Atmospheric vs. turbo boiler with DHW preparation: which type is right for you

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Atmospheric vs. turbo boiler with DHW preparation: which type is right for you?

When a customer is deciding between an atmospheric boiler and a turbo boiler with instantaneous domestic hot water (DHW) heating, the most common mistake we see in practice is putting too much emphasis on the purchase price and too little on the installation conditions, the layout of the housing unit, and long-term operating costs. This article will help you understand the operating principle of both technologies in depth – not at the level of a marketing leaflet, but at the level of a technician who installs and services these boilers and ultimately advises customers on what to buy.

We are talking about the category of low-temperature combi boilers with instantaneous DHW heating – devices that, in a single unit, heat water both for the heating circuit and, on demand, for domestic hot water. There is no storage tank in this case – the water is heated continuously every time a tap is opened. This category of boilers is currently dominant on the Slovak market for family houses and flats in apartment buildings, and this is where most decisions about choosing between an atmospheric and a turbo boiler are made.

Basic principle: how an atmospheric boiler works and how a turbo boiler works

Before we can compare them, we first need to clearly define what we are talking about. Both technologies are low-temperature gas boilers – they operate with a circuit water temperature typically of 40–75 °C and are suitable for both radiator systems and underfloor heating. The fundamental difference lies in the way combustion air is supplied and flue gases are removed.

Atmospheric boiler – open combustion chamber

The atmospheric boiler (labelled atmoTEC in the Vaillant catalogue) has an open combustion chamber. It draws combustion air directly from the room in which the boiler is installed. Flue gases are removed by natural draught through a vertical chimney flue. The boiler is dependent on the chimney – it needs a functional, certified, regularly cleaned chimney flue with sufficient draught.

The air for combustion comes from the air in the room. This means the room must have sufficient air flow – it must not be hermetically sealed, must not be under negative pressure (for example from a kitchen extractor hood), and there must be no backdraught situation. This dependence on room air and chimney draught is key when choosing the installation location.

Turbo boiler – closed combustion chamber

The turbo boiler (Vaillant turboTEC) has a closed combustion chamber. It draws combustion air from outside – from the exterior – through a separate plastic pipe. Flue gases are also discharged outside, through a second pipe (or a double-walled coaxial pipe). The boiler is equipped with a fan that actively controls the entire process of drawing in air and expelling flue gases. That is why a turbo boiler is also called a boiler with forced flue gas discharge, or a boiler with a closed combustion chamber.

The turbo boiler does not depend on a chimney flue. Flue gas discharge is handled horizontally or in a combined way through the perimeter wall, or possibly through a shaft with special plastic liners. The boiler can be placed in a room with no relation to a chimney at all, even in a hermetically sealed bathroom or in a cabinet.

The diagram above shows the fundamental difference in the way air is supplied and flue gases are discharged. The atmospheric boiler depends on chimney draught and room air; the turbo boiler secures its own air supply and flue gas discharge independently through the perimeter wall or a shaft.

DHW preparation in a combi boiler: instantaneous heating

Both variants – atmoTEC and turboTEC – can be equipped with instantaneous DHW preparation. This means that when the hot water tap is opened, the boiler automatically switches priority to DHW heating: primary water from the heating circuit passes through a plate heat exchanger and heats the water from the mains supply. As soon as you close the tap, the boiler switches back to heating. The whole process takes a fraction of a second.

The output allocated for DHW preparation in combi boilers is usually higher than the output for heating – precisely so that the flow of hot water is sufficient. For example, the Vaillant atmoTEC pro VUW SK 200/3-3 has a maximum DHW output of 21 kW, which at a standard inlet of 10 °C and outlet of 40 °C means a flow of around 8–9 litres per minute. For comparison, the Vaillant turboTEC pro VUW SK 202/3-3 offers the same output class, but with a closed combustion chamber.

A more detailed comparison of outputs and hot water flow rates is covered in a separate article, What hot water flow rate does a boiler with instantaneous DHW heating provide, in this Knowledge Centre – we recommend reading it before making your final choice.

Where an atmospheric boiler is suitable: real-world scenarios

An atmospheric boiler is not an outdated technology. It is a well-proven, reliable solution that has clear advantages in many situations. Let's look at when atmoTEC is the right choice.

Family house with a well-preserved brick or fireclay chimney

This is the most common case we encounter. A house from 1970–1995, originally heated with solid fuel or a boiler room with natural gas and a first-generation atmospheric boiler. The chimney is well preserved, regularly cleaned by a chimney sweep, and has good draught. In this case, an atmospheric boiler is the natural choice – you make use of existing infrastructure, no additional construction work is needed, and installation is straightforward.

For example: a customer from Záhorie, a 120 m² family house, boiler room in the basement, a brick chimney flue 200×200 mm, chimney height 8 m. They replaced an old Junkers boiler with a Vaillant atmoTEC pro VUW SK 240/3-3 – 24 kW output, instantaneous DHW heating. The whole replacement took one day, there was no need to change the chimney, and the boiler works without any problems.

Situations where an atmospheric boiler does NOT work well

It is important to be honest with the customer. An atmospheric boiler is not suitable in these cases:

Modern low-energy houses with heat recovery ventilation – the building is hermetically sealed, there is no combustion air available, and chimney draught is unstable.
A bathroom/utility core in a prefab flat without a chimney – here the only option is a turbo boiler through the outer wall.
Rooms with a strong extractor hood or negative-pressure ventilation – an extractor hood creates negative pressure which "reverses" the chimney draught, causing flue gases to enter the room. Risk of CO poisoning.
Locations with poor chimney draught – boiler on the ground floor, short chimney, surrounding trees or neighbouring buildings creating air turbulence.
A cabinet or enclosed space without ventilation – an atmospheric boiler must be in a room with a sufficient volume of air.

Where a turbo boiler is suitable: real-world scenarios

Today, the turbo boiler is the dominant choice for new builds, renovated flats in prefab buildings, and anywhere a chimney is not available or where dependence on its condition is undesirable. Let's look at specific situations.

Prefab flat – no chimney, perimeter wall available

This is probably the most common case in Slovak cities. A three-room flat in a prefab building, heated by district heating or electric direct heaters, and the customer has decided to switch to a gas boiler. There is no chimney in the flat – the boiler must discharge flue gases through the perimeter wall. The only technically correct choice is a turbo boiler. A coaxial pipe (80/125 mm or 60/100 mm) is run through the perimeter wall via a hole about 130 mm in diameter, with a cover and grille fitted on the outside. The entire flue gas discharge installation is done within an hour.

New-build low-energy family house

A low-energy or passive house has a facade with thick thermal insulation, triple-glazed, tightly sealed windows, and a heat recovery ventilation unit. Such a building is essentially a "thermos flask" – no air for the combustion process comes from outside naturally. The turbo boiler is the only correct choice here, because it draws air directly from the exterior and does not depend on internal air pressure.

Heating renovation in a house with an unusable chimney

A house from the 1980s where a chimney exists but is in poor condition – cracked joints, insufficient draught, and the chimney sweep's report recommends against using it for gas. Two options: relining the chimney (cost 800–1,800 € depending on height and diameter) or a turbo boiler with flue gas discharge through the wall. In practice, if the boiler room is on the ground floor or in the basement and the perimeter wall is accessible, the turbo boiler works out cheaper in the overall calculation.

For such cases, we recommend, for example, the Vaillant turboTEC pro VUW SK 242/3-3 – 24 kW, closed combustion chamber, instantaneous DHW preparation, fully modulating burner for maximum gas savings.

Technical parameters and differences in numbers

Let's look at a concrete numerical comparison of the two technologies. The efficiency difference between an atmospheric and a turbo boiler in low-temperature boilers is not as dramatic as many people think.

Combustion efficiency

Both types achieve an efficiency of around 90–92% at nominal output (in low-temperature mode, not condensing boilers). An atmospheric boiler has slightly higher flue gas losses, because flue gases leave the boiler at a temperature of around 130–160 °C (necessary for sufficient chimney draught). A turbo boiler can have a lower flue gas temperature, because the air is controlled by a fan and does not depend on draught.

In practice, we're talking about an efficiency difference of 1–3 percentage points in favour of the turbo boiler. At a consumption of 1,500 m³ of gas per year and a price of €0.08/kWh, this can amount to a difference of around €25–45 per year. Efficiency itself is therefore not a decisive argument – installation conditions are far more important.

Equipment and installation cost

An atmospheric boiler is usually €80–150 cheaper to purchase than an equivalent turbo boiler (same output, same manufacturer). For example, atmoTEC pro 200/3-3 vs. turboTEC pro 202/3-3 – both 20 kW, instantaneous DHW, typically differ in price by €100–120.

Installation costs, on the other hand, can be lower with a turbo boiler if there is no functional chimney available (no need for relining). If a chimney exists and is functional, installing an atmospheric boiler is simpler and cheaper.

Noise level

The turbo boiler contains a fan – this is a source of acoustic noise. Modern Vaillant turboTEC boilers have a noise level of around 38–44 dB(A) at full output. The atmospheric boiler is quieter (30–36 dB(A)), as it has no fan noise. For installation in a living space (kitchen, hallway), the difference can be noticeable, especially at night when using DHW.

Servicing and maintenance requirements

Both types of boilers require an annual service inspection. The turbo boiler additionally has a fan as a moving part subject to wear – the typical lifespan of a fan motor is 10–15 years, with replacement costing €150–250. The atmospheric boiler has no fan motor, so maintenance is somewhat simpler and cheaper. The chimney of an atmospheric boiler requires regular cleaning and inspection by a chimney sweep.

Safety aspects: what you need to be aware of

This topic is often underestimated in practice. An atmospheric boiler with an open combustion chamber is a safe device – if it is properly installed and operated. Problems arise when conditions in the building change. The most common cause of flue gas leakage into a living space with an atmospheric boiler is:

Installation of a new hermetic window or door – disrupts natural air flow in the house.
Adding a powerful extractor hood without compensating air supply – creates permanent negative pressure.
Blockage or damage to the chimney flue – a bird's nest, a crack in the joints.
A renovation that changed the layout of the house – the boiler room ends up in a "dead-end" room with no air flow.

The turbo boiler is immune to these situations – it takes combustion air from outside and discharges flue gases outside, regardless of what happens inside the building.

Binding Slovak technical standards (STN EN 483, STN 06 1008) clearly define the ventilation requirements for a room with an atmospheric boiler – minimum ventilation opening cross-section, minimum room volume. These requirements should be discussed with an installer before choosing an atmospheric boiler.

Vaillant atmoTEC pro vs. turboTEC pro: specific product lines

Vaillant has long been the strongest brand on the Slovak market in this category. Let's compare the specific pro and plus product lines:

atmoTEC pro line

The atmoTEC pro series is available in outputs of 20 kW (VUW SK 200/3-3) and 24 kW (VUW SK 240/3-3). Both models have an open combustion chamber, natural flue gas discharge, and instantaneous DHW preparation. This is a proven design with a modulating burner, digital controller, and built-in diagnostics. Dimensions: width 440 mm, depth 298 mm, height 720 mm – a compact design that fits into virtually any boiler room.

atmoTEC plus line

The Vaillant atmoTEC plus VUW CZ/SK 240/3-5 is a higher-class atmospheric boiler with instantaneous DHW heating. Compared to the pro version, the difference lies in extended control, compatibility with the eRELAX thermostat via eBUS, better burner modulation at the lower end of the output range, and a wider range of accessories for system integration. A detailed comparison can be found in the article Vaillant atmoTEC pro vs. atmoTEC plus: what's the difference and which to choose.

turboTEC pro line

The turboTEC pro series offers the same output classes of 20 kW (VUW SK 202/3-3) and 24 kW (VUW SK 242/3-3), but with a closed combustion chamber, a fan, and coaxial air supply/discharge through the wall or a shaft. In terms of dimensions, turboTEC boilers are slightly deeper (338 mm) due to the combustion chamber, but are otherwise comparable to atmoTEC. Both turboTEC pro series are certified for air supply from the exterior via the LAS (Luft-Abgas-System) system – allowing pipe runs even over longer horizontal or vertical distances.

The diagram above shows a simplified principle of instantaneous DHW heating: when the tap is opened, the diverter valve redirects the flow of primary water through the plate heat exchanger, where it transfers heat to the cold water from the mains. The heating circuit is temporarily shut off during DHW preparation (for a maximum of 20–30 seconds).

How to decide: a practical decision-making process

Based on experience from dozens of installations, we recommend the following process:

Step 1 – Chimney: Do you have a chimney flue that is functional and has a valid chimney sweep's report? → An atmospheric boiler is technically possible. If there is no chimney, or it is not in good condition → turbo boiler.
Step 2 – Building type: Is it a low-energy building or one with heat recovery ventilation? → Turbo boiler. An older family house with natural infiltration? → An atmospheric boiler is fine.
Step 3 – Installation location: Is the boiler room in a room with an extractor hood or strong extraction? → Turbo boiler. Will the boiler be in an enclosed cabinet? → Turbo boiler with LAS system.
Step 4 – Future plans: Are you planning to make the house more airtight in the future (new windows, facade, heat recovery ventilation)? → Consider a turbo boiler now to avoid future problems or replacement.
Step 5 – Budget: If the chimney is functional and none of the above complications apply, an atmospheric boiler will save €80–150 on the purchase price. Over a 15-year horizon, with savings on chimney sweep servicing and no fan replacement, it can save even more.

For the correct boiler output, we also recommend reading the article What boiler output with DHW heating do I need for my house – output is the second key parameter after the type of combustion chamber, and incorrect sizing is just as problematic as an incorrect choice of technology.

Summary of the strengths of each type

Atmospheric boiler – main advantages

Lower purchase price.
Simpler construction, lower probability of fan failure.
Quieter operation.
Ideal with an existing functional chimney.
Long-proven technology with readily available spare parts.

Turbo boiler – main advantages

Independence from the chimney and from room air.
Can be installed anywhere – even in a hermetically sealed space.
Safer in situations with a risk of negative pressure in the room.
Greater flexibility in placement within the building.
Lower flue gas temperature losses, slightly higher efficiency.
Does not require a chimney sweep inspection of the flue (only inspection of the coaxial pipe).

Frequently Asked Questions (FAQ)

Can I replace an old atmospheric boiler with a turbo boiler without any work on the chimney?

Yes, a turbo boiler does not need a chimney. If your boiler room has a perimeter wall with access to the exterior (typically 300–500 mm from the boiler), all that's needed is to drill a 125 mm diameter hole for the coaxial outlet, and the old chimney flue can be left unused or bricked up. The installer will assess the specific coaxial route and pipe length – standard Vaillant coaxial extensions allow a horizontal run of up to 8 metres (with an output reduction of about 0.5 kW for every 1 m of extension).

Is a turbo boiler suitable for an older family house with a chimney?

Yes, you can install a turbo boiler even in a house where a functional chimney exists. You simply won't use the chimney for flue gas discharge, and it will instead be routed through the wall via a coaxial pipe. Some installations use the chimney flue as a shaft for running coaxial plastic pipes (LAS system in the chimney) – this is an elegant solution if the flue is free and sufficiently large (minimum diameter 160 mm for an 80/125 coaxial pipe).

Are atmospheric boilers banned, or is a ban planned?

As of the publication date of this article, atmospheric boilers are not banned on the Slovak market or in the EU. The EU Energy Performance of Buildings Directive (EPBD) talks about a gradual phase-out of the sale of purely fossil-fuel boilers (including gas boilers) with a horizon of 2040–2050, which applies equally to both atmospheric and turbo boilers. Condensing boilers are more energy efficient, but low-temperature boilers (both atmo and turbo) remain a fully legal and installable solution.

Can an atmospheric boiler harm me – CO leakage?

Modern Vaillant atmospheric boilers have a draught safety sensor (a flue gas safety thermostat) that shuts down the boiler in the event of backdraught or insufficient chimney draught. The risk of CO leakage exists, but is minimised by correct installation, annual servicing, and a functional CO detector in the room (recommended for every installation with an atmospheric boiler). Problems arise from unprofessional interventions or from additional building modifications made without consulting a heating engineer.

What output size should I choose – 20 kW or 24 kW?

The basic rule for family houses: 20 kW is sufficient for a house with a heat loss of up to 14 kW and DHW needs for 2–4 people. 24 kW is suitable for houses with a heat loss of 14–18 kW or for households with 4+ people with higher DHW consumption. A detailed calculation procedure can be found in the article What boiler output with DHW heating do I need for my house. We also do not recommend an oversized boiler – it short-cycles unnecessarily and wears out faster.

Is instantaneous DHW preparation sufficient, or is a storage tank better?

Instantaneous DHW preparation is convenient and hygienically advantageous (no storage tanks, no Legionella). The downside is limited simultaneous flow – typically 8–11 litres per minute (depending on boiler output and temperature difference). If your household has two bathrooms in simultaneous use, or a large bathtub you want to fill quickly, a storage water heater may be more advantageous. A comparison of both solutions can be found in the article Instantaneous DHW heating vs. storage water heater: advantages and disadvantages with a low-temperature boiler.

Conclusion: the decision depends on the building, not on preference

Atmospheric vs. turbo boiler is not a question of "which is better" – it's a question of "which is right for your specific situation". If you have a functional chimney, an older family house without heat recovery ventilation, and a well-ventilated boiler room, an atmospheric boiler is just as reliable and economically sound a solution as a turbo boiler. If you don't have a chimney, the house is low-energy, or the installation is planned in an apartment building, a turbo boiler is the only correct choice.

When choosing a specific model, we recommend starting with the flue gas discharge conditions, continuing with a calculation of the required output, and only then deciding between specific models. All Vaillant models from the category of low-temperature boilers with instantaneous DHW heating are equipped as standard with a modulating burner, digital diagnostics, and full support for original accessories – so in terms of quality and lifespan they don't differ from each other, only in the flue gas discharge technology.

If you're not sure, consult your choice with an installer on site – an assessment of the chimney, air supply, and boiler room layout is irreplaceable, and no article can fully substitute for it. A correctly chosen boiler works without problems for 15–20 years. An incorrectly chosen boiler brings problems from the very first season.

Do you have a question on this topic?

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