Method of providing heat

A method of providing heat to promote an endothermic reaction or be extracted as sensible heat, the method comprising providing a first passage having a first inlet thereto and a first outlet therefrom, when considering fluid flow along said first passage towards the first outlet from the first inlet said first passage having an upstream portion leading towards a downstream portion of said first passage, a second passage having a second inlet thereto and a second outlet therefrom, a third passage having a third inlet thereto and a third outlet therefrom, a heat conducting wall provided between said upstream portion of the first passage and the second passage and between said downstream portion of the first passage and the third passage, introducing combustible gas and oxidant into the upstream portion of the first passage wherein said combustible gas and oxidant are heated by heat conducted through said wall from the second passage, burning the combustible gas with said oxidant in said downstream portion of the first passage, supplying the resultant products of combustion to the second inlet to the second passage for said products of combustion to flow along the second passage to said second outlet wherein the products of combustion give up heat which is conducted through said wall to said upstream portion of the first passage to heat further introduced said combustible gas and oxidant, and either (i) introducing one or more fluid substances into the third passage through the third inlet to undergo an endothermic chemical reaction using heat conducted through said wall from said downstream portion of the first passage such that products of the reaction leave through said third outlet or (ii) introducing a heatable fluid into the third passage through the third inlet to receive heat conducted through said wall from said downstream portion of the first passage such that the heatable fluid becomes heated and thereafter leaves through the third passage bearing sensible heat.