Abstract: A hot gas water heater has a housing, and at least a pair of concentric coils in the housing through which liquid to be heated is sequentially passed. Each coil has a helical tube with adjacent turns connected by a web member such that the tube and web member form an imperforate coil wall. Hot gas passes sequentially between the coils in co-current flow with liquid flow in one adjacent coil and counter-current flow with liquid flow in another adjacent coil.