Rule of thumb · applies

Third-party pipe only by dimension table, never by nominal size – why a transition to a foreign system depends on outside diameter and wall thickness

Third-party pipe only by dimension table – never by nominal size.

A rule of thumb replaces neither the design calculation nor the manufacturer’s instructions.

Category: robust rule
Standards referenced: EN ISO 21003-2, EN ISO 15875-2, EN ISO 15876-2 (permissible third-party pipes per Geberit TI 972.124.00.0) – manufacturer data

Rule of thumb

Whether a transition fitting grips a foreign pipe tightly is decided by the pipe itself – measured outside diameter and wall thickness –, not by the number printed on it. For multilayer pipes “16” means 16 × 2.0 or 16 × 2.2, and those are two different transition groups.

Where does the rule come from?

The rule comes from the manufacturer’s approval list: for the FlowFit transition to third-party systems Geberit has seven transition groups by d × s (16 × 2.0–2.2, 16–17 × 2.5–2.75, 20 × 2.0–2.25, 20 × 2.8–2.9, 25 × 2.5–2.7, 25 × 3.5–3.7, 26 × 3) and assigns exactly one group to each third-party pipe – with the note that the dimensions must always be checked because of tolerances. The calibration mandrel of the transition is group-specific; a pipe of the wrong group can be calibrated and pressed and is still not approved.

What is technically correct?

A press transition seals between the support sleeve (inside) and the press sleeve (outside). The support sleeve needs the inside diameter, which is d minus 2 × s; the press sleeve needs the outside diameter. Two pipes with the same d and a 0.2 mm difference in s differ by 0.4 mm in inside diameter – for an O-ring on the support sleeve that is the difference between preload and gap. That is why every approval states d × s, and why the nominal size is not enough. Measure with a calliper at two points 90° apart (ovality), the wall thickness at a square cut or from the manufacturer’s print if legible.

Where does the rule end?

Within ONE system the rule is not a checking effort: system pipe and system fitting are built for each other, the dimension suffices.
Compression fittings with a range (Multigebo measures outside and inside diameter with the gauge from the kit, repair bands cover ranges) need the dimension too – just with a wider band.
Metal pipes and coated metal pipes are not permitted on the FlowFit transition at all – no dimension table helps there, only a threaded or flanged transition.
The approval list has an expiry date (Geberit: 2027-04-01) – a pipe listed today may be missing in the next issue.

What does the standard say?

The pipe standards Geberit admits for third-party pipes – EN ISO 21003-2 (multilayer), EN ISO 15875-2 (PE-X), EN ISO 15876-2 (PB) – fix outside diameters and wall thickness series with tolerances; they do not regulate the connection to a foreign fitting. The approval is therefore always manufacturer data, and it states d × s because the standard allows several wall thicknesses per outside diameter. We do not hold the standard text; the data follow the technical information.

DIN/EN versus ASME/ASTM

In the ASME/ASTM world multilayer and PE-X pipes have their own standards (ASTM F1281 multilayer, ASTM F876 PE-X) with inch dimensions by nominal tube size – a “½ inch” PE-X tube has 15.9 mm outside diameter and fits none of the seven EN groups. The rule applies there all the more: measure, do not translate.

Worked example

A maintenance fitter wants to connect an existing pipe “d 20” of unknown make to FlowFit. Measured: 20.0 mm outside, 2.25 mm wall – group 20 × 2.0–2.25, transition 620.711.00.1. Had he ordered by “20” and the pipe been a 20 × 2.8 (group 620.712.00.1), the support sleeve would sit on an inside diameter 1.1 mm smaller – not approved, and nobody would carry the warranty.

Zickwolff reference

From daily business: enquiries about transitions come with photos of prints or with “about 20”. We ask for measured d × s and, if legible, manufacturer and trade name – with that we check the approval list. What is not in it we solve via thread, flange or Multigebo, not via hope.

Checked today

Verdict: applies (class A). The rule follows the manufacturer approval literally (d × s per group, tolerance note); it is an installation truth, not an interpretation of a standard.

Frequently asked questions

Is “multilayer pipe 20” enough for the enquiry?

No – we need d × s measured or from the print, ideally with manufacturer and trade name. 20 × 2.0 and 20 × 2.8 are two transition groups.

May I calibrate and press a pipe of the neighbouring group?

Technically it often works – approved it is not. The fitting manufacturer’s warranty applies only to the named group.

What if my pipe is in no list?

Then there is no approved press transition: threaded transition of the existing system, compression fitting on the measured pipe (Multigebo) or dismantling back to the next metal connection.

All rules of thumb

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