Experiments have been performed on fully premixing multiport burners closely representative of those used in modern domestic heating systems to study the ways in which the detail design of the burner head affects the incidence of unwanted thermoacoustic oscillations in practical installations. The effects of changes in the spacing of the flame ports, the velocity profile of the mean flow within the ports and the topography of the firing surface on the acoustic impedance of the burner and the transfer function of the flame array are analysed separately. In some of the experiments diagnostic changes have also been made to the fuel:air ratio of the mixture burned. The results confirm practical experience that small changes to the head design can have very major influences on the stability of complete systems but show also that the effects of these design changes are not well represented by simple models.