1. An apparatus for producing hot fluids, comprising a heat exchanger and burner means associated therewith, said burner means including a perforated screen of refractory material, means for directing a stream of fuel and primary air against one side of said screen, means for directing a stream of secondary air against the opposite side of said screen and comprising a substantially mushroom-shaped hollow body connected with air supply means and having an outlet facing said screen, the two streams being deflected along said screen to form a substantially annular flame at the circumference thereof, said burner means being mounted at the upper end of a substantially vertically disposed gas passageway in said heat exchanger, said passageway having a substantially annular cross-section and having its upper end adjacent the circumference of said mushroom-shaped body, the lower end of said passageway communicating with the lower end of an outer combustion gas passageway having an annular crosssection co-axially surrounding said first-named combustion gas passageway, the upper end of said outer combustion gas passageway extending along the wall of said mushroom-shaped body and communicating with an outlet, means defining an outer fluid space between the outer wall of said first-named combustion gas passageway and the inner wall of said outer combustion gas passageway, and means defining an inner fluid space within the inner wall of said first-named combustion gas passageway, said fluid spaces communication with each other.