How to choose a low-temperature boiler with the option of connecting a storage tank

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How to choose a low-temperature boiler with the option to connect a cylinder

Low-temperature boilers with the option to connect a hot water cylinder represent a solution that combines economical heating with comfortable hot water supply for the entire household from a single source. At first glance it sounds simple – buy a boiler, connect a cylinder, done. In practice, however, it is a decision that will affect everyday comfort, monthly energy costs, and the lifespan of the entire system for decades to come. That's why it's worth taking the time to choose and understand what actually lies behind the term "option to connect a cylinder".

This article will guide you through all the key aspects – from a basic understanding of how a low-temperature boiler works, through the differences between atmospheric and turbo variants, to the specific technical parameters that determine the right output and configuration for your home.

What is a low-temperature boiler and why is it suitable with a cylinder

A low-temperature boiler is a gas boiler that can operate efficiently at heating water temperatures lower than the standard for old systems (60–90 °C). Low-temperature boilers are designed so that they can operate at a boiler water temperature in the range of 35–75 °C without causing condensation of flue gases on the heat exchanger. This distinguishes them from condensing boilers, which are directly designed to utilize condensation heat, and also from old standard boilers, where low temperatures caused corrosion.

When we talk about the "option to connect a cylinder", we mean that the boiler is equipped with a three-way valve (or preparation for one), its own circulation pump for the cylinder circuit, or control electronics that allow alternating cylinder heating and space heating – so-called priority control. Not every boiler you find on the market has this connection physically present. That's why the "option to connect a cylinder" category is an important filter when choosing.

The advantage of this arrangement is essential: the household always has hot water at hand regardless of whether heating is currently running. The boiler simply receives a signal from the cylinder thermostat, the three-way valve redirects the heated water to the cylinder, heats it to the required temperature, and then returns to heating. All this happens automatically and does not burden the owner with any operation.

Atmospheric or turbo boiler – which to choose with a cylinder?

This is one of the most frequently asked questions when choosing. And the answer depends on the specific conditions of your installation, not on a general recommendation. In short: if you have a functional chimney flue, an atmospheric boiler is more affordable and easier to install. If you don't have a chimney or it is unsuitable, a turbo boiler with a closed combustion chamber is a necessity. This topic is covered in more detail in a separate article in our Knowledge Center "Atmospheric vs. turbo low-temperature boiler – which to choose with a cylinder".

Atmospheric low-temperature boilers with a cylinder

An atmospheric boiler draws combustion air directly from the room where it is installed and discharges flue gases into a chimney with natural draught. A typical representative in this category is the Vaillant atmoTEC plus VU 200/3-5 series, which offers an output of 20 kW and is directly equipped with preparation for an external cylinder – it has an integrated three-way valve and circulation pump for the cylinder circuit. A similar situation applies to the atmoTEC plus VU 240/3-5 with an output of 24 kW and to the atmoTEC plus VU 280/3-5 with an output of 28 kW.

Advantages of atmospheric boilers with a cylinder:

Lower purchase price compared to turbo variants
Easier installation where a chimney exists
Independence from electricity with natural draught (the boiler starts with electricity, but combustion does not require forced venting)
Quiet operation – no flue gas fan
Long tradition and proven reliability in Central European conditions

Disadvantages to consider:

Need for a functional, inspected chimney with sufficient draught
The boiler room (utility room) must have sufficient volume and ventilation
Not suitable for apartments without a chimney
Chimney draught fluctuates with weather changes – in some situations it may cause ignition problems

Turbo low-temperature boilers with a cylinder

A turbo boiler (also referred to as a boiler with a closed combustion chamber, "C-type") has its own fan that actively discharges flue gases and supplies combustion air through a dual-pipe system (coaxial pipe or twin-pipe) directly from outdoors. The boiler room therefore does not need special ventilation, and the boiler can also be installed in rooms without a traditional chimney.

Typical representatives in this category are the Vaillant turboTEC plus VU 122/3-5 with an output of 12 kW and the Vaillant turboTEC plus VU 202/3-5 with an output of 20 kW. Both models are equipped with an integrated preparation for a hot water cylinder.

Advantages of turbo boilers with a cylinder:

Can be installed practically anywhere – apartment, new build without a chimney, utility room
Independence from chimney draught quality
Higher flexibility in flue pipe routing (horizontal, vertical, combined)
Closed combustion chamber means higher safety (flue gases cannot leak into the room)

Disadvantages:

Higher purchase price
Dependence on electricity – the fan does not work without power
Slightly noisier than atmospheric versions
More expensive repair in case of fan failure

Key technical parameters when choosing

When someone tells you "choose a boiler according to the size of the house", that's a correct piece of advice, but far from sufficient. For a low-temperature boiler with a cylinder, you must consider several interrelated parameters at once.

Heating output

The boiler's output is determined based on the heat loss of the building. As a rough calculation: a well-insulated family house needs approximately 40–60 W/m² of floor area, an older uninsulated building 80–120 W/m². Practical examples:

House 150 m², low-energy construction: heat loss ≈ 7–8 kW → 12 kW boiler (turboTEC 122/3-5) with reserve for cylinder heating
House 150 m², standard 1990s construction: heat loss ≈ 12–14 kW → 20 kW boiler (atmoTEC 200/3-5 or turboTEC 202/3-5)
House 200 m², older brick building without insulation: heat loss ≈ 18–22 kW → 24–28 kW boiler (atmoTEC 240/3-5 or 280/3-5)

Important note: Never choose a boiler with an output substantially larger than your heat loss. An oversized boiler operates in short on/off cycles (so-called "cycling"), which accelerates wear and reduces efficiency. Conversely, slight oversizing (10–20% above heat loss) is fine, since the boiler will also work to heat the cylinder.

Output for hot water heating

Here a difference arises compared to an ordinary heating boiler: when the boiler heats the cylinder, it temporarily stops heating (cylinder priority). Therefore, when sizing cylinder output, it also matters what cylinder volume you have and how quickly you want to heat it. A 20 kW boiler output can heat a 150-liter cylinder from 15 °C to 60 °C in approximately 45–55 minutes. At 28 kW it will be around 30–35 minutes. If you have a larger household and more simultaneous draws, invest in a larger cylinder and possibly higher boiler output – more on this topic in the article "What hot water cylinder volume do I need for a low-temperature boiler".

System pressure and heating system type

Low-temperature boilers generally operate in closed heating systems with an expansion vessel and safety valve. The operating pressure is typically 1.5–2.5 bar. Before installing the boiler, it is therefore necessary to verify that your existing heating system is suitable for closed operation (no open expansion vessels in the attic). If you are renovating from an old gravity system, it needs to be adapted before installation.

Circulation pump – integrated vs. external

Models like atmoTEC plus or turboTEC plus have a circulation pump integrated directly in the boiler. This pump serves the heating circuit. The cylinder circuit (if the boiler doesn't have a directly built-in cylinder circuit) usually makes do with the same pump via the three-way valve. Check the technical data sheet of the specific model to see whether the boiler includes:

Integrated three-way valve for the cylinder (yes/no)
Check valve for the cylinder circuit
Cylinder thermostat (sensor or input for external one)
Control electronics with cylinder priority

Sizing the cylinder for a low-temperature boiler

The cylinder is just as important to the system as the boiler itself. A cylinder that is too small will cause the boiler to heat the cylinder too often, increasing system heat losses and causing the boiler to cycle excessively. A cylinder that is too large will have unnecessarily large heat losses even in summer, and the heating time will be long.

Practical recommendations for common households:

1–2 people: 80–100 liter cylinder
3–4 people: 120–150 liter cylinder
5–6 people or a house with a bathtub and two bathrooms: 200 liter cylinder
Larger family or holiday house: 250–300 liter cylinder

For a low-temperature boiler, it is important that the cylinder has a coil heat exchanger ("coil") with sufficient surface area. At heating water temperatures of 65–75 °C (which is the typical upper limit of a low-temperature boiler), the cylinder heat exchanger surface area is critical. A cylinder with a small coil will heat up slowly even with a powerful boiler. Therefore, always check the "cylinder charging output at 80/60 °C" parameter in the cylinder's technical data sheet.

A detailed guide to calculating the required cylinder volume can be found in the article "What hot water cylinder volume do I need for a low-temperature boiler" in our Knowledge Center.

Cylinder priority – how it works in practice

Cylinder priority control means that the boiler electronics automatically prioritizes cylinder heating over space heating. When the cylinder thermostat signals a temperature drop below a set value (e.g. 55 °C), the boiler interrupts heating and switches the three-way valve to the cylinder. Once the target cylinder temperature is reached, heating resumes.

In practice, you may notice this as a brief delay in the rise of room temperature in winter mornings after the whole family has showered. This is usually 20–40 minutes – acceptable for a correctly sized system. If this takes too long or happens too often, the solution is either a larger cylinder or setting a lower hysteresis on the cylinder thermostat.

Some controllers also allow setting a maximum cylinder priority time – e.g. "the cylinder may have priority for a maximum of 60 minutes, after which the boiler must heat for at least 10 minutes". This function prevents the house from cooling down too much in extreme situations due to cylinder heating. More on setting up this function can be found in the article "Setting cylinder heating priority on a low-temperature boiler".

What to watch out for when buying – practical field experience

Over years of contracting practice, the same mistakes keep occurring. Here are the most common ones:

Mistake #1: Cylinder purchased without checking compatibility

Many customers buy the boiler and cylinder separately, from different manufacturers, without verifying whether they are compatible. A problem arises when connecting the control electronics: a Vaillant boiler communicates with the cylinder thermostat through a specific electrical interface. If you buy a cylinder with a thermostat from another manufacturer, it may work, but you won't have advanced functions (e.g. Legionella protection, consumption statistics, remote control). The simplest solution is to choose a cylinder from the same manufacturer as the boiler – e.g. Vaillant uniSTOR cylinders for atmoTEC and turboTEC models.

Mistake #2: Underestimating water quality

Hard water is common in Slovakia. Calcium and magnesium precipitate during heating and settle on the cylinder's and boiler's heat exchanger. At temperatures above 60 °C, this process is extremely intensive. For low-temperature boilers, the recommended water hardness is usually a maximum of 15 °dH (German degrees). If you have harder water, you need to invest in a water softener or at least an inhibitor dosing unit. Neglected hard water shortens the lifespan of the cylinder's heat exchanger by several years.

Mistake #3: Underestimating dimensions and placement

A 150–200 liter cylinder is a physically large appliance. An upright cylinder 120–150 cm high and 50–60 cm in diameter also requires service space around it. Before ordering, always measure the boiler room, check the ceiling height (including space for insulated piping) and the access doors. The cylinder must be brought into the boiler room before installation – through the door or possibly a window.

Mistake #4: Ignoring the cylinder's service interval

Cylinders with a steel tank must have regular inspection and replacement of the protective anode (magnesium anode) – usually every 2 years. If this anode is not replaced in time, the cylinder will start to corrode rapidly from the inside. This is cheap prevention (the anode costs €20–40), but neglecting it can lead to replacing the entire cylinder for several hundred euros. More about servicing can be found in the article "Maintenance and servicing of a low-temperature boiler with a cylinder – what and when to check".

Mistake #5: Choosing a boiler based on price, not parameters

A cheaper boiler without an integrated three-way valve may end up costing you more later – you will need an external valve, an additional pump, and expansion of the control system. Always compare the total cost of the system (boiler + cylinder + accessories + installation), not just the price of the boiler alone.

Installation and legislation – what you need to know

Installation of a gas boiler may only be performed by a person with the appropriate professional qualification – in accordance with the applicable Slovak regulations (Decree of the Ministry of Transport, Construction and Regional Development of the Slovak Republic No. 508/2009 Z.z. and related STN standards). Amateur installation is not only dangerous but also grounds for voiding the warranty of the boiler and cylinder.

After installation, the installer must issue a pressure test report, commission the boiler, set the CO₂ level in the flue gases, and hand you a warranty card with the installer's section completed. Keep these documents – you will need them for future servicing as well as for any insurance claims.

The boiler should be inspected every year (at least every 2 years) by a service technician authorized by the manufacturer. For Vaillant models, this is provided by a network of authorized service partners. Regular servicing is generally a condition for maintaining the warranty. Details about servicing can be found in the articles "Installing a low-temperature boiler with a cylinder connection step by step" and "Maintenance and servicing of a low-temperature boiler with a cylinder – what and when to check".

Energy efficiency and savings in practice

A low-temperature boiler is not a condensing boiler – it does not have the same high energy efficiency (a condensing boiler achieves an efficiency of 96–109%, a low-temperature one typically 89–93%). Nevertheless, the boiler + cylinder system can bring significant savings compared to an old solution.

Where savings occur:

Elimination of an instantaneous water heater or electric boiler – heat from gas is cheaper than electricity
Possibility of using night tariff for top-up heating (if the cylinder has an electric heater as a backup)
Optimal cylinder sizing reduces heat losses in standby mode
Modern control (weather-compensated control) adjusts the heating water temperature to the outdoor temperature – the boiler does not unnecessarily heat to maximum temperature during transitional periods

In practice, we see savings of 15–25% in gas consumption when switching from an old standard boiler (85–90 °C, no modulation) to a low-temperature boiler with a cylinder and modern control. Specific figures depend on the state of the house's insulation, system settings, and household behavior. More on this topic can be found in the article "Energy savings when combining a low-temperature boiler and a hot water cylinder".

Overview of available models and their use

In the Slovak market, the Vaillant range dominates the category of low-temperature boilers with the option to connect a cylinder, combining quality workmanship, accessible service, and a long tradition. Here is a brief overview of the models available at atria.sk:

Vaillant turboTEC plus VU 122/3-5 – turbo boiler, 12 kW, ideal for smaller houses and apartments up to 100 m² (low-energy construction) or as a supplement to an alternative source. Closed combustion chamber, can be installed without a chimney.
Vaillant turboTEC plus VU 202/3-5 – turbo boiler, 20 kW, universal solution for family houses of 120–180 m² with standard thermal insulation. Sufficient output to heat a 120–150 liter cylinder with a comfortable heating time.
Vaillant atmoTEC plus VU 200/3-5 – atmospheric boiler, 20 kW, for houses with an existing chimney. A more affordable variant for the same category of houses as the turboTEC 202.
Vaillant atmoTEC plus VU 240/3-5 – atmospheric boiler, 24 kW, suitable for houses of 160–220 m² or for houses with a larger heat loss and a 150–200 liter cylinder.
Vaillant atmoTEC plus VU 280/3-5 – atmospheric boiler, 28 kW, the most powerful in the range, for larger houses over 200 m² or holiday properties with a larger cylinder (200–300 l).

All models have an integrated three-way valve for the cylinder, a circulation pump, and control electronics with cylinder priority. They differ in the type of flue gas discharge (atmospheric / turbo) and output. If you are unsure about your choice, we also recommend reading the article "Connecting a Vaillant atmoTEC and turboTEC boiler with a hot water cylinder".

Frequently Asked Questions (FAQ)

Can I connect a cylinder to any low-temperature boiler?

No. A cylinder can only be connected to those boilers that have a built-in three-way valve and control electronics with a cylinder priority function. Boilers without this preparation are intended exclusively for heating. That's why the "option to connect a cylinder" category is an important filter – all boilers in this category at atria.sk include this connection directly from the factory.

Do I have to buy the cylinder from the same manufacturer as the boiler?

It's not a requirement, but it is recommended. A cylinder from the same manufacturer (Vaillant uniSTOR for atmoTEC/turboTEC) guarantees full control compatibility, maintains the warranty, and allows the use of advanced functions (e.g. Legionella protection, remote control via a mobile app). A cylinder from another manufacturer may work, but compatibility needs to be verified at the level of the cylinder thermostat and connection.

What is the lifespan of a low-temperature boiler with a cylinder?

With regular servicing, Vaillant atmoTEC and turboTEC boilers have a real lifespan of 15–20 years. A cylinder with a steel tank (enameled) will also last 15–20 years with regular anode replacement and water quality up to 15 °dH. A stainless steel cylinder can last considerably longer – 25 years or more, but is more expensive. Regular servicing every 1–2 years is the most reliable approach.

What to do when the cylinder is not enough – the water is lukewarm or cold?

There can be several causes: worn anode, limescale buildup on the cylinder heat exchanger, incorrect cylinder temperature setting on the boiler (too low), cylinder volume too small, faulty cylinder thermostat, or a stuck three-way valve. The solution depends on the cause – start with a service inspection. More details can be found in the article "Common faults of low-temperature boilers with a connected cylinder and their solutions".

Is it more advantageous to have an instantaneous water heater or a cylinder with a low-temperature boiler?

For a family house with multiple hot water draw points (bathroom + kitchen + possibly another draw), a cylinder is significantly more advantageous. An instantaneous gas water heater must have an output of 18–28 kW just for water heating, which places the same demands on the boiler as a cylinder – but without the comfort of a reserve. An instantaneous electric water heater is the most energy-expensive. A 120–150 liter cylinder will cover the needs of a 4-member family with a comfortable reserve, and the boiler works in a more favorable, longer cycle when heating the cylinder.

Can I connect solar collectors to a low-temperature boiler for cylinder heating?

Yes, this is a very advantageous combination. During spring–summer–autumn, solar collectors cover most of the hot water needs and the boiler practically doesn't switch on for the cylinder. In winter, the boiler tops up the cylinder heating. However, this combination requires a cylinder with two heat exchangers (one for solar, one for the boiler) or a bivalent cylinder. The boiler must be set up for correct cooperation with the solar system – this requires somewhat more complex control, but the result is worth it.

Conclusion: How to proceed with the selection?

Choosing a low-temperature boiler with the option to connect a cylinder can be summarized in a few clear steps. First, have the building's heat loss calculated – this is the basis, without which even an experienced salesperson cannot recommend the correct output. Then decide whether you have a functional chimney (if yes, consider the atmospheric variant; if not, a turbo boiler is your choice). Next, determine the number of people and their hot water consumption – this will determine the cylinder volume. Finally, check the local conditions (space, gas access, electrical connection, water hardness) and agree with an experienced installer on the specific solution.

The decision should not be rushed – a correctly sized and installed system will serve you without problems for 15–20 years. Conversely, a poorly chosen boiler or an incorrectly designed cylinder circuit can cause problems from the first winter. If you have specific questions about any of the models or your specific situation, our team is available to you – and if you're looking for more information, we also recommend reading "Frequently asked questions about low-temperature boilers with the option to connect a cylinder" in our Knowledge Center.

Do you have a question on this topic?

Can't decide or are you dealing with a specific situation in your household? Write to us - we'll be happy to advise.