Gas valves have evolved over time - a traditional fixed output central heating boiler uses a multi functional gas valve; the purpose of which is threefold:
1- To provide a means of shutting off the gas supply in a no gas situation.
2- To control the gas supply to the burner on temperature change.
3- To control the gas pressure supplied to the burner.
The valve will consist of a manually operated flame supervision device, a solenoid operator and a constant pressure governor.
The manual flame supervision device is a thermocouple which is located in the pilot flame burner and an electromagnet unit which is placed in the inlet to the gas valve. The thermocouple is made of two different metal parts that produce an electrical current of 30mV D.C when heated. The electromagnet unit contains a solenoid that creates a magnetic field when electrically charged - it is this magnetic field that holds the gas valve in the open position. If the pilot light is extinguished the thermocouple cools and consequently the electromagnet de-energises and the gas valve is shut by a return spring.
Modern boilers now use flame rectification or ionisation for flame proving.
Flame rectification works on the principal by way of a small electrical current (micro-amp A.C.)which is passed through a flame by a relatively high voltage. As the small electrical current passes through the flame an ionisation process takes place where the current is converted to D.C. and it is returned to the PCB by either the neutral or the earth circuit. The PCB then can detect that the returning micro-amp has been rectified and therefore has passed through a flame.
Solenoid operator fitted to gas valves are operated by a magnetic coil. When an electrical current passes through the coil it becomes magnetic which consequently lifts an armature and opens a pathway for the gas to flow. Combination boilers normally have two for safety just in case one solenoid sticks open. Some gas valves use the second solenoid as a step up modulator. A gas valve with a fixed output is not suitable for a combination boiler as we need the gas valve to adjust its output very accurately to ensure that the hot water delivery temperature remains constant. This is achieved by replacing the constant pressure governor with a modulating gas regulator.
Modulating Gas controls (Modu-Regs) are an electronically operated solenoid coil which is fitted to the top side of a constant pressure governor replacing the spring loading. A rod passing through the coil moves in relation to the voltage changes applied to the coil. The voltage changes are made through the thermistor and the potentiometer by way of the PCB. The movement of the rod either increases or decreases the loading on the governor - when there is no voltage the Modu-Reg is in a minimum gas rate position and when the maximum voltage is applied the Modu-Reg is in a maximum gas rate position.