Classification, regulations and selection criteria
In the design and installation of flue systems, one of the most critical — and often underestimated — technical aspects is the pressure class of the exhaust duct. Not all flue systems operate under the same pressure conditions: the difference between a natural draft system and a positive pressure system — typical of condensing boilers, pellet stoves, and many industrial generators — leads to substantially different technical requirements for the duct. A high-pressure flue pipe for boilers consists of components specifically designed and certified to operate under positive pressure conditions, ensuring tightness, safety, and regulatory compliance even in the most demanding installations.
Materials and construction: stainless steel and plastic systems
High-pressure flue pipes for boilers and stoves are available in two main material families.
Stainless steel systems — AISI 316L for acidic flue gases and condensate, AISI 304 for lower-acidity applications — are the preferred solution for pellet stoves, wood stoves, non-condensing gas boilers, and systems with flue gas temperatures above 120°C. These systems are CE-marked according to EN 1856-1 (rigid ducts) or EN 1856-2 (flexible ducts and connectors) and must indicate pressure class, temperature class, and other declared performance characteristics. Systems designed for positive pressure differ by the presence of P1-rated gasketed joints and certified tightness at 200 Pa.
Plastic systems — polypropylene (PP), flame-retardant polypropylene (PPS), or PVDF — are typically used for gas condensing boilers up to approximately 70 kW, where flue gas temperatures do not exceed 120°C. These systems comply with EN 14471 and are available in single-wall or coaxial configurations. The P1 pressure class (200 Pa) is fully compatible with these materials when installed according to manufacturer specifications.
High-pressure flue pipes for boilers: requirements and applications
Type C condensing boilers — sealed chamber units drawing combustion air from outside — typically require a P1-certified high-pressure flue pipe. The internal fan pushes flue gases through the duct, generating positive pressure.
According to UNI 7129:2015, flue systems for condensing appliances installed indoors must have a pressure class of at least P1. If the duct is external and detached, P2 may be acceptable. If the duct runs inside the building under positive pressure, it must be installed within a ventilated shaft to allow dispersion of any gas leakage.
Collective condensing systems may also operate under positive pressure, with internal pressure limits typically not exceeding 25 Pa, again requiring P1-certified systems.
High-pressure flue pipes for stoves: pellet stove specifics
Pellet stoves often operate under positive pressure due to their internal exhaust fan. This requires the use of a certified high-pressure flue pipe.
Using ducts designed for natural draft systems (class N1) can lead to leakage of combustion gases, including carbon monoxide, especially at joints not designed for positive pressure.
Choosing the correct high-pressure flue pipe is a safety issue
Selecting a properly certified high-pressure flue pipe is essential for system safety. Improperly rated ducts can lead to undetectable carbon monoxide leakage.
MT Camini offers certified flue systems suitable for positive pressure applications. Our technical team supports designers and installers in system selection, compatibility verification, and compliance documentation.
Contact us for technical consultation.