The flame tube-flue tube boiler has two horizontal flame tubes (3) side by side leading to a smoke box (2, 4, 6) inside the boiler. The upper portion (6) of the box wall forms a single curve over both flame tubes whilst the lower portion (2) consists of two sections each of which is curved cylindrically over at least 180 deg. around one flame tube. The sections are connected (4) so as to form a downwardly open tunnel (8) between, and extending to the upper wall level of, the tubes (3). The top (4) of the tunnel may be connected to the top of the box wall by tubes (5). The tubes (5) may be welded to steel spacing strips to form a tube wall baffle.
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