When choosing ducts, people often ask whether flexible or rigid ones are better. Yet that comparison is incomplete without the route and air quantities. One particular flexible duct may be intended for a distribution system, another only for a short connection, while a rigid pipe can also be selected or installed incorrectly. The decision is supported by calculation, product data and the complete installation detail, not merely an impression of its material.
The actual internal diameter matters
A product's external dimension does not always show the airflow area. Two ducts of similar size can have different internal diameters and surfaces. The particular model's data is therefore needed for comparison. The same label on the packaging does not establish that both will have the same resistance. This is especially important in smaller-diameter distribution routes.
Also check the product's intended purpose. An ordinary flexible connection and a semi-rigid distribution system specified by a manufacturer are not the same. They must not be substituted merely because both can bend. System compatibility covers connections, sealing, permitted bends and other documented requirements.
Pressure losses depend on airflow
The Lindab LFPE technical document provides airflow and pressure-loss diagrams for the particular product. They demonstrate the method: losses are assessed according to air quantity and the data for a section or bend. This diagram is not a table for all flexible ducts and cannot be applied to another model without justification.
Straight-run losses and losses at individual components must be assessed together. An elbow, branch, box, valve and grille can each have their own resistance. The price per metre or a straight-pipe figure alone does not describe the whole route. The proposal needs to identify the critical route used to select the unit's operating point.
Bend shape changes the result
Flexibility makes routing more convenient, but is not permission to compress or kink the duct. A small bend radius and deformation can reduce the area and increase resistance. The shapes permitted by the manufacturer must be maintained through to final covering. A photograph before finishing helps check later how the system was actually routed.
Geometry also matters in a rigid arrangement. The shortest route with many abrupt, unselected bends will not necessarily be best for losses and sound. All parts must be assessed together. Saying that 'metal offers less resistance' is insufficient if the final route has not been calculated or its connections leak.
The division of airflow must be calculated
Where several smaller lines run to one room, their number and lengths are selected according to demand and the particular system. The same number of pipes cannot automatically be assigned to every room. Room use, air quantities and route lengths differ. Actual adjustment and verification are still required after installation.
The PNNL air-movement explanation helps explain the importance of route resistance. If one branch offers an easier route while another is highly restricted, the overall fan percentage does not show the result in every room. A larger unit does not always reasonably compensate for unsuitable distribution selection. The layout itself must be put right first.
Illustrative example: a long branch to the bedroom
Suppose the bedroom is furthest from the technical room. The drawing specifies an air quantity, but installation lengthens the route around a beam and introduces extra bends. This change must be assessed before the ceiling is covered. It cannot simply be assumed that opening the air valve fully later will be enough.
The designer or responsible contractor checks the new resistance and, if necessary, adjusts the route or its selection. Actual airflow is measured after installation. This sequence helps avoid a situation where it is noisy beside the unit while the distant room still receives too little air. The home's overall capacity figure alone does not reveal that problem.
Airtightness and cleanliness are not minor extras
Connections must be installed according to the system manufacturer. A homemade connection that looks strong is not necessarily airtight or suitable for long-term operation. The Komfovent installation manual treats ducts as part of the whole system. A particular unit cannot independently guarantee a result if its connection does not match the intended solution.
During construction, openings must be protected from contamination. A duct interior should not be treated as a convenient place to leave dust, because the home's air will flow through it later. Before covering, also check maintenance possibilities. The routine cleaning method must suit the actual system, not a promise that 'we will clean it somehow later'.
Insulation depends on location
The insulation requirement for a duct in a cold or warm section depends on the air and environmental conditions. Whether the pipe is flexible or rigid does not answer that question. Thermal protection and, where necessary, vapour-tight protection must be provided according to the design. An incorrectly made joint in the insulation layer can cause local condensation.
Mark routes in the loft, service shaft or through an unheated zone separately. The proposal must make clear where insulation is included and its scope. A cheap metre rate without these components is not a complete solution. When comparing proposals, the same technical brief matters, not just an identical total route length.
The as-built layout must show alterations
After the work, keep not only the initial drawing, but also the actual route arrangement. If structures required changes in length or bends, these must be marked. Later diagnostics will be more accurate when distribution-box positions and connection access are known. Link photographs to room names rather than leaving them unexplained in a general phone collection. This is particularly useful before suspended ceilings are fitted, when the entire system is visible for the last time.
What to request
Ask for data on the particular ducts, a calculated route, an installation layout and a plan for final airflow verification. The airflow selection article explains the importance of the operating point. In your enquiry, include a plan showing possible routes and structural obstacles. The flexible or rigid solution can then be selected for the actual property, not according to one general claim.
Information sources
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