Functional Claim Limits: “Configured so that” Must Be Achieved by the Claimed Component, and “Substantially Unchanged” Is Invalid Without an Ascertainable Criterion
1. Introduction
Case: Kohler Mira Ltd v Norcros Group (Holdings) Ltd (Triton) [2025] EWCA Civ 1670
Court: England and Wales Court of Appeal (Civil Division)
Date: 22 December 2025
This appeal concerned UK Patent No. 2466504 entitled “Ablutionary installation”, filed on 24 December 2008, relating particularly (though not exclusively) to instantaneous water heaters used in electric showers. The patentee (Mira) succeeded at first instance in the Intellectual Property Enterprise Court (IPEC), where HHJ Melissa Clarke held Triton’s “DuElec” dual-outlet electric showers infringed and the patent was valid.
The central dispute on appeal was the meaning and validity of claim language requiring a diverter valve downstream of a heater tank, “configured so that flow rate is substantially unchanged during changeover” between outlets. Triton argued (i) the claim required the diverter valve’s configuration itself to deliver that result (not other system elements), and (ii) “substantially unchanged” was uncertain because the specification provided no criterion to draw a boundary.
2. Summary of the Judgment
The Court of Appeal (Arnold LJ giving the leading judgment, with Falk LJ and Asplin LJ agreeing) allowed Triton’s appeal and:
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Reversed the IPEC’s construction: the phrase “the diverter valve is configured so that” requires the result to be achieved by the diverter valve (at least together with other components of the claimed instantaneous water heater), not by elements outside the claimed heater.
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Rejected Mira’s proposed criterion for “substantially unchanged” based on a “risk of scalding” threshold, because claim 1 is not limited to showers and even in showers the purpose extends beyond scalding avoidance.
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Held no infringement: Triton’s admitted temperature stability during changeover was attributable to a flow stabilisation valve and thermal cut-off, not the diverter valve’s geometry/configuration as such, and in any event the operative “substantially unchanged” test was not established.
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Revoked the patent for insufficiency/uncertainty under section 72(1)(c) of the Patents Act 1977 because, without an ascertainable criterion, the boundary of “substantially unchanged” was “impossible to determine”.
Grounds relating to excessive breadth and obviousness did not require determination given the decisive findings on construction, non-infringement, and insufficiency.
3. Analysis
3.1 Precedents Cited
The Court reiterated that claim construction is purposive: what the skilled person would understand the patentee to mean from the claim language in the context of the specification. The judgment relied on Lord Hoffmann’s formulation at [34] that “the question is always what the person skilled in the art would have understood the patentee to be using the language of the claim to mean”.
This underpinned two key appellate moves: (i) focusing on the claim’s actual subject-matter (an “instantaneous water heater” not necessarily a “shower”), and (ii) refusing to treat the specification’s shower-centric safety narrative as automatically importing a “scalding” metric into a general claim term (“substantially unchanged”).
(b) Nokia GmbH v IPCom GmbH & Co KG [2009] EWHC 3482 (Pat)
Cited for the principle that where a claim uses general language, it is generally illegitimate to restrict it to a preferred embodiment. The Court used this to reject Mira’s attempt to tie “substantially unchanged” to the shower embodiment’s scalding-risk discussion.
(c) Optis Cellular Technology LLC v Apple Retail UK Ltd [2022] EWCA Civ 792
The Court relied on Optis (approving Nokia v IPCom) to reinforce the same interpretive discipline: the claim’s breadth, chosen by the patentee, cannot be re-written by importing embodiment-specific limitations unless the language and context demand it.
(d) Anan Kasei Co Ltd v Neo Chemicals & Oxides Ltd [2019] EWCA Civ 1646, [2020] FSR 8
This was the controlling authority on “uncertainty” as a species of insufficiency under section 72(1)(c). The Court applied Anan Kasei’s distinction between:
- Fuzzy boundaries (permissible: the boundary is imprecise but ascertainable in principle), and
- Unascertainable boundaries (impermissible: it is impossible to determine what is within the claim).
The Court treated “substantially unchanged” as falling into the second category once Mira’s only proposed criterion (“risk of scalding”) was rejected and no alternative metric was advanced: the skilled person could not know whether 1%, 5%, 25% or 50% flow change was “substantial”.
3.2 Legal Reasoning
(a) The claim’s subject-matter controls the interpretive lens
A pivotal step was the Court’s insistence that claim 1 is to an “instantaneous water heater”, not to a shower. Only claim 19 is a “shower installation” claim. Therefore:
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The scope of claim 1 must be determinable without reference to how the heater is used or intended to be used (e.g., shower, dishwasher, power washer).
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Infringement cannot depend on intended use such that the “same” heater could be infringing/non-infringing depending on context—memorably labelled “Schrödinger’s patent infringement”.
(b) “Configured so that” ties the functional result to the diverter valve (within the claimed heater)
The Court held that the words “the diverter valve is configured so that” mean the diverter valve’s configuration must be responsible for delivering the “substantially unchanged” flow-rate outcome. Mira was permitted a limited concession: the result could be achieved by the diverter valve in conjunction with other components of the instantaneous water heater. But the Court drew a firm boundary: the result cannot be satisfied by components that are merely part of a broader installation outside the heater as claimed.
This matters because Triton’s DuElec showers maintained temperature/flow behaviour during outlet changeover through a combination of other mechanisms (flow stabilisation valve and thermal cut-off), not the diverter valve geometry itself. On the Court’s construction, that was insufficient for infringement.
(c) “Substantially unchanged” could not be saved by importing a “scalding-risk” criterion
The IPEC accepted Mira’s “risk of scalding criterion” (flow is “substantially unchanged” if, during changeover, any temperature rise does not create a scalding risk). The Court of Appeal rejected that approach for three reasons:
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Embodiment-locking is impermissible where the claim is drafted in general terms (Nokia GmbH v IPCom GmbH & Co KG; Optis Cellular Technology LLC v Apple Retail UK Ltd).
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Non-shower applications exist within claim 1’s scope, where “scalding risk” may be irrelevant or even contrary to purpose (e.g., appliances where very hot water is useful).
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Even in showers, the specification’s purpose was broader than scalding avoidance: it spoke of avoiding “any sudden unexpected change in temperature”, “especially” scalding increases, meaning decreases or non-dangerous but uncomfortable increases also matter.
(d) Uncertainty as insufficiency: the absence of a workable boundary test was fatal
Having rejected the only offered metric, the Court held claim 1 insufficient/uncertain: there was no disclosed or inferable standard by which the skilled person could decide what degree of change remained “substantial”. That moved the claim from permissible approximation into impermissible indeterminacy per Anan Kasei Co Ltd v Neo Chemicals & Oxides Ltd.
As claims 4 and 17 depended on claim 1’s disputed requirement, they fell with it. The Court therefore ordered revocation.
3.3 Impact
(a) Drafting and prosecution: “substantially” demands an anchor
The decision is a drafting warning: terms like “substantially unchanged” may survive if the specification (or common general knowledge in context) supplies an operational yardstick. But where the patentee chooses broad, application-agnostic claiming (here, a heater not confined to showers), the yardstick cannot be borrowed from a single use-case unless the claims are correspondingly limited.
(b) Functional claiming: “configured so that” is not a free pass to rely on external system features
The judgment reinforces that functional language attached to a particular claimed component typically requires that component (within the claimed combination) to be responsible for the function. A defendant may avoid infringement where the system-level result is achieved by other elements not fairly attributable to the claimed component’s configuration.
(c) Litigation strategy: if your primary construction fails, plead a workable alternative metric
The Court’s uncertainty finding turned on a practical forensic point: Mira advanced only one criterion for “substantially unchanged”, which was rejected, and offered no alternative case. Future patentees/claimants can expect increased pressure to articulate (and evidence) an objective boundary test if they rely on qualitative modifiers.
(d) Product design: safety-control elements can decouple performance from claim language
In dual-outlet electric showers, stability during changeover can be achieved by closed-loop controls (flow stabilisation, thermal cut-out, thermostatic systems). This case suggests that where claims attribute a result to a particular valve configuration, defendants may be able to show the result is delivered elsewhere in the control architecture.
4. Complex Concepts Simplified
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Purposive construction: interpreting patent claims by asking what a skilled reader would understand the patentee to mean in context, rather than reading words with strict literalism.
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“Configured so that” (functional limitation): language that ties a component’s configuration to achieving a stated outcome. The Court treated this as requiring the stated outcome to be delivered by that component’s configuration (within the claimed device), not merely observed in the overall product due to other features.
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“Substantially” terms: modifiers like “substantially unchanged” can be valid when they imply an objective tolerance the skilled person can work with. They become invalid when no one can tell where the boundary lies.
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Insufficiency by uncertainty (Patents Act 1977, s72(1)(c)): a patent can be revoked if its claims are so unclear that it is impossible to determine what falls inside or outside the monopoly.
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Fuzzy vs unknowable boundary: “fuzzy” means there is a boundary but it is not mathematically precise; “unknowable” means the boundary cannot be located at all. Only the latter is fatal.
5. Conclusion
The Court of Appeal’s decision both constricts and clarifies functional patent scope. It holds that where a claim attributes an outcome to a specific component (“the diverter valve is configured so that…”), the patentee cannot rely on other, external control features to satisfy that outcome. More importantly, it confirms—applying Anan Kasei Co Ltd v Neo Chemicals & Oxides Ltd—that qualitative language such as “substantially unchanged” can invalidate a patent if the specification provides no workable criterion and the court cannot legitimately import one from a preferred embodiment or a particular use-case. The result was decisive: no infringement and revocation for insufficiency/uncertainty.