation, every time you open the hot water tap, you have to wait for the cold water in the pipe from the tank to the tap to be replaced by hot water (so-called "wasting the first liters"). If the distances in the house are large (e.g. boiler room in the basement, bathroom upstairs), circulation shortens the wait for hot water. The circulation pump is connected via a special control circuit, typically with a timer, so that it does not run continuously. Thermona boilers support this function as standard.
How long does the expansion vessel last in a system with a built-in tank and when does it need to be replaced?
The expansion vessel (pressure membrane) typically has a lifespan of 7 – 12 years. Its failure manifests itself as repeated dripping from the heating system's safety valve, because the vessel loses its function of dampening pressure fluctuations. The condition of the vessel is checked by every service technician during the annual inspection – by measuring the pre-charge pressure (the air pressure in the section without water should be approx. 1 – 1.5 bar for most systems). Replacing the expansion vessel (8 – 18 liters) costs 50 – 120 € including labor, and it is an investment that prevents costly failures.
Conclusion
Low-temperature boilers with a built-in tank are a proven, reliable technology that, with proper selection, professional installation and regular maintenance, will serve trouble-free for 15 – 25 years. The key to satisfaction with this equipment is correct sizing (boiler output + tank volume), quality installation in accordance with applicable standards, and a disciplined service plan – especially checking the anode and the expansion vessel.
If you are considering solar support, the Thermona THERM PRO 14 TKX or Thermona THERM PRO 14 TKX (second variant) models offer a built-in second heat exchanger without the need for an external tank and are an elegant solution for those who want to combine gas with solar energy. For larger houses with higher demands, the Thermona THERM 28 LXZ.A 5 with its larger tank and 28 kW output is the choice.
Further information from the Knowledge Center: a detailed comparison with condensing boilers, a guide to choosing the tank volume, service procedures and other practical topics are available in the other articles in this section. If you have doubts about a specific solution for your home, contact a sales advisor – making the right choice when buying will save you much more than the price of the boiler itself.
Do you have a question on this topic?
Can't decide or are dealing with a specific situation in your household? Write to us - we will be happy to advise you.