Atmospheric vs. Turbo Low-Temperature Boiler – Which to Choose with a Storage Tank

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Atmospheric vs. turbo low-temperature boiler – which one to choose with a hot water storage tank

When a customer comes into the store and says "I want a low-temperature boiler with a storage tank," the first question to ask isn't "how many litres should the tank be" – it's "which type of boiler even comes into consideration in the first place." Atmospheric or turbo? Both types exist in a low-temperature version, both are commonly combined with a hot water storage tank, but their physical, installation and operational characteristics are so different that a decision made "by eye" can generate unnecessary costs and problems for years to come.

This article discusses both types in detail, compares them in the context of a storage tank combination, and helps you make an informed decision – whether you are a homeowner, a designer, or an installer.

Basic principle: what makes a boiler "atmospheric" and what makes it "turbo"

The term refers exclusively to the way combustion air is supplied and flue gases are discharged – not to power parameters or the temperature regime. This is exactly where many people make a mistake, thinking that "turbo" generally means a more powerful or more modern boiler.

Atmospheric boiler – open combustion chamber

An atmospheric boiler (also referred to as type B according to the European classification) burns gas in an open burner, where the combustion air comes directly from the room in which the boiler is installed. Flue gases leave through the chimney by natural draught – i.e. thanks to the physical effect of hot gas rising. The boiler doesn't need any fan to move the flue gases and it "draws" air directly from the boiler room.

The consequences of this solution are fundamental: the boiler room must have a permanently ensured supply of fresh air (usually a grille with a minimum free cross-sectional area according to the boiler's output), and the chimney must be functional with sufficient draught. The chimney must not be made of plastic – it must withstand flue gas temperatures and possible condensation. With a low-temperature atmospheric boiler, condensation in the chimney is a real possibility, because flue gases leave the boiler at a lower temperature than with a conventional boiler.

Turbo boiler – closed combustion chamber

A turbo boiler (type C) has a closed combustion chamber that is hermetically separated from the installation space. Air comes from the outdoor environment through a coaxial (double-pipe) or twin-pipe duct, and flue gases are actively discharged outside by means of a fan (hence "turbo"). The boiler manages its own flow of combustion air and flue gas discharge, independently of the chimney draught and the air quality in the room.

This solution brings higher safety (no risk of flue gas backdraught into the room), the possibility of installation in practically any environment including tightly sealed new buildings, and greater flexibility in placement – the outlet can go through a side wall or through the roof. The price for this is a higher purchase price and the need for electricity to power the fan (several dozen watts continuously during operation).

Diagram: Comparison of the air supply and flue gas discharge principle – atmospheric boiler (left) vs. turbo boiler (right). The atmospheric boiler uses the chimney's natural draught and air from the room; the turbo boiler has a closed chamber with a coaxial connection and a fan.

What "low-temperature" specifically means in combination with a storage tank

A low-temperature boiler operates with a heating water flow temperature typically in the range of 40–70 °C, and is structurally optimised for lower temperatures – the return temperature is around 30–50 °C. At such temperatures, water vapour from the flue gases starts to condense in the heat exchanger (condensation), which is intentional in a condensing boiler, but in a low-temperature boiler it is a phenomenon that must be addressed by suitable design (resistant materials, condensate drainage).

When combined with a hot water storage tank, a specific operating regime occurs: the boiler must be able to heat the tank to a temperature of 55–60 °C (the minimum temperature for hygienic safety – legionella elimination), which is a higher temperature than normal heating operation. A low-temperature boiler can handle this, but during tank heating it operates at higher output and higher temperature, which is important to take into account when designing the whole system.

You can find more about choosing the right boiler for your situation in the article How to choose a low-temperature boiler with the option of connecting a storage tank, which also discusses the heat losses of the tank and the power requirements.

Specific models: atmoTEC plus vs. turboTEC plus from Vaillant

On the Slovak market, the most widespread series in the category of low-temperature boilers with the option of connecting a storage tank are the Vaillant atmoTEC plus and Vaillant turboTEC plus series. These boilers are available in proven, well-serviceable versions and have an extensive service network. Let's take a look at specific models from both ranges.

The atmoTEC plus series – atmospheric low-temperature boilers

The Vaillant atmoTEC plus VU CZ/SK 200/3-5 is a boiler with a nominal output of 20 kW, the Vaillant atmoTEC plus VU CZ/SK 240/3-5 has an output of 24 kW, and the Vaillant atmoTEC plus VU CZ/SK 280/3-5 has an output of 28 kW. All three are heating-only boilers (not combi units – they don't have a built-in instantaneous water heater), designed precisely for cooperation with an external storage water heater.

The atmoTEC plus boilers have a steel heat exchanger, an atmospheric burner, natural flue gas discharge, and are structurally simple with a minimum of moving parts. The advantage is long service life, lower failure rate, and simpler servicing. The disadvantage is the need for a functional chimney and a sufficient air supply to the boiler room – conditions that may not be easily met in many older as well as new buildings.

In terms of output – 20, 24 and 28 kW – the atmoTEC plus series covers family houses with a floor area of roughly 100–250 m², depending on the thermal insulation standard. For tank heating, the decisive factor is the output at a flow temperature of 75 °C, at which the boiler delivers its nominal output; at a lower heating temperature the output changes slightly.

The turboTEC plus series – turbo low-temperature boilers

On the turbo boiler side, the Vaillant turboTEC plus VU CZ/SK 122/3-5 with an output of 12 kW and the Vaillant turboTEC plus VU CZ/SK 202/3-5 with an output of 20 kW are available. These are also heating-only boilers without a built-in tank – hot water is heated in an external storage tank.

The fan in the turboTEC plus actively controls the flow of flue gases and air, which allows output modulation in the range of roughly 30–100% of nominal output. This feature is especially valuable when combined with a storage tank: during the tank heating cycle the boiler rises to full output, while for regular heating it operates at partial output as needed. The coaxial outlet (diameter DN 60/100 or DN 80/125 depending on the output and length of the run) is installed through a side wall, without the need for a chimney.

Chart of power variants – the atmoTEC plus series offers 20, 24 and 28 kW; the turboTEC plus series 12 and 20 kW. Correct boiler sizing is essential, especially when combined with a storage tank.

Installation conditions: where you can use which boiler

This is, in practice, the most common deciding factor. It's not about which boiler is "better" in an absolute sense – it's about which boiler you can install at all in a given building and a given space.

Conditions for the atmoTEC plus atmospheric boiler

Functional chimney – brick, ceramic-lined or stainless steel, with sufficient draught (at least 4–6 Pa of static underpressure). The chimney must be resistant to condensate – moisture is a real occurrence in low-temperature operation.
Permanent air supply to the boiler room – a grille or opening with a minimum area according to the relevant standards. For a 20 kW boiler this is typically a free cross-section of at least 150 cm², for a 28 kW boiler at least 200 cm² and more. No airtight new building without a ventilation solution automatically meets these conditions.
The boiler room is not a living space – an atmospheric boiler must not be installed in bathrooms, bedrooms or other permanently inhabited rooms without a special solution.
The chimney draught must not be disrupted – heat recovery units, central vacuum systems, kitchen extractor hoods and other equipment creating underpressure in the building can cause flue gas backdraught. In passive and low-energy houses with controlled ventilation, this is a real risk.

Conditions for the turboTEC plus turbo boiler

Possibility of routing the coaxial pipe outside – through the perimeter wall or roof. The maximum equivalent duct length depends on the model (usually 4–10 m horizontally or the equivalent for a vertical outlet).
No chimney needed – this is a key advantage in renovations where the chimney is missing, non-functional, or its restoration would be too costly.
The electrical connection must be reliable – during a power outage the boiler does not function (fan). In locations with an unstable grid, a backup connection or UPS is advisable.
Possibility of installation in living spaces – with type C (closed chamber), installation is also permitted in bathrooms (with certain restrictions depending on zones), hallways and other rooms, which gives designers greater flexibility.

Diagram: Typical connection of a low-temperature boiler (atmoTEC or turboTEC) with a hot water storage tank. The three-way valve (3V) switches between tank heating and space heating. The circulation pump (P) ensures circulation in the primary circuit. EV = expansion vessel.

Boiler and storage tank cooperation: what matters in practice

Most of the problems we see in service practice with low-temperature boilers and storage tanks do not arise from a fault of the boiler itself – but from a poorly designed or poorly set up connection system. That's why this aspect is extremely important.

Storage tank heating priority

If the boiler serves both heating and the storage tank at the same time, a priority must be set. By default, priority is given to the tank (when the temperature in the tank drops below the set threshold, the boiler switches its entire output to heating the tank and heating is temporarily interrupted). On Vaillant atmoTEC plus and turboTEC plus boilers, this is set via a controller (VRC, VR, calorMATIC) and hydraulic connection through a motorised three-way valve.

This topic is discussed in detail in the article Setting the storage tank heating priority on a low-temperature boiler, where you can also find specific controller parameters for the Vaillant series.

Hydraulic connection – pump, three-way valve, mixing valve

In systems with a lower heating temperature (underfloor heating, low-temperature radiators) and a storage tank heated to 55–60 °C, the hydraulics must be designed so that the temperature for the tank is sufficient, while the heating branch does not take on excessive temperatures. Common solution:

The boiler operates at 65–70 °C during tank heating.
The heating branch is mixed down to the required temperature (e.g. 40–45 °C for underfloor heating) via a motorised mixing valve.
After the tank is filled, the boiler switches back to the heating temperature (e.g. 50–55 °C).

With atmoTEC plus atmospheric boilers, it is important to ensure that the return temperature does not drop too far below 40 °C – otherwise intensive condensation occurs in the steel heat exchanger, which can damage the boiler over a longer period. The turboTEC plus's metal heat exchanger is also sensitive on this point – a low-temperature, non-condensing boiler is not structurally designed for long-term condensation. If your system is primarily underfloor heating with return temperatures regularly below 40 °C, you should consider a condensing boiler rather than a low-temperature one.

Tank volume and boiler output – sizing

A too-small tank with a too-powerful boiler = short heating cycles, frequent switching, burner wear. A too-large tank with a weak boiler = insufficient heating, long waiting times, dissatisfied occupants. Recommended combinations for a family house:

atmoTEC plus 200/3-5 (20 kW) + 120–150 l tank: suitable for 3–4 people
atmoTEC plus 240/3-5 (24 kW) + 150–200 l tank: suitable for 4–5 people
atmoTEC plus 280/3-5 (28 kW) + 200–250 l tank: suitable for 5–6 people or higher hot water demand
turboTEC plus 122/3-5 (12 kW) + 80–120 l tank: small house or flat for up to 3 people
turboTEC plus 202/3-5 (20 kW) + 120–150 l tank: family house for 3–4 people

You can find more about sizing the tank and boiler output in the article What storage tank volume do I need for a low-temperature boiler.

Comparison aspects: where they differ in everyday operation

Energy efficiency

Both types of boilers are structurally low-temperature and have comparable efficiency at the nominal point – usually 89–93% Hs (lower heating value). The atmospheric boiler has no fan, so it doesn't consume electricity to drive the flue gas system (only the circulation pump draws power). The turbo boiler has continuous fan consumption (15–50 W depending on speed), which over a heating season represents an increase in electricity consumption on the order of 30–100 kWh – not a dramatic difference, but a measurable one.

On the other hand, a modulating turbo boiler can regulate output more precisely (e.g. the turboTEC plus in the range of 40–100%), which, with correct controller settings, leads to less frequent cycles and higher overall operating efficiency.

Noise level

The atmospheric boiler is quieter during combustion – the burner emits a mild combustion sound, but no fan noise. The turbo boiler adds fan noise, which is audible near the boiler. In practice this is usually not a problem if the boiler is in a boiler room or utility room – but when installed in a living space (hallway, entrance hall), the fan noise may bother the customer.

Dependence on electricity

The atmospheric boiler needs electricity only for the control electronics, the pump, and the three-way valve actuator – minimal requirements. In the event of a shorter power outage, you may be able to run the system with a smaller backup power source. The turbo boiler needs electricity additionally for the fan – during a power outage it doesn't work at all, neither the pump nor the burner. In locations with an unstable distribution grid, this is a factor worth attention.

Serviceability and maintenance costs

The atmospheric boiler has a simpler design: fewer moving parts, no fan (which wears out), no coaxial system seals. Annual maintenance (burner cleaning, flue gas inspection, seal inspection) is usually cheaper and faster. The turbo boiler additionally requires inspection of the fan and bearings, cleaning and inspection of the coaxial system, and checking the tightness of the closed combustion chamber. Overall service costs are somewhat higher.

You can find a detailed maintenance procedure for both types of boilers in the article Maintenance and servicing of a low-temperature boiler with a storage tank – what and when to check.

Decision tree: atmospheric or turbo

The decision tree helps narrow down the choice based on the real conditions of the building. In practice, most new buildings and low-energy houses are predestined for a turbo boiler.

Typical scenarios from practice

Scenario 1: Renovation of a family house from the 1980s, functional chimney

A brick house, floor area 140 m², a 4-member family. The existing chimney is brick-built, inspected, with sufficient draught. The boiler room is in the basement with natural ventilation through a small window. In this case, the atmospheric Vaillant atmoTEC plus VU CZ/SK 240/3-5 (24 kW) with a 150-litre tank is the ideal solution. Simple installation, lower purchase price, reliable operation for the coming decade. The boiler is connected to the existing chimney via a single-flue pipe DN 130–150, and the tank is placed next to the boiler.

Scenario 2: New terraced housing, no chimney, solid facade

A three-room flat or terraced house without a chimney structure. An outlet through the side wall is possible – a 60 cm concrete wall with a DN 110 core drill. In this case, the only reasonable choice is the Vaillant turboTEC plus VU CZ/SK 202/3-5 (20 kW) with a 120-litre tank. Coaxial outlet DN 60/100 through the side wall, run length 1.5 m – no problems. An atmospheric boiler is not an option here, even if you wanted it.

Scenario 3: Cottage or small house with a gas connection, 2 people

A small house or cottage, floor area 60 m², 2 people, a space-limited boiler room. An 80–100 l tank will suffice. Here the Vaillant turboTEC plus VU CZ/SK 122/3-5 (12 kW) is an interesting choice – compact dimensions, lower output matching the small area, without chimney dependence. If the cottage is in an area with an unstable electrical grid, the turbo boiler's dependence on electricity needs to be taken into account.

Scenario 4: Existing house, chimney requires lining, the customer is deciding

A brick chimney in poor condition – it needs a stainless steel liner (lining cost €1,000–2,000 depending on height). The customer is considering whether it's cheaper to line the chimney and use an atmospheric boiler, or use a turbo boiler without lining. In practice: if lining is cheaper than the price difference between the boilers and coaxial installation is complicated (e.g. an 80 cm thick wall, routing through multiple rooms), investing in lining and an atmospheric boiler pays off in the long run. If the chimney route is extremely long or complicated, a turbo boiler with a side outlet through the perimeter wall is a simpler and cheaper overall solution.

Safety aspects – what you should know

Flue gas safety is a critical issue with atmospheric boilers. Every year there are carbon monoxide (CO) poisonings due to flue gas backdraught – an atmospheric boiler is more prone to this phenomenon if there is equipment in the building creating underpressure (extractor hoods, heat recovery ventilation, central vacuum systems). A turbo boiler with a closed combustion chamber eliminates this problem.

For safe operation of an atmospheric boiler, we recommend:

Installing a CO detector in the boiler room and in adjacent living spaces.
Regularly checking the chimney draught (once a year during a service inspection).
Never blocking or covering the boiler room's ventilation grilles.
Assessing the impact of other air-consuming appliances in the house (kitchen extractor hood, fireplace, range hood).

A turbo boiler does not eliminate safety risks entirely – a leak in the coaxial system or damage to the tightness of the combustion chamber can lead to flue gas escaping into the boiler room. That's why we also recommend installing a CO detector and regularly checking the tightness of the coaxial ducting with turbo boilers.

Integration with the storage tank and controls – the practical side

Both types of boilers – atmoTEC and turboTEC – are designed to work with digital Vaillant controllers (VRC 700, VRC 470, calorMATIC 470). These controllers can manage tank priority, weather-compensated control, time programmes and boiler status monitoring. The connection runs via the eBus communication protocol, which allows convenient integration without external modules.

It is important that the tank has a built-in temperature sensor (NTC resistance sensor), which signals to the boiler when the tank is sufficiently heated and when a heating cycle needs to be started. Standard storage water heaters for Vaillant boilers have this sensor built in directly – when buying a tank from a different manufacturer, check the compatibility of the connector and the resistance characteristic of the sensor.

You can find more about the technical connection in the article Connecting a Vaillant atmoTEC and turboTEC boiler with a hot water storage tank, which also describes the correct setting of boiler parameters for tank heating.

Purchase and operating costs – real figures

The atmoTEC plus atmospheric boiler is generally cheaper to purchase than the turboTEC plus at a comparable output. The difference is on the order of hundreds of euros. Installation costs and any chimney work must be added to this. Overall cost comparison for a typical family house:

atmoTEC plus 240/3-5 + 150 l tank + storage water heater, installation, chimney inspection: typically €2,500–3,800 in total (excluding lining costs, if needed)
turboTEC plus 202/3-5 + 150 l tank + coaxial outlet, installation: typically €2,800–4,200 in total (no chimney work needed – none is required)

Operating costs are similar for both types – the decisive factors are the gas price, the operating temperature profile, and the quality of the control system. The difference in electricity consumption for the turbo boiler's fan is negligible in annual costs (on the order of €5–15/year).

Frequently Asked Questions (FAQ)

Can I connect a storage tank to an atmoTEC plus atmospheric boiler without a Vaillant controller?

Yes, it is technically possible with an external tank thermostat and a mechanical three-way valve, but you will lose advanced functions such as weather-compensated control, convenient time programmes and diagnostics via the controller. For simpler installations, an external tank thermostat that switches the three-way valve actuator directly is sufficient. In practice, however, we recommend at least a basic Vaillant controller for better energy savings.

Can the turboTEC plus operate during a power outage?

No. The turbo boiler is fully dependent on electricity – the fan, control electronics, circulation pump – everything requires a 230 V supply. In the event of a power outage, the boiler stops operating immediately and provides neither heating nor tank heating. If you live in an area with frequent outages (storm-prone areas, rural areas with less reliable distribution), we recommend a backup UPS power source with a capacity of at least 300–500 Wh for several hours of operation.

What temperature must the tank reach and does this affect the choice of boiler?

For hygienic reasons, the tank must regularly (at least once a week) reach a temperature of 60 °C to protect against legionella. A low-temperature boiler – whether atmospheric or turbo – can manage this, because the nominal flow temperature is 65–75 °C. A problem arises if the boiler is permanently set to a low temperature (e.g. 50 °C for underfloor heating without mixing) and the tank never reaches 60 °C – there is then a health risk. The solution is a thermal disinfection function in the controller, which raises the boiler temperature to maximum once a week and runs a full tank heating cycle up to 60 °C.

Is the atmoTEC plus suitable for a passive or low-energy house?

In most cases, no – and this is not about output, but about safety. Passive houses and low-energy new buildings have extremely airtight envelopes with controlled ventilation (heat recovery). In such an environment it is not possible to ensure a sufficient and permanent air supply for an atmospheric boiler without compromising the building envelope. In addition, heat recovery units can create underpressure indoors and cause backdraught of flue gases from the atmospheric boiler. For passive and low-energy houses, the only correct choice is the turboTEC plus or a condensing boiler with a closed chamber (type C).

Can I replace an old atmospheric boiler with a turbo boiler if I have a functional chimney?

Yes, you can. Replacing an atmospheric boiler with a turbo boiler is a common renovation – the boiler is disconnected from the chimney (the chimney is sealed off or kept as a backup) and a new outlet is created through the side wall using a coaxial duct. This is even a popular solution in old boiler rooms where the chimney requires costly lining – instead of investing in the chimney, the customer invests in a turbo boiler with a side outlet.

What if I later want to add solar panels or a heat pump – does this affect the choice of boiler?

Yes, if you are planning a future hybrid system (boiler + solar collectors for hot water heating, or boiler + heat pump), it is advantageous to take this into account already when choosing the tank (a bivalent tank with two coils) and the controls (compatibility with multi-energy management). The choice between atmoTEC and turboTEC is not a decisive factor in terms of hybrid integration – what matters more is the compatibility of the tank and controller with future energy sources. More in the article Saving energy when combining a low-temperature boiler and a hot water storage tank.

Conclusion: atmospheric or turbo – it depends on conditions, not on bias

After reading this article, it should be clear that the question "atmospheric or turbo" has no universal answer. Both types of low-temperature boilers – atmoTEC plus and turboTEC plus – are quality, proven and long-term reliable products, if correctly installed and operated.

The atmoTEC plus atmospheric boiler is ideal where a functional chimney exists, air supply to the boiler room is possible, and the customer is looking for reliability with lower technological complex

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 you.