"but it vented to the atmosphere"

... and that combined with the ready access to the oxygen caused a draught that intensified the heat, like in a blast furnace, an object which is also larger that your defintion of 'confined space', but gets so hot internally when fired by low temperature burning fuels that it even melts steel rather nicely...

Confined

Confine \Con*fine"\ (k[o^]n*f[imac]n"), v. t. [imp. & p. p.
   {Confined}; p. pr. & vb. n. {Confining}.] [F. confiner to
   border upon, LL. confinare to set bounds to; con- + finis
   boundary, end. See {Final}, {Finish}.]
   To restrain within limits; to restrict; to limit; to bound;
   to shut up; to inclose; to keep close.
   [1913 Webster]

         Now let not nature's hand
         Keep the wild flood confined! let order die! --Shak.
   [1913 Webster]

         He is to confine himself to the compass of numbers and
         the slavery of rhyme.                   --Dryden.
   [1913 Webster]

   {To be confined}, to be in childbed.

   Syn: To bound; limit; restrain; imprison; immure; inclose;
       circumscribe; restrict.
       [1913 Webster]

        -- From The Collaborative International Dictionary of English v.0.48

confined \confined\ adj.
   1. having movement restricted to within a certain area; --
      usually a building. Opposite of {unconfined}.

   Note: [Narrower terms: {claustrophobic}; {close, confining};
         {homebound, housebound, shut-in}; {in
         childbed(prenominal)}; {pent, shut up(predicate)};
         {snowbound}; {weather-bound}; {stormbound,
         storm-bound}]
         [WordNet 1.5]

   2. deprived of liberty; especially placed under arrest or
      restraint.
      [WordNet 1.5]

   3. having movement restricted to within an enclosed outdoor
      area; -- of animals.

   Syn: fenced in, penned.
       [WordNet 1.5]

   4. (Med.) not invading healthy tissue.
      [WordNet 1.5]

   5. held prisoner.

   Syn: captive, imprisoned, jailed.
       [WordNet 1.5]

   6. having movement or progress restricted to a certain area;
      as, an outbreak of the plague confined to one quarter of
      the city; wildfires confined to within the canyon.
      [PJC]

        -- From The Collaborative International Dictionary of English v.0.48

confined
    adj 1: not free to move about [ant: {unconfined}]
    2: enclosed by a confining fence [syn: {fenced in}, {penned}]
    3: not invading healthy tissue [ant: {invasive}]
    4: deprived of liberty; especially placed under arrest or
       restraint
    5: in captivity [syn: {captive}, {imprisoned}, {jailed}]

        -- From WordNet (r) 2.0

113 Moby Thesaurus words for "confined":
   angustifoliate, angustirostrate, angustisellate, angustiseptal,
   authoritative, barred, bedfast, bedridden, beleaguered, beset,
   besieged, blockaded, bound, bounded, boxed in, cabined, caged,
   circumscribed, cloistered, close, close-fitting, closed-in,
   conditioned, constricted, cooped, copyrighted, cordoned,
   cordoned off, corralled, cramp, cramped, cribbed, crowded,
   detained, disciplined, down, enclosed, expert, feature, featured,
   fenced, finite, geographically limited, hedged, hedged about,
   hedged in, hemmed, hemmed in, hospitalized, icebound, immured,
   impounded, imprisoned, in childbed, in confinement, in hospital,
   incapacious, incarcerated, incommodious, insular, invalided,
   isthmian, isthmic, jailed, kept in, knowledgeable, laid up,
   landlocked, leaguered, limited, local, localized, meager, mewed,
   moderated, narrow, near, of a place, paled, parochial, patented,
   penned, pent, pent-up, prescribed, proscribed, prostrate,
   provincial, qualified, quarantined, railed, restrained, restricted,
   scant, scanty, shut-in, sick abed, slender, snowbound, specialist,
   specialistic, specialized, stinted, strait, straitened, technical,
   tight, topical, under restraint, vernacular, walled, walled-in,
   windbound




        -- From Moby Thesaurus II by Grady Ward, 1.0


'Ya burn 70 tons, and whadda ya get?'