to the valve 33 causing the latter to be opened to supply gaseous fuel when pressure above atmospheric conditions exist in the associated conduit 36 conversely when the pressure in the associated conduit 36 is below atmospheric pressure the valve 33 closes to discontinue supply of gaseous fuel to the respective combustor regenerator section 11 the work or load 51 to be heated by the burner system 10 is schematically illustrated in fig1 the hot surface 31 of each combustor unit 11 formed by the respective screens 21 and underlying porous material 20 exposed at the holes of such screen face the work 51 preferably so that rays normal to these hot surfaces are aimed to the extent possible at such work in the illustrated case the hot surfaces 31 are substantially flat operation of the combustor regenerator system 10 is commenced by igniting a combustible mixture of air or oxygen and gaseous fuel such as natural gas in one of the combustor regenerator units 11 by a conventional ignitor not shown adjacent the associated eventually hot surface it will be apparent from the foregoing that air is delivered through the burner cold side 38 and porous regenerator zone 17 by the supply blower 39 through the valve 37 gaseous fuel is delivered by the associated valve 33 responding to pressure in the respective conduit 36 combustion occurs in the porous combustor zone 16 and its temperature rises to operational levels combustion continues at.