Water heater having highly efficient and compact heat exchanger

A heat exchanger comprising:

• an inner coil having a plurality of helically wound inner coil turns, the inner coil defining a central axis extending therethrough;
• an outer coil having a plurality of helically wound outer coil turns, the outer coil positioned circumferentially around the inner coil relative to the central axis;
• an end member positioned over a last one of the plurality of inner coil turns of the inner coil and a last one of the plurality of outer coil turns of the outer coil;
• a first baffle overlaying spaces between adjacent turns of the plurality of inner coil turns;
• a second baffle including a wall positioned between the inner coil and the outer coil, the second baffle extending in a first direction parallel to the central axis from a first one of the plurality of inner coil turns and a first one of the plurality of outer coil turns toward the end member to an edge that is spaced below at least a portion of the last one of the plurality of inner coil turns;
• and
• a flue gas bypass path defined between the edge of the second baffle and the end member, the flue gas bypass path having a width at least equal to a helical pitch of the inner coil;
• wherein flue gases are configured to flow radially outward over the inner coil, in the first direction along the second baffle wall, through the flue gas bypass path, and then in a second direction opposite the first direction over the outer coil to heat water flowing through the inner coil and the outer coil, and
• wherein the heat exchanger further comprises a transfer conduit connected between an end of the last one of the plurality of outer coil turns and an end of the last one of the plurality of inner coil turns.

A method of heating water using a heat exchanger, the method comprising:

• providing an inner coil of an inner water tube having a plurality of helically wound inner coil turns, the inner coil defining a central axis extending therethrough, an outer coil of an outer water tube having a plurality of helically wound outer coil turns, the outer coil positioned circumferentially around the inner coil relative to the central axis, an insulation layer positioned over a last one of the plurality of inner coil turns of the inner coil and a last one of the plurality of outer coil turns of the outer coil, and a drum baffle positioned between the inner coil and the outer coil;
• conducting the water first through the outer coil and subsequently through the inner coil;
• generating flue gases with a burner;
• flowing the flue gases radially outward over the inner coil to heat the water flowing through the inner coil;
• flowing the flue gases in a first direction parallel to the central axis along a wall of the drum baffle toward the insulation layer;
• directing the flue gases through a flue gas bypass path arranged between the insulation layer and a terminal edge of the drum baffle;
• and
• flowing the flue gases in a second direction opposite the first direction over the outer coil to heat the water flowing through the outer coil,
• wherein conducting the water first through the outer coil and subsequently through the inner coil comprises flowing the water through the outer coil at a first flow velocity and flowing the water through the inner coil at a second flow velocity greater than the first flow velocity.