A gas heater includes a premix burner, a supply channel for supplying a mixture of a gaseous fuel and air to the burner, and a check valve placed in the supply channel upstream of the burner. The check valve includes a moveable valve body having a first end position in which the check valve is opened and a second end position in which the check valve is closed. In a first pressure condition in which a pressure upstream is higher than a pressure downstream, the valve body is urged to the first end position. In a second pressure condition in which the pressure upstream is lower than the pressure downstream the valve body is urged to the second end position. After closure of the check valve the check valve remains closed for a delay time.