Air regulator (1) upstream of the burner comprising a detector having a diaphragm (3) suspended in a chamber (4) which is subjected on one side (12) to the pressure of the air supply to the burner and which is subjected on the other side (11) to the pressure of the air or gas flow downstream of the burner. A change in pressure in the combustion chamber of the burner causes a change in the equilibrium position of the diaphragm (3). There is a stepping motor connected to the regulator (1) whereby movement of said motor causes a change in the amount of air flowing through the regulator and means (7, 8) for actuating the stepping motor in response to a change in the equilibrium position of the diaphragm (3) or a change in the equilibrium position of the diaphragm above a predetermined level so that the pressure difference, which may be zero, between the air supply to the burner and the air or gas flow downstream of the burner remains substantially constant.