Instantaneous sanitizing water heating unit

Claim 1

I claim instantaneous sanitizing water heating unit comprising: a. a base; b. an air-vented firebox supported on said base; c. a heat exchanger having continuous side and end walls upstanding from said base above said firebox, an open bottom in heat-receiving communication with the firebox, and a cover topped by a flue; d. an opposed pair of said walls each of which comprises a pair of inner and outer horizontally spaced vertical plates defining a water plenum between each pair of plates; e. a plurality of vertically spaced rows of horizontal water tubes extending through a heating zone between the two inner plates and providing communication between said water plenums; f. each plenum being vertically divided by horizontal separators into a series of plenum compartments enabling flow of water from an upper, inlet plenum compartment to a lower, outlet plenum compartment in a series of back and forth flows through progressively decreasing numbers of said tubes to progressively increase the velocity and turbulence of water in the tubes from the top to the bottom of the heat exchanger; g. a recycle conduit containing a water pump having its inlet and outlet connected respectively to said outlet and inlet plenum compartments; h. a cold water makeup input conduit connected to said inlet plenum compartment; i. makeup water valve control means in said input conduit; j. a hot water output conduit connected to said pump outlet; k. output water valve control means in said output conduit; l. combustion means in said firebox; and m. means for controlling said combustion means in response to temperature of the heated water.