Source : Webster's Revised Unabridged Dictionary (1913)

Water back \Wa"ter back`\
   See under 1st {Back}.

Back \Back\, n. [F. bac: cf. Arm. bak tray, bowl.]
   1. A large shallow vat; a cistern, tub, or trough, used by
      brewers, distillers, dyers, picklers, gluemakers, and
      others, for mixing or cooling wort, holding water, hot
      glue, etc.

   {Hop back}, {Jack back}, the cistern which receives the
      infusion of malt and hops from the copper.

   {Wash back}, a vat in which distillers ferment the wort to
      form wash.

   {Water back}, a cistern to hold a supply of water; esp. a
      small cistern at the back of a stove, or a group of pipes
      set in the fire box of a stove or furnace, through which
      water circulates and is heated.

   2. A ferryboat. See {Bac}, 1.

Source : WordNet®

water back
     n : water heater consisting of a tank or pipes set at the back
         of a fireplace or in the firebox of a stove