(57) The apparatus has a horizontal tunnel-like passage for hot combustion gases from
a fuel burner (52) to a flue outlet (57). The passage is defined by several adjacent
but discrete helical coils (59) each comprising from 3 to 12 turns of water conduit,
such as copper tubing. A common source (54) feeds water separately to all the coils
which in turn discharge heated water separately to a common outlet (55). In one embodiment,
an additional outer set of coils (60), similarly connected, may create an annular
space around the tunnel-like passage and an exterior water jacket (50) a second, outermost
annular space. These spaces and the tunnel-like passage can be connected in various
ways to impart series or parallel flow to the hot combustion gases therein. The apparatus
is suitable for use in central heating systems.
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