Can a flue be inserted into an existing duct?

08/04/2022 – “Is it possible to use an old flue to insert another?”. This is the question posed in community of Edilportale by a user who is carrying out seismic improvement interventions on a load-bearing masonry house.

An intervention of this kind is known by the term intubation.

According to UNI 10845the reference standard for systems for the evacuation of the combustion products of gas systems for domestic useis defined intubation the operation through which, in the context of the restructuring of a system and through the introduction of one or more suitable ducts, a new system is created from a chimney, flue or technical compartment.

The ducted duct instead, it is the ducting duct inserted in the chimney, flue or existing technical compartment.

When is this technique used?

I reasons they may be different, like the case of our reader who collided with the impossibility of creating elsewhere the new flue and would like to exploit the existing one.

Or when the pre-existing duct:
– is made with non-compliant materials with current legislation;
– is no longer suitable for its function and has functional problems, such as a bad draft or the formation of condense;
– presents mechanical problems.

The answer to the question it is given by the legislation that defines the cases and procedures of application.

First of all it should be emphasized that an operation of this type cannot be the result of do-it-yourself; such interventions they must be designed by professionals ed performed by specialized companieshaving specific technical expertise.

Ducting, the characteristics of the pre-existing duct

The existing flue becomes practically a cavedio, that is a technical room which, according to UNI, must be exclusively dedicated to the use of the ducted duct. Inside pwill get through only the duct for the evacuation of combustion products, no other type of piping, much less cables of any kind.

According to UNI 10845 ducting is allowed if the chimney, flue or existing technical compartment complies with the following general criteria:
– be of fire reaction class “0” material (UNI 9177);
– have a sufficient section to ensure easy and adequate insertion of the new duct (or of the new ducts, in the case of multiple systems);
– be free from structural damage, obstructions or constrictions such as to compromise its integrity;
– be free of slag or soot;
– be protected with devices that prevent the penetration of atmospheric agents;
– be used for the exclusive use of the new system.

Ducted duct, types and operation

A ducted duct can be made with operation in negative and positive pressure:
– in a functioning fireplace a positive pressure the internal pressure is greater than the external pressure;
– in a functioning fireplace a negative pressure the internal pressure is less than the external pressure.

The difference between the two pressures, the one external to the flue and the internal one, is indicative of the draft. A negative pressure chimney is a natural draftwhile those with positive pressure are a forced draft that is, they exploit the head supplied by the heat generator.

In the operation in negative pressurethe old flue, which now serves as a technical compartment, must always be ventilated from above. There are no useful sections to be respected, but in the ducted duct, according to the direction of the fumes, it is not allowed to restrict the section. The ducted duct must have a predominantly vertical course and only two changes of direction with an angle of less than 30 ° are allowed. At the base there must be a collection chamber and an inspection opening.

In the positive pressure operationon the other hand, the technical compartment must always be ventilated both from below and from above. This type of installation allows the use of smaller sections, for the ducted duct, compared to ducting under negative pressure. Furthermore, the forced draft allows to create systems of considerable height and with different changes of direction.

Technical Handbook © Beretta Caldaie

In this case it is essential to realize a ventilation gap between the ducted duct and the chimney / host compartment. It is a safety measure that avoids the stagnation of any condensation along the development of the ducts. This precaution is not required if a duct is made in a pre-existing pipe outside the building.

The ducts must have junctions crimped. Possible narrowings of the section and / or changes in direction and the angle of incidence with the vertical, must be verified by a calculation or expressly declared suitable by the manufacturer. The air ventilation gap must be open both at the base and at the top.

The interspace section must be suitably sizedaccording to the parameters established by UNI, in any case the net free section of the ventilation gap must be at least equivalent to that of the duct itself.

Can a flue be inserted into an existing duct? Technical Handbook © Beretta Caldaie

Finally, in both cases, the existing chimney, flue or technical compartment must be prepared for carrying out the necessary maintenance, verification and control operations. Ducting must be carried out with the aid of products and technologies suitable for the purpose, expressly declared suitable by the manufacturer.

As for the materials, for the condensing boilers are used PP plastic ducts (polypropylene). The metal it is suitable for all types of fuel, therefore gas and solid fuels, that is traditional boilers e fireplaces and stovesbut it is forbidden to use flexible metal hoses.

Do you need a project?

In addition to the cases seen so far, it is possible to carry out the intubation of several ducts inside a single chimney, flue or existing technical compartment; in this case the system that is created it is said multiple because inside it can coexist several systems even of different types.

In the event that the system to be restored or renovated is enslaved by a single appliance with a nominal heat input not exceeding 35 kW, the project is not necessary and can be replaced by a technical report; on the contrary the project is needed in multiple or collective systems.

Source: Le ultime news dal mondo dell'edilizia by www.edilportale.com.

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