Heating apparatus for rooms or premises having a frame delimiting a combustion chamber of a fuel, said frame including a main part as well as a door constrained to said main part and movable with respect to the latter between an open position and a closed position of said combustion chamber, said apparatus further comprising a transmission circuit of primary air to be heated and conveyed in an environment external to said heating apparatus as well as a circuit for conveying combustion fumes generated in said combustion chamber, said transmission circuit and said conveying circuit being at least for part of their extension in heat exchange ratio between them, so that the air conveyed in said transmission circuit is heated by said combustion fumes in said conveying circuit, said transmission circuit comprising at least one first section and at least one second section, while said conveying circuit comprises at least one first portion in heat exchange ratio with said at least one first section as well as at least one second portion in a heat exchange ratio with said at least one second section, wherein said at least one first section and said at least one first portion extend along a first direction (A-A), while said at least one second section and said at least one second portion extend or have a main direction along a second direction (B-B) inclined with respect to the first direction (A-A) of main development of said at least one first section and of said at least one first portion, and/or the relative arrangement between said at least one first section and said at least one first portion is different from that between said at least one second section and said at least one second portion, whereby the primary air in the passage from the first to the second section and the combustion fumes in the passage from the first to the second portion undergo a variation not only or not necessarily directional, but also relative to the shape and/or size of the passage section of the respective circuit (5, 6), said apparatus being characterized in that - said at least one first portion is internal to said at least one first section and said at least one second section is internal or in any case not enveloping or not including said at least one second portion, or - said at least one first section is internal to the first portion and the second portion is internal or in any case not enveloping or not including said at least one second section, or - said at least one second portion is internal to said at least one second section and said at least one first section is not internal and not enveloping or not including said at least one first portion, or - said at least one second section is internal to said at least one second portion and said at least one first section is not internal and not enveloping or not including said at least one first portion. said at least one first section and said at least one first portion extend along a first direction (A-A), while said at least one second section and said at least one second portion extend or have a main direction along a second direction (B-B) inclined with respect to the first direction (A-A) of main development of said at least one first section and of said at least one first portion, and/or the relative arrangement between said at least one first section and said at least one first portion is different from that between said at least one second section and said at least one second portion, whereby the primary air in the passage from the first to the second section and the combustion fumes in the passage from the first to the second portion undergo a variation not only or not necessarily directional, but also relative to the shape and/or size of the passage section of the respective circuit (5, 6), said apparatus being characterized in that - said at least one first portion is internal to said at least one first section and said at least one second section is internal or in any case not enveloping or not including said at least one second portion, or - said at least one first section is internal to the first portion and the second portion is internal or in any case not enveloping or not including said at least one second section, or - said at least one second portion is internal to said at least one second section and said at least one first section is not internal and not enveloping or not including said at least one first portion, or - said at least one second section is internal to said at least one second portion and said at least one first section is not internal and not enveloping or not including said at least one first portion. - said at least one first portion is internal to said at least one first section and said at least one second section is internal or in any case not enveloping or not including said at least one second portion, or - said at least one first section is internal to the first portion and the second portion is internal or in any case not enveloping or not including said at least one second section, or - said at least one second portion is internal to said at least one second section and said at least one first section is not internal and not enveloping or not including said at least one first portion, or - said at least one second section is internal to said at least one second portion and said at least one first section is not internal and not enveloping or not including said at least one first portion.