Court Opinion

ID: 7797352
Source: CourtListenerOpinion
Date Created: 2022-08-03 15:00:38.135914+00
Date Added: 2024-06-11T16:28:36.482210
License: Public Domain

Case: 21-2222   Document: 44     Page: 1   Filed: 08/03/2022

        NOTE: This disposition is nonprecedential.

   United States Court of Appeals
       for the Federal Circuit
                 ______________________

    A. O. SMITH CORPORATION, AOS HOLDING
                    COMPANY,
                 Plaintiffs-Appellees

                            v.

        BRADFORD WHITE CORPORATION,
              Defendant-Appellant
             ______________________

                       2021-2222
                 ______________________

     Appeal from the United States District Court for the
 District of Delaware in No. 1:18-cv-00412-LPS, Judge
 Leonard P. Stark.
                 ______________________

                 Decided: August 3, 2022
                 ______________________

     S. EDWARD SARSKAS, Michael Best & Friedrich LLP,
 Milwaukee, WI, argued for plaintiffs-appellees. Also rep-
 resented by KENNETH M. ALBRIDGE, III, Madison, WI; KYLE
 GLENDON HEPNER, Washington, DC.

     JAMES R. BARNEY, Finnegan, Henderson, Farabow,
 Garrett & Dunner, LLP, Washington, DC, argued for de-
 fendant-appellant. Also represented by RYAN VALENTINE
 MCDONNELL.
Case: 21-2222     Document: 44     Page: 2    Filed: 08/03/2022

 2                                 A. O. SMITH CORPORATION v.
                                BRADFORD WHITE CORPORATION

                   ______________________

      Before DYK, REYNA, and TARANTO, Circuit Judges.
     Opinion for the court filed by Circuit Judge TARANTO.
        Concurring opinion filed by Circuit Judge DYK.
 TARANTO, Circuit Judge.
     AOS Holding Company, which is wholly owned and
 controlled by A. O. Smith Corporation, owns U.S. Patent
 No. 8,375,897, titled “Gas Water Heater.” The two compa-
 nies (collectively, A.O. Smith), the only persons permitted
 to practice or profit from the invention embodied in the pa-
 tent, brought the present action against Bradford White
 Corporation in the U.S. District Court for the District of
 Delaware, alleging infringement of the patent’s claim 1
 (the patent’s sole claim), which identifies a “method of in-
 terfacing a natural convection vent construction with a wa-
 ter heater.” ’897 patent, col. 6, lines 9–10. The district
 court held that Bradford White infringed claim 1 and that
 the claim was not invalid. Bradford White appeals both
 determinations. We affirm.
                               I
     The short specification of the ’897 patent describes the
 operation of a hot-water heater system, using a burner con-
 taining a blower, a flue tube running through a water tank,
 and exhaust components. A burner burns a “fuel/air mix-
 ture,” and the resulting “products of combustion” are
 forced, by the burner’s blower, into a “flue tube 65 under
 positive pressure.” ’897 patent, col. 3, lines 15–18. The flue
 tube (which may have bends in it to increase surface area)
 extends through the tank and functions as a heat ex-
 changer to heat the surrounding water. Id., col. 4, lines 54–
 60; see fig. 2. At the outlet of the flue, the products enter
 an exhaust plenum, where their pressure drops to “near or
 below atmospheric pressure, and the products of combus-
 tion are therefore able to rise out of the plenum 70 and into
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 A. O. SMITH CORPORATION v.                                  3
 BRADFORD WHITE CORPORATION

 the draft hood 75 substantially entirely under the influence
 of natural convection.” Id., col. 4, lines 5–9. In the draft
 hood, the products mix with ambient air. Id., col. 4, lines
 9–11. The invention may also have a baffle in the flue tube
 “to reduce the velocity and pressure of the products of com-
 bustion as they approach the outlet end.” Id., col. 3, lines
 54–57.
     The plenum and baffle, the specification says, “may be
 said to uncouple the flow of the products of combustion
 from the power burner”—“[i]n other words, the products of
 combustion enter the plenum 70 under the influence of the
 power burner 60, but exhaust from the plenum 70 without
 the influence of the power burner 60.” Id., col. 4, lines 12–
 17. The specification immediately adds: “Thus, the present
 water heater 10 can be retrofitted into a Category I venting
 system despite the fact that the water heater 10 utilizes a
 power burner 60.” Id., col. 4, lines 17–19. That language
 refers to the standard industry classification of venting sys-
 tems, embodied in the National Fuel Gas Code, which ex-
 plains that a Category I vented appliance is one with
 nonpositive vent static pressure and a vent gas tempera-
 ture that avoids excessive condensate production. J.A. 242;
 J.A. 28. 1
     Claim 1 reads:
     1. A method of interfacing a natural convection
     vent construction with a water heater, the method
     comprising:

     1   The Code describes Categories I through IV as the
 four possible combinations of (a) vent static pressure that
 is either nonpositive or positive (relative to atmospheric
 pressure) and (b) vent gas temperature that either avoids
 or may cause excessive condensation. See J.A. 28–29; J.A.
 242.
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 4                                 A. O. SMITH CORPORATION v.
                                BRADFORD WHITE CORPORATION

     providing a water heater having a burner, a blower,
     and a flue;
     creating products of combustion with the burner;
     forcing the products of combustion into the flue un-
     der positive pressure with the blower;
     interposing an exhaust plenum between the flue
     and the natural convection vent construction;
     dropping the pressure of the products of combus-
     tion to near atmospheric pressure within the ple-
     num; and
     permitting the products of combustion to rise out of
     the plenum and into the natural convection vent
     construction substantially entirely under the influ-
     ence of natural convection;
     wherein the natural convection vent construction
     includes a draft hood, the method further compris-
     ing mixing ambient air with the products of com-
     bustion as the products of combustion flow into the
     draft hood.
 ’897 patent, col. 6, lines 9–27.
     A.O. Smith sued Bradford White for infringement in
 March 2018. It alleged that Bradford White was directly
 and indirectly infringing claim 1 through sales and other
 activities involving several Bradford White models of water
 heaters. J.A. 154, 160. Bradford White counterclaimed for
 invalidity and non-infringement.
     The parties first sought an early claim-construction
 hearing addressing the phrase “substantially entirely un-
 der the influence of natural convection” in the permitting
 limitation. Bradford White argued that the phrase was in-
 definite—thus invalidating the claim—because a relevant
 artisan would not understand the degree of influence that
 the power burner and other factors exert on the products
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 A. O. SMITH CORPORATION v.                                   5
 BRADFORD WHITE CORPORATION

 as they enter the vent. The court disagreed with Bradford
 White’s contention and instead adopted A.O. Smith’s pro-
 posed construction: “[a]t a pressure near or below atmos-
 pheric pressure and without the influence of the power
 burner, such that a Category I venting system can be used.”
 J.A. 118. The court determined that the claim, read in light
 of the specification, particularly the specification’s refer-
 ence to Category I venting, provides sufficient instruction
 to a relevant artisan. J.A. 119–20. Subsequently, the par-
 ties sought a construction of the term “natural convection”
 within the same claim phrase. The court construed “natu-
 ral convection” to mean “fluid motion compatible with use
 of a Category I venting system,” consistent with its earlier
 construction of the entire claim phrase. J.A. 110–11.
     The district court then held a bench trial on infringe-
 ment and on invalidity for anticipation or obviousness (as
 well as on damages issues, including willfulness, not before
 us). The court found that Bradford White directly and in-
 directly infringed claim 1. The infringement dispute
 turned on whether Bradford White (and its customers) per-
 formed, using specified Bradford White water heaters, the
 limitation regarding “permitting the products of combus-
 tion to rise out of the plenum . . . substantially entirely un-
 der the influence of natural convection” (the “permitting”
 limitation). J.A. 68–69. The court found that Bradford
 White (and its customers) performed the permitting limi-
 tation, relying on evidence, including expert testimony,
 that “the negative static pressure of the products of com-
 bustion leaving the plenum” and the accused products’
 “Category I certification” demonstrated that the products
 of combustion in the accused products rise out of the ple-
 num and into the vent “at a pressure near or below atmos-
 pheric pressure and without the influence of the power
 burner, such that a Category I venting system can be used.”
 See J.A. 42, 68–73.
     The court also rejected Bradford White’s invalidity
 challenges, including the contention that claim 1 is invalid
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 6                                A. O. SMITH CORPORATION v.
                               BRADFORD WHITE CORPORATION

 for anticipation by a prior-art patent to Tam (U.S. Patent
 No. 5,228,413). J.A. 94–97. Only the Tam-based anticipa-
 tion argument is before us, as to which it suffices for us to
 focus on whether Tam teaches the “dropping” limitation of
 the ’897 patent’s claim 1—requiring “dropping the pres-
 sure of the products of combustion to near atmospheric
 pressure within the plenum.” The district court held that
 the dropping limitation requires that the products of com-
 bustion be at a positive pressure before they enter the ple-
 num, so that the claim does not encompass a situation in
 which the products are already negative in pressure when
 they enter the plenum and drop to a more negative (but
 still “near atmospheric”) pressure. J.A. 62–64. Under that
 construction, the district court found that Tam does not
 disclose the dropping limitation (or render a modification
 to perform that limitation obvious, an issue not raised on
 appeal here). J.A. 95–99.
    Bradford White timely appealed. We have jurisdiction
 under 28 U.S.C. § 1295(a)(1).
                              II
     Claim construction is ultimately a question of law, de-
 cided de novo on review, as are the intrinsic-evidence as-
 pects of a claim-construction analysis. Teva Pharms. USA,
 Inc. v. Sandoz, Inc., 574 U.S. 318, 331–32 (2015). Subsidi-
 ary factual determinations based on extrinsic evidence are
 reviewed for clear error. Id.
                              A
                              1
      For infringement, the only dispute involves the permit-
 ting limitation. Bradford White challenges an aspect of the
 district court’s construction of that limitation, namely, the
 construction of “natural convection” as meaning “fluid mo-
 tion compatible with use of a Category I venting system.”
 J.A. 110; Bradford White’s Br. at 25; id. at 2 (statement of
 first issue); id. at 22 (summary of argument). Bradford
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 A. O. SMITH CORPORATION v.                                 7
 BRADFORD WHITE CORPORATION

 White argues that this construction improperly departs
 from the ordinary meaning of the claim term “natural con-
 vection” shown in a dictionary definition, without the as-
 sertedly required redefinition or disavowal of that
 dictionary meaning. Id. at 25–26. We reject this conten-
 tion.
     The premise of Bradford White’s argument is that the
 phrase “natural convection” is to be read in isolation to de-
 termine its ordinary meaning, with a dictionary definition
 of the phrase in isolation to be given controlling force un-
 less overcome by a clear enough redefinition or disavowal.
 That premise is incorrect. “‘[T]he context of the surround-
 ing words of the claim . . . must be considered in determin-
 ing the ordinary and customary meaning of those terms.’”
 Phillips v. AWH Corp., 415 F.3d 1303, 1314 (Fed. Cir. 2005)
 (en banc) (quoting with approval ACTV, Inc. v. Walt Disney
 Co., 346 F.3d 1082, 1088 (Fed. Cir. 2003)). Moreover, “the
 person of ordinary skill in the art is deemed to read the
 claim term not only in the context of the particular claim
 in which the disputed term appears, but in the context of
 the entire patent, including the specification.” Id. at 1313;
 see id. at 1315–16 (stressing central importance of specifi-
 cation). Here, these principles justify the district court’s
 construction of “natural convection” in the challenged re-
 spect.
      The claim phrase at issue, “natural convection,” is part
 of the requirement of permitting the combustion products
 to rise out of the plenum into the vent structure “substan-
 tially entirely under the influence of natural convection.”
 The phrase as a whole was construed to mean “[a]t a pres-
 sure near or below atmospheric pressure and without the
 influence of the power burner, such that a Category I vent-
 ing system can be used.” J.A. 118. Bradford White does
 not challenge the portion of that construction requiring
 pressure near or below atmospheric pressure; nor does it
 challenge the “influence” portion of the construction or the
 district court’s elaboration of that portion to preclude the
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 8                                 A. O. SMITH CORPORATION v.
                                BRADFORD WHITE CORPORATION

 burner from being a “significant influence” on the rising
 process at issue, J.A. 120; see also J.A. 111, which must oc-
 cur “substantially entirely” under the influence of natural
 convection, J.A. 119. This surrounding claim language, as
 construed in unchallenged respects, provides essential con-
 text for understanding the “natural convection” term as
 used in the integrated set of terms linking pressure and the
 degree of influence of the burner on the rising process. In-
 deed, Bradford White recognizes that the issue is not the
 construction of the phrase “natural convection” by itself but
 also the “such that a Category I venting system can be
 used” construction of the permitting limitation as a
 whole—which Bradford White has to challenge as well for
 its challenge to the construction of “natural convection” to
 matter. See Bradford White’s Br. at 33 n.3. As the district
 court explained, this claim-language context invites clari-
 fication from the specification, without the need for what
 would amount to redefinition or disclaimer of a meaning
 already plain from the claim itself. See J.A. 109–10 (look-
 ing to specification for “a baseline” for the “substantially
 entirely” requirement and “for determining whether pres-
 sure has been dropped [to] near atmospheric pressure”).
      The specification in this case provides that clarifica-
 tion, and it compellingly supports the district court’s read-
 ing of “natural convection” in context. The specification
 discloses the relationship of natural convection and atmos-
 pheric venting. See, e.g., ’897 patent, col. 4, lines 5–11.
 More pointedly, the specification is explicit that “the pre-
 sent water heater [] can be retrofitted into a Category I
 venting system despite the fact that the water heater [] uti-
 lizes a power burner.” Id., col. 4, lines 17–19; see also id.,
 col. 5, lines 26–29 (explaining that the invention can re-
 place “an existing atmospheric water heater”). A.O. Smith
 did not need to repeat itself to make the point clear in this
 short and to-the-point specification: A requirement of the
 claimed invention was compatibility with the installed
 base of Category I venting systems into which the water
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 A. O. SMITH CORPORATION v.                                  9
 BRADFORD WHITE CORPORATION

 heater operating according to the invented method could be
 “retrofitted.” 2
      For the foregoing reasons, we affirm the district
 court’s inclusion of its requirement concerning Category I
 compatibility in its claim construction.
                               2
     In Bradford White’s brief, the statement of the issue
 and the argument headings regarding the permitting limi-
 tation are limited to disputing the inclusion of the Category
 I requirement in the claim construction, as discussed
 above, and asserting that such inclusion, if error, was
 harmful, requiring a new trial. See Bradford White’s Br.
 at v–vi (headings); id. at 2 (statement of issues); id. at 25–
 33 (arguing that the claim construction of the permitting
 limitation is incorrect); id. at 33–39 (arguing that correct-
 ing the claim-construction error requires a new trial); see
 also id. at 22 (summary of argument). Bradford White does
 not present a separate argument to this court that the evi-
 dence cannot support the finding that it practices (or in-
 duces practice of) the permitting limitation even if we
 conclude that the Category I construction is correct, as we
 do.

     2   The district court adopted its claim construction
 before trial, but trial evidence later supported what the
 specification itself indicates about the relationship be-
 tween Category I compatibility and the claim. For exam-
 ple, A.O. Smith’s expert explained that Category I venting
 standards, which require a nonpositive vent pressure, J.A.
 242, are equivalent to exhaustion by natural convection
 and atmospheric venting, see J.A. 706–07; J.A. 714–15; J.A.
 747, and a Bradford White witness testified that “atmos-
 pheric vent[ing]” is synonymous with Category I venting,
 J.A. 780–81.
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 10                                A. O. SMITH CORPORATION v.
                                BRADFORD WHITE CORPORATION

     In its harmful-error argument, however, Bradford
 White contends that certain language in the district court’s
 findings and conclusions on the permitting limitation sug-
 gests a view—which Bradford White asserts is legal error,
 given the ’897 specification—that the “velocity or flow” of
 combustion products (in contrast to pressure) is simply im-
 material under the ’897 patent. Bradford White’s Br. at
 36–38. We need not and do not decide whether Bradford
 White has adequately raised a legal argument along these
 lines, or whether it is correct in such an argument, for con-
 sideration independently of its challenge to the Category I
 venting claim construction, which we have rejected. Even
 if we so assume, we see no basis to set aside the court’s
 determination that the permitting limitation was met here.
 We read the district court’s opinion as relying inde-
 pendently on an evidentiary finding that reasonably as-
 signed little if any probative weight to the evidence
 regarding velocity presented in this case and instead found
 that the evidence based on pressure, together with the
 meaning of the Category I certification of the accused prod-
 ucts, proves satisfaction of the permitting limitation.
      The district court held that both “the negative static
 pressure of the products of combustion leaving the plenum”
 in the accused products, and the products’ “Category I cer-
 tification,” persuasively prove satisfaction of the permit-
 ting step. J.A. 42. The court determined that, consistent
 with industry standards and Category I requirements, “an-
 alyzing pressure ‘is a definitive way’ of determining
 whether the products of combustion are substantially en-
 tirely under the influence of natural convection.” J.A. 22–
 23. It also credited A.O. Smith’s expert testimony that
 pressure measurements in the accused products indicated
 that the products of combustion dropped within the plenum
 to a pressure that would allow them to rise into the vent
 substantially entirely under the influence of natural con-
 vection. J.A. 41. And it explained that “[t]he negative pres-
 sure at the plenum exit means that there is no longer any
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 A. O. SMITH CORPORATION v.                                 11
 BRADFORD WHITE CORPORATION

 significant power burner influence on the products of com-
 bustion at that point.” J.A. 68–69. In addition to consider-
 ing pressure measurements, the court found that Bradford
 White’s relevant products have been certified as Category
 I compliant. J.A. 69. And it found that, if the power burner
 exerted significant influence in the plenum of the accused
 products, there would be “spilling, leakage, and an inability
 to obtain Category I compliance.” Id.
     The district court also gave evidentiary reasons, with
 sufficient record support, for not crediting the velocity
 measurements introduced by Bradford White. The district
 court credited testimony from A.O. Smith’s expert that, in
 the view of a relevant artisan, “velocity cannot be used to
 judge the influence of the blower,” J.A. 72, because velocity
 can “be a product of both natural convection and anything
 else,” J.A. 38 (quoting J.A. 868), rather than an indication
 of blower influence specifically. The district court also re-
 jected Bradford White’s velocity measurements because its
 expert, who did not qualify as a relevant artisan at the pri-
 ority date, see J.A. 26, 70, did not measure velocity in the
 products’ “steady-state, intended operation,” J.A. 73.
      We have been shown no clear error in the foregoing
 findings, for which the district court pointed to clear sup-
 porting evidence. More generally, we have been shown no
 clear error in the overall finding that the Category-I-com-
 pliant accused products permit venting by natural convec-
 tion without a significant influence of the burner, thus
 meeting the permitting limitation. For these reasons, we
 affirm the district court’s determination that Bradford
 White directly and indirectly infringes claim 1.
                               B
      For invalidity, the only dispute is over anticipation by
 Tam. The dispositive issue is whether Tam teaches the
 dropping limitation of the ’897 patent’s claim 1. The dis-
 trict court held that it does not, relying on its construction
 of the dropping limitation—“dropping the pressure of the
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 12                                A. O. SMITH CORPORATION v.
                                BRADFORD WHITE CORPORATION

 products of combustion to near atmospheric pressure
 within the plenum”—as requiring the pressure of the com-
 bustion products to be positive before entering the plenum.
 J.A. 63, 96. Bradford White challenges that claim construc-
 tion. Bradford White’s Br. at 39. We reject this challenge.
      We agree with the district court that a “drop[] . . . to
 near atmospheric pressure within the plenum” is most rea-
 sonably read to mean a drop from outside to inside the
 range defined by “near atmospheric pressure.” See J.A. 63–
 64. That is the ordinary-language meaning, which does not
 naturally encompass a situation where the pressure before
 the combustion products enter the plenum is already below
 atmospheric, i.e., negative (in the “near atmospheric”
 range), and the pressure then drops to still further below
 atmospheric pressure (but still in the “near atmospheric”
 range) when the products are in the plenum. Expert testi-
 mony explaining how a relevant artisan would interpret
 the claim phrase, credited by the district court, made the
 same point. J.A. 63–64; J.A. 959–60. This construction is
 also consistent with the specification. The patent explains
 that the products of combustion are forced into the flue “un-
 der positive pressure,” ’897 patent, col. 3, lines 15–18, and
 as they exit the flue, they “enter the plenum 70 under the
 influence of the power burner 60, but exhaust from the ple-
 num 70 without the influence of the power burner 60,” id.,
 col. 4, lines 14–17. This central description of the overall
 operation of the invention suggests that the products are
 at positive pressure (outside the “near atmospheric” range)
 before entering the flue due to burner influence, and drop
 to negative pressure (in the “near atmospheric” range) only
 in the plenum, so that the process satisfying the permitting
 limitation may occur. See J.A. 31; J.A. 866; J.A. 966.
     Bradford White contends that the district court’s
 adopted construction reads out the “creep and spill” embod-
 iment from the scope of claim 1. But we agree with the
 district court that the claim language at issue here is clear
 enough that the proper construction would not be altered
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 even if one embodiment set forth in the specification were
 not covered. See J.A. 64 (citing TIP Sys., LLC v. Phillips &
 Brooks/Gladwin, Inc., 529 F.3d 1364, 1373 (Fed. Cir. 2008)
 (claims often do not encompass all embodiments)). Moreo-
 ver, Bradford White has not shown that a relevant artisan
 would have understood that, in the identified embodiment,
 the pressure of the combustion products before they enter
 the plenum would actually be negative. Bradford White
 relies on certain testimony from its expert, but, as already
 noted, the district court determined that the expert did not
 qualify as a relevant artisan at the priority date, a finding
 unchallenged on appeal. And the testimony was only that,
 in the embodiment at issue, the products would have “very
 little” energy and pressure and would be “near” atmos-
 pheric pressure at the flue exit, J.A. 1316–17, testimony
 not tied to the claim construction of “near atmospheric
 pressure,” J.A. 109–10, a construction that is not chal-
 lenged on appeal. This testimony does not justify depart-
 ing from the claim construction of the dropping limitation
 adopted by the district court and used to reject anticipation
 by Tam.
     There is no dispute in this appeal that, if the district
 court’s construction of the dropping limitation is correct,
 Tam does not anticipate the challenged claim. We thus af-
 firm the court’s judgment of no invalidity.
                              III
     For the foregoing reasons, we affirm the court’s final
 judgment.
     The parties shall bear their own costs.
                        AFFIRMED
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         NOTE: This disposition is nonprecedential.

    United States Court of Appeals
        for the Federal Circuit
                   ______________________

     A. O. SMITH CORPORATION, AOS HOLDING
                     COMPANY,
                  Plaintiffs-Appellees

                              v.

         BRADFORD WHITE CORPORATION,
               Defendant-Appellant
              ______________________

                         2021-2222
                   ______________________

     Appeal from the United States District Court for the
 District of Delaware in No. 1:18-cv-00412-LPS, Judge
 Leonard P. Stark.
                 ______________________

     DYK, Circuit Judge, concurring.

     I join section II.B of the majority’s opinion finding no
 invalidity of the dropping limitation. With respect to the
 permitting limitation, I agree with the result reached by
 the majority to affirm the district court’s infringement find-
 ing but by a different path.
     The district court’s original construction of the permit-
 ting limitation came inbi response to defendant Bradford
 White Corporation’s (“Bradford”) indefiniteness challenge.
 The construction appeared to make Category I compliance
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 2                                 A. O. SMITH CORPORATION v.
                                BRADFORD WHITE CORPORATION

 a necessary, but not sufficient, condition of the permitting
 limitation. J.A. 118 (“At a pressure near or below atmos-
 pheric pressure and without the influence of the power
 burner, such that a Category I venting system can be
 used.”) (emphasis added). The same is true for the district
 court’s subsequent construction of the permitting limita-
 tion before trial. J.A. 110 (“[F]luid motion compatible with
 use of a Category I venting system.”) (emphasis added).
       After trial, the district court appeared to hold for the
 first time that satisfying the Category I compliance stand-
 ard itself established infringement of the permitting limi-
 tation. It found that “if the products of combustion were
 still under the influence of the power burner . . . the Ac-
 cused Products could not be certified as Category I compli-
 ant and, consequently, a Category I venting system could
 not be used,” J.A. 42 (internal quotations omitted), ulti-
 mately concluding that “[t]he evidence demonstrate[d] that
 . . . there [was, at the time of venting] no longer . . . any
 remaining significant influence from the power burner on
 the products of combustion; natural convection has taken
 over such that Category I compliance can be achieved,” J.A.
 69. See also J.A. 70 (“The patent is about enabling Cate-
 gory I venting. . . .”).
     Unlike the majority, which approves of the post-trial
 Category I construction, this seems to me to be an incorrect
 claim construction. The claim language on its face requires
 that the influence of the blower be substantially entirely
 eliminated so that the products of combustion rise out of
 the plenum “substantially entirely under the influence of
 natural convection.” ’897 patent, col. 6, ll. 22–23. That the
 permitting limitation as a matter of claim construction can-
 not be satisfied simply by the gasses being at a negative
 pressure is shown by the fact that the claim includes an
 additional and separate limitation (the “dropping” limita-
 tion) requiring negative pressure. See id., col. 6, ll. 18–19.
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      The specification is not to the contrary. Its sole refer-
 ence to Category I venting merely states that the water
 heater is able to achieve Category I compliance because it
 “can be retrofitted into a Category I venting system,” id.,
 col. 4, ll. 17–18, not that Category I compliance is sufficient
 for the products of combustion to rise out of the plenum
 without the influence of the power burner.
     Nonetheless, as the majority points out, the district
 court did not rely solely on claim construction as to Cate-
 gory I compliance to establish infringement. Its conclusion
 that the power burner’s influence had been sufficiently
 eliminated was also based on a factual finding that evi-
 dence of negative pressure showed a lack of influence from
 the power burner. The district court credited expert testi-
 mony that “analyzing pressure is a definitive way of deter-
 mining whether the products of combustion are
 substantially entirely under the influence of natural con-
 vection,” J.A. 23 (internal citations and quotations omit-
 ted), and tests revealing “positive static pressure of the
 products of combustion entering the plenum” and “negative
 static pressure of the products of combustion leaving the
 plenum [],” J.A. 41. According to the district court, the
 measured negative pressure showed that “there [was] no
 longer any significant power burner influence . . . at that
 point.” J.A. 68–69 (internal citations omitted).
     This factual determination seems to me open to ques-
 tion since the patent itself appears to assume that negative
 pressure does not preclude influence of the power burner.
 But as the majority points out, Bradford does not challenge
 this fact finding on appeal. Under these circumstances, the
 district court’s factual finding is sufficient to affirm the
 judgment of infringement with respect to the permitting
 limitation.