The Collaborative International Dictionary of English v.0.48:

Water back \Wa"ter back`\
   See under 1st Back.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Back \Back\, n. [F. bac: cf. Arm. bag, bak a bark, D. 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.
      [1913 Webster]

   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.
      [1913 Webster]

   2. A ferryboat. See Bac, 1.
      [1913 Webster]

WordNet (r) 3.0 (2006):

water back
    n 1: water heater consisting of a tank or pipes set at the back
         of a fireplace or in the firebox of a stove