What Flue and Chimney Does a Low-Temperature Boiler Need

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What chimney and flue does a low-temperature boiler need

A low-temperature (classic) gas boiler is not only about performance and price – it is equally important how and where the exhaust gases are vented out of the house. Unlike condensing boilers, where plastic concentric flues are almost exclusively used today, we encounter two different solutions with low-temperature boilers – the atmospheric variant connected to a masonry chimney and the turbo variant with forced draft. The difference between them is not cosmetic: it determines whether you can even install the boiler in a given apartment or house, what type of chimney or flue you need, and what to expect if you consider replacing it with a condensing boiler in the future. In this article, we will explain both systems, what to watch out for with a masonry chimney, and what accessories you will need for the turbo variant.

Atmospheric vs. turbo low-temperature boiler – difference in exhaust gas removal

Low-temperature boilers are divided into two basic types based on the combustion method and exhaust gas removal, which cannot be interchanged or modified from one to the other without major changes to the entire system.

Atmospheric boiler (open combustion chamber) draws the air needed for combustion directly from the room where it is installed. The exhaust gases are then vented through a masonry chimney using natural draft – that is, without the help of a fan, only based on the physical difference in temperature and density between the hot exhaust gases and the outside air. This type of boiler works reliably only if the room has sufficient fresh air supply (typically through a grille in the door or wall) and if the chimney is in good technical condition with sufficient draft. An atmospheric boiler is a traditional solution that you will encounter mainly in older houses where a masonry chimney is already built and functional.

Turbo boiler (closed combustion chamber) works completely differently – it does not draw air for combustion from the room, but instead draws it from outside through a concentric pipe (two tubes in one – the inner one carries exhaust gases out, the outer one draws in fresh air). The exhaust of exhaust gases and the intake of air are ensured by a fan directly in the boiler, so it is referred to as forced draft. The advantage is obvious – a turbo boiler does not need a classic masonry chimney at all, it can be installed even in apartments and houses where a chimney was never built, it is enough to have a penetration through the perimeter wall outside. That is why the turbo variant is a common choice for apartments in panel buildings or for modern new builds without a chimney.

The choice between the atmospheric and turbo variant therefore largely depends not only on preference, but on what the building actually offers – a functional masonry chimney, or the possibility to run a concentric pipe through the facade.

Check points for atmospheric connection to a chimney.

What to watch out for with a masonry chimney

If you are deciding on an atmospheric low-temperature boiler or already have one at home, the masonry chimney is a key component of the entire heating system – it is not just a "hole in the wall", but a component that must meet certain technical requirements in order for the boiler to operate safely and efficiently.

Sufficient draft. Natural chimney draft is created due to the temperature difference between the hot exhaust gases inside the chimney and the cooler outside air. If the chimney is too short, has an unsuitable flue opening diameter, or is exposed to adverse weather conditions (for example, the proximity of a taller building that disrupts air flow), the draft may be insufficient. The result is poor exhaust of exhaust gases, which is not only inefficient, but also a safety risk.

Technical condition of the chimney. An older masonry chimney should be free of cracks in the body and chimney lining, without loose or damaged plaster, and not clogged with soot. A clogged or damaged chimney worsens the draft and increases the risk that the exhaust gases will not be vented as they should. For older chimneys without a suitable lining, it is recommended to check whether the chimney lining is suitable for the type of appliance and fuel used.

Flue material. An important difference compared to condensing boilers – with a low-temperature (classic) boiler, the temperature of the exhaust gases is significantly higher, typically in the range of approximately 110 to 150 °C, while with a condensing boiler it is only around 20 to 50 °C. That is why a thin plastic flue, as you know from condensing boilers, is never used with a classic boiler – it would simply not withstand such exhaust gas temperatures. The exhaust of exhaust gases must be solved by a masonry chimney with a suitable (typically ceramic or stainless steel) lining, or in the case of a turbo variant, a metal or stainless steel concentric system that is dimensioned for high exhaust gas temperatures.

Regular chimney inspection by a chimney sweep. Regardless of how old or new your chimney is, regular inspection by a professional – a chimney sweep – is a basic requirement for safe operation. A chimney sweep assesses the condition of the lining, the flue opening and the draft itself, and will point out any deficiencies before they manifest as faults or safety risks. The frequency and scope of inspections are determined by current regulations and the manufacturer's recommendations, so it is worth following the schedule recommended by the chimney sweep during the last inspection.

Accessories for the turbo variant

With a turbo low-temperature boiler (closed combustion chamber, forced draft), you will need a concentric chimney set instead of a masonry chimney, which ensures simultaneous air intake and exhaust of exhaust gases through one penetration in the wall or roof. Important – this accessory belongs exclusively to the turbo variant of the boiler with forced draft, not to the atmospheric boiler connected to a masonry chimney. An example of a commonly used chimney set for turbo boilers:

Protherm chimney set Ø 60/100 mm S3K – concentric chimney set with a diameter of 60/100 mm, intended for turbo boilers with forced draft (closed combustion chamber). It ensures simultaneous intake of combustion air from outside and exhaust of exhaust gases through the boiler fan, without the need for a masonry chimney. It is not suitable or intended for an atmospheric boiler with natural draft. Price from 67.00 €.

When selecting the specific length and diameter of a concentric flue system, always follow the type and performance of your specific boiler and the manufacturer's recommendations – different models of turbo boilers may require different pipe diameters (e.g., 60/100 mm or 80/125 mm) and maximum route length. In case of any uncertainties, we recommend consulting with an installation company or our customer service before purchasing.

What changes when switching to a condensing boiler

If you currently have an atmospheric low-temperature boiler connected to a masonry chimney and are considering replacing it with a condensing boiler in the future, there is one important change to consider – a masonry chimney cannot be directly connected to a condensing boiler. The reason is simple: a condensing boiler operates with significantly lower flue gas temperatures (approx. 20 – 50 °C), at which water vapor in the flue gases condenses. The resulting condensate is slightly acidic and an old masonry chimney – especially if it does not have a suitable corrosion-resistant lining – could be gradually damaged by it.

A commonly used solution in practice is to use the existing masonry chimney as a shaft – that is, only as a space through which a new plastic flue pipe suitable for condensate and lower flue gas temperatures is run. In this case, the masonry chimney loses its function as a flue gas exhaust and becomes only a protective casing around the new pipe. This option is usually feasible in most cases and does not require the chimney to be demolished – however, the passability and dimensions of the existing shaft must always be verified before purchasing a specific boiler and flue kit. If the masonry chimney for any reason cannot be used in this way (e.g., due to insufficient internal diameter), an alternative is to route the exhaust through the façade, similar to what is common with turbo versions of low-temperature boilers.

Summary: when switching from a low-temperature to a condensing boiler, the material and type of flue pipe (plastic instead of masonry or metal) will almost always change, even though the masonry chimney as a structural element can remain in place in many cases and serve as a shaft.

Basic difference between both variants.

Two changes compared to the original low-temperature solution.

Situations where the turbo variant is worthwhile.

Frequently asked questions

Does a gas boiler always need a chimney?
No. An atmospheric low-temperature boiler requires a chimney, as it relies on natural draft to exhaust flue gases. A turbo low-temperature boiler and most condensing boilers do not require a masonry chimney – they use forced draft to exhaust flue gases through a concentric pipe that is directly routed through the façade.

Can an old chimney be used for a condensing boiler?
It is not recommended to connect a condensing boiler directly to a masonry chimney – the condensate from the lower flue gas temperature could damage the chimney. However, in many cases, the chimney can be used as a protective shaft through which a new plastic flue pipe suitable for a condensing boiler is run.

How often should a chimney be inspected by a chimney sweep?
We recommend regular chimney inspections by a chimney sweep according to current regulations and the last inspection's recommendations – the chimney sweep will assess the draft, the condition of the lining and the air intake, and point out any deficiencies.

What is the difference between an atmospheric and a turbo boiler in terms of installation?
An atmospheric boiler requires a room with sufficient air supply and a functional masonry chimney with good draft. A turbo boiler only needs a passage through the external wall for the concentric pipe and does not require a masonry chimney or a separate air supply to the room, as it draws air directly from outside.

Can I keep the masonry chimney if I don't use it for a turbo boiler?
Yes, the masonry chimney can remain on the building without changes if it is not used by the turbo boiler – it may serve as a reserve or for another appliance. The turbo boiler itself does not need it for operation.

Is a concentric chimney set the same for all turbo boilers?
No, the diameter and maximum length of the concentric pipe vary depending on the specific model and boiler output. Always follow the manufacturer's recommendations for the boiler in question, or consult with an installation company when selecting it.

Do you have a question about choosing a boiler?

Having trouble deciding or dealing with a specific situation in your household? Write to us – we are happy to help.