A fluid heating apparatus for use in water heaters, boilers, cooking applications or the like comprising a body. The fluid heating apparatus also comprises a primary heat exchanger arranged within the body and a secondary heat exchanger arranged adjacent to the primary heat exchanger within the body. The apparatus also comprises a burner arranged within the primary heat exchanger. The primary heat exchanger comprises a plurality of concentric doughnut shaped plates arranged over the burner. A blocking plate is arranged adjacent to one of the concentric plates of the primary heat exchanger. The secondary heat exchanger comprises a plurality of concentric doughnut shaped plates arranged adjacent to the blocking plate on a side opposite of the plates of the primary heat exchanger. The apparatus further comprises a manifold in fluid communication with the plurality of concentric plates and blocking plates of both the secondary heat exchanger and primary heat exchanger.