The Honeywell VR4205 direct-spark ignition (DSI) combination gas valve shown in Figure 5-51 contains a safety shutoff, a manual valve, two automatic operators, a pressure regulator, and a pilot adjustment device. Some models use a single electrode for spark ignition and flame sensing; others use separate electrodes for spark ignition and flame sensing. The Honeywell VR4205 gas control is also designed to provide on-off manual control of gas flow. In the off position, gas flow to the main burner is mechanically blocked. In the on position, gas flows to the main burner under control of the thermostat, the direct-spark ignition (DSI) module, and the two automatic main valves.
The operation of a Honeywell VR4205 valve gas control is illustrated in Figures 5-52 and 5-53. When the thermostat calls for heat, the DSI module is energized. The module activates the first and second automatic valves of the gas control, which allow gas to flow to the main burner. At the same time, the DSI module generates a spark at the igniter-sensor to light the main burner. The second automatic valve diaphragm, controlled by the servo pressure regulator, opens and adjusts gas flow as long as the system is powered. The servo pressure regulator monitors outlet pressure to provide an even flow of gas to the main burner.
Loss of power (thermostat satisfied) deenergizes the DSI module and closes the automatic valves. The first automatic valve and the second automatic valve operator close, bypassing the regulator(s) and shutting off the main burner and the pilot. As pressure inside the gas control and underneath the automatic valve diaphragm equalizes, spring pressure closes the second automatic valve to provide a second barrier to gas flow. The system is now ready to return to normal service when the thermostat again calls for heat and power is restored.
Some gas control modules are offered with slow-opening or step-opening regulation of the gas flow. Slow-opening gas controls function the same as standard models except that when the thermostat calls for heat, the second automatic valve opens gradually. Step-opening gas controls actually combine two pressure regulators, one for the low pressure and one for the full-rate pressure.