An experimental study of the effects of using triangular burner exit ports in place of the standard circular ports in a laminar partially premixed gas burner is presented. A natural gas burner employed in a small residential-scale heating system rated at 5.86 kW was modie ed. Air entrainment into the nonburning jets, emission indices of nitrogen oxides and carbon monoxide, and ine ame temperature and composition proe les were measured. Flowe eld in the near-nozzle region was visualized by shadowgraphy. Results show that triangular ports increase air entrainment by 30%, decrease nitrogen oxides emission by less than 15%, and increase carbon monoxide emission by 20%. The effects are explained in terms of the changes in e ow structure and instabilities caused by the noncircular geometry and sharp corners of the burner exit port. Nomenclature D = area equivalent diameter of burner exit port EI = emission index HHV = heating value M = molecular weight n = number of moles PA = primary air r = radial distance from the centroid SA = secondary air T = temperature Tri = triangular port X = mole fraction Subscripts a = air f = fuel