Abstract: A control system for a gas burner of a domestic hot water system has a normally closed gas valve for controlling the supply of gas to the burner, and an electric solenoid for actuating the valve when energized by current of a first predetermined value. The solenoid can be held open by a current of a second predetermined value lower than the first. The solenoid current is derived from a mains a.c. supply through two thermistors in series. One of these thermistors has a negative temperature coefficient of resistance and provides a short delay to the operation of the solenoid when heat is initially demanded. The second thermistor, having a positive temperature coefficient of resistance, controls the valve lift time, that is to say, the limited time during which gas is supplied to the burner for enabling an attempt to ignite the burner to be made. The valve lift time is automatically reduced if the system is still hot from a previous operation, thereby preventing a possible explosion hazard.