Purposive construction showers Schrödinger’s water heater

The Kohler Mira v Norcros [2025] EWCA Civ 1670 judgment from the Court of Appeal is worth reading if you draft patents. 

Two problems arose because of the patent’s breadth. Although the focus of the patent in this case was on shower installations, the claim language was broader than this, to an ‘instantaneous water heater’ without limitation to a shower installation. Firstly, this meant that how other components of a shower installation functioned could not determine whether the instantaneous water heater part infringed (meaning there was no infringement). Secondly, the shower context could not provide purpose for the interpretation of “substantially unchanged” (meaning the claim was invalid for uncertainty insufficiency).

The dispute was about whether Norcros/Triton’s dual-outlet electric showers (think overhead and hand-held hoses) infringed Mira’s patent. Mira’s patent claimed an ‘instantaneous water heater’ comprising a diverter valve downstream of the heater tank, in which “the diverter valve is configured so that” the flow rate (of the water) “is substantially unchanged during changeover” between outlets.

At first instance, HHJ Melissa Clarke held that Mira’s patent was valid and infringed. The Court of Appeal (CA) explained that the judge had construed the claim language incorrectly, leading to error on both counts. 

There was no real disagreement between the judge and the CA on the applicable principles in respect of construction. The judge’s summary of them had been accurate, but Arnold LJ added:

“…the overarching principle is that claims are to be purposively construed. The inventor's purpose in using the language of the claim is to be ascertained from the specification as a whole. As Lord Hoffmann explained in Kirin-Amgen Inc v Hoechst Marion Roussel Ltd [2004] UKHL 46, [2005] RPC 9 at [34], "[t]he 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". The correct interpretation of a claim is a question of law for the court. In general, expert evidence as to the meaning of the claim (as opposed to the common general knowledge with which the claim is read and the meaning of any technical terms employed in the claim) is inadmissible.”

The purposive construction principle had not been properly applied by the judge when reaching her conclusions of both infringement and validity.

No infringement followed from purposive construction 

Arnold LJ gave the leading judgment. He explained that the claim was to an instantaneous water heater, not an instantaneous water heater for a shower. Therefore the claim was not limited to instantaneous water heaters for showers. So it must be possible to determine whether any particular instantaneous water heater fell within the claim or not without reference to any larger apparatus in which the instantaneous water heater might be incorporated or intended to be incorporated.

Otherwise, the same instantaneous water heater could be both infringing and non-infringing (a situation that could aptly be described as “Schrödinger's patent infringement").

When changing between outlets, Triton’s diverter valve operated in a way that meant the cross-sectional area of the open outlets of the diverter valve significantly reduced. At one point, only about a third of the area of the fully open outlet port was open. The evidence was that this would not affect the water flow if the minimal cross-sectional area was the same as or larger than the minimum aperture somewhere else in the flow path within the installation. Also that it was the upstream flow stabilisation valve that generally determined the flow rate of at the narrowest point. 

Triton’s expert accepted that, consistently with Triton’s marketing assertions, Triton’s range of showers “seamlessly divert from one outlet to the other”. However, in accordance with the purposive construction of the claim (which was simply to an instantaneous water heater), the establishment of infringement could not depend on the design of the diverter valve in conjunction with the design of other elements of the product in which the water heater was (or might be) incorporated. Therefore the judge’s finding of infringement, which had been reached on such basis, was overturned.

Invalidity followed from purposive construction

The claim language also required that the flow rate be “substantially unchanged during changeover” between outlets. Triton contended that the integer was uncertain (and therefore the claim was invalid) because the specification provided no criterion for determining what was and what was not substantial. Indeed, it was common ground that the specification did not explain what was meant by the term or provide any criterion for determining it. 

Mira contended that the term meant that when water was flowing at a selected water temperature while the heater was energised, any increase in temperature during changeover from one or more selected outlets to another selected one or more outlets did not give rise to a risk of scalding. 

Arnold LJ explained that if general language is used in a claim then it is not normally legitimate to restrict the claim to a preferred embodiment. In the present case the language of claim 1 was very general and there was no basis for limiting it to the specific embodiment described in the specification. Claim 1 was to an instantaneous water heater that might be incorporated into a product, e.g. a dishwasher or power washer, and in some such products scalding the user was not necessarily a concern. In the specific context of showers, it was apparent from the patent specification that the purpose of the integer was “to avoid any sudden unexpected change in temperature, especially an increase in temperature that might give rise to scalding”. This included a sudden unexpected decrease in temperature, which would be uncomfortable to the user even if not dangerous.

Consequently, Triton was correct that the risk of scalding criterion was not the applicable criterion for determining whether the flow rate was “substantially unchanged”. The patent was therefore invalid for uncertainty insufficiency.