What Flue and Chimney Does a Low-Temperature Boiler Need
What Flue and Chimney Does a Low-Temperature Boiler Need
A low-temperature (classic) gas boiler isn't just about output and price – it's equally important how and where it discharges flue gases from the house. Unlike condensing boilers, which today almost always use a plastic concentric flue, low-temperature boilers come in two distinct configurations – an atmospheric variant tied to a masonry chimney, and a turbo (fan-assisted) variant with forced draught. The difference between them isn't cosmetic: it determines whether you can even install the boiler in a given flat or house, what type of chimney or flue you'll need, and what awaits you if you consider switching to a condensing boiler in the future. In this article we explain both systems, what to watch out for with a masonry chimney, and what accessories you'll need for the turbo variant.
Atmospheric vs. Turbo Low-Temperature Boiler – The Difference in Flue Gas Discharge
In terms of combustion and flue gas discharge, low-temperature boilers fall into two basic types that cannot be interchanged or converted from one to the other without intervening in the entire system.
An atmospheric boiler (open combustion chamber) draws the air needed for combustion directly from the room in which it is installed. It then discharges the flue gases through a masonry chimney using natural draught – that is, without the help of a fan, relying purely on the physical difference in temperature and density between the hot flue gases and the outside air. This type of boiler works reliably only if the room has an adequate supply of fresh air (typically via a grille in the door or wall) and if the chimney is in good technical condition with sufficient draught. The atmospheric boiler is the traditional solution you'll mainly find in older family houses where a masonry chimney is already built and functional.
A turbo boiler (closed combustion chamber) works completely differently – it does not draw air from the room, but instead sucks it in from outside through a concentric pipe (two pipes, one inside the other – flue gas flows out through the inner pipe, while fresh air is drawn in through the outer one). Both the flue gas discharge and the air intake are handled by a fan built directly into the boiler, which is why this is called forced draught. The advantage is clear – a turbo boiler doesn't need a classic masonry chimney at all; it can be installed even in flats and houses where a chimney was never built, requiring only a duct through the outer wall. That's exactly why the turbo variant is the common choice for flats in panel buildings or in modern new builds without a chimney.
The choice between an atmospheric and a turbo variant therefore largely depends not just on preference, but on what the building actually offers – a functioning masonry chimney, or the possibility of routing a concentric pipe through the façade.
Checkpoints for an atmospheric chimney connection.
What to Watch Out for with a Masonry Chimney
If you're choosing an atmospheric low-temperature boiler, or already have one at home, the masonry chimney is a key part of the whole heating system – it's not just a "hole in the wall", but a component that must meet certain technical requirements for the boiler to work safely and efficiently.
Sufficient draught. Natural chimney draught is created by the temperature difference between the hot flue gases inside the chimney and the cooler air outside. If the chimney is too short, has an unsuitable flue diameter, or is exposed to unfavourable weather effects (for example, being close to a taller building that disrupts airflow), the draught may be insufficient. The result is poor flue gas discharge, which is not only inefficient but above all a safety risk.
Technical condition of the chimney. An older masonry chimney should be free of cracks in the body and the liner, without loose or damaged plaster, and free of soot build-up. A clogged or damaged chimney reduces draught and increases the risk that flue gases won't be discharged as they should. For older chimneys without a suitable liner, it's advisable to check whether the liner is suitable for the given appliance type and fuel.
Flue material. An important difference compared with condensing boilers – in a low-temperature (classic) boiler, the flue gas temperature is considerably higher, typically around 110 to 150 °C, whereas in a condensing boiler it's only around 20 to 50 °C. That's exactly why a classic boiler never uses the thin plastic flue you know from condensing boilers – it simply couldn't withstand such flue gas temperatures. The flue gas discharge must be handled by a masonry chimney with a suitable (typically ceramic or stainless steel) liner, or in the case of the turbo variant, by a metal or stainless steel concentric set designed for high flue gas temperatures.
Regular chimney sweep inspection. Regardless of how old or new your chimney is, a regular inspection by a professional chimney sweep is a basic requirement for safe operation. The chimney sweep assesses the condition of the liner, the flue, and the draught itself, and will flag any deficiencies before they turn into a fault or safety risk. The frequency and scope of inspections are set by applicable regulations and the boiler manufacturer's recommendations, so it's worth following the schedule recommended by the chimney sweep at the last inspection as well.
Accessories for the Turbo Variant
With a turbo low-temperature boiler (closed combustion chamber, forced draught), instead of a masonry chimney you'll need a concentric flue set that simultaneously handles air intake and flue gas discharge through a single duct in the wall or roof. Important – this accessory belongs exclusively to the turbo (forced-draught) variant of the boiler, not to an atmospheric boiler connected to a masonry chimney. An example of a real flue set commonly used with turbo boilers:
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Protherm flue set Ø 60/100 mm S3K – a concentric flue set with a 60/100 mm diameter, designed for turbo (forced-draught) boilers with a closed combustion chamber. It simultaneously draws in combustion air from outside and discharges flue gases via the boiler's fan, without needing a masonry chimney. It is not suitable or intended for an atmospheric boiler with natural draught. Price from €67.00. |
When choosing the specific length and diameter of the concentric set, always follow the type and output of your particular boiler and the manufacturer's recommendation – different turbo boiler models may require a different pipe diameter (for example 60/100 mm or 80/125 mm) as well as a different maximum run length. If in doubt, we recommend consulting 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 switching to a condensing boiler in the future, you need to account for one important change – a masonry chimney cannot be connected directly to a condensing boiler. The reason is simple: a condensing boiler operates at a considerably lower flue gas temperature (approx. 20–50 °C), at which water vapour in the flue gases condenses. The resulting condensate is slightly acidic, and an old masonry chimney – especially if it lacks a suitable resistant liner – could gradually be damaged by it.
The solution commonly used in practice is to use the existing masonry chimney as a shaft – that is, purely as a space through which a new plastic flue, designed for condensate and lower flue gas temperatures, is routed. The masonry chimney itself thus loses its function of discharging flue gases in this case and becomes merely a protective casing around the new pipe. In most cases this option is feasible and doesn't require demolishing the chimney – however, you should always verify the clearance and dimensions of the existing shaft before buying a specific boiler and flue set. If the masonry chimney cannot be used this way for any reason (for example, insufficient internal diameter), an alternative is to route the discharge through the façade, similar to what's common with the turbo variant of low-temperature boilers.
Summary: when switching from a low-temperature to a condensing boiler, the flue material and type will therefore almost always change (plastic instead of masonry or metal), even though the masonry chimney itself, as a structural element, can in many cases remain in place and serve as a shaft.
The basic difference between the two variants.
Two changes compared with the original low-temperature setup.
Situations where the turbo variant pays off.
Frequently Asked Questions
Does a gas boiler always need a chimney?
No. An atmospheric low-temperature boiler needs a masonry chimney because it discharges flue gases by natural draught. A turbo low-temperature boiler, like most condensing boilers, doesn't need a masonry chimney – it discharges flue gases by forced draught through a concentric pipe routed directly through the façade.
Can an old chimney be used for a condensing boiler?
Connecting a condensing boiler directly to a masonry chimney is not recommended – the condensate from the lower flue gas temperature could damage the chimney. In many cases, however, the chimney can be used as a protective shaft through which a new plastic flue designed for a condensing boiler is routed.
How often should the chimney be inspected by a chimney sweep?
We recommend having the chimney regularly inspected by a chimney sweep in line with applicable regulations and the recommendation from the sweep's last inspection – the sweep assesses the draught, the technical condition of the liner and the flue, and flags any deficiencies.
What's the difference between an atmospheric and a turbo boiler in terms of installation?
An atmospheric boiler requires a room with an adequate air supply and a functioning masonry chimney with good draught. A turbo boiler only needs a duct through the outer wall for the concentric pipe and doesn't need a masonry chimney or a special air supply to the room, since it draws air directly from outside.
Can I keep a masonry chimney for a turbo boiler even if I'm not using it?
Yes, a masonry chimney can remain on the building unchanged if the turbo boiler doesn't use it – it can serve as a backup or for another appliance. The turbo boiler itself doesn't need it for operation.
Is the concentric flue set the same for all turbo boilers?
No, both the diameter and the maximum length of the concentric pipe vary depending on the specific model and output of the boiler. Always follow the recommendation of the boiler's manufacturer, or consult your choice with an installation company.