Written on: November 4, 2020 by Timmie McElwain
Part One appeared in the September/October 2020 Indoor Comfort.
Operation
The VR8345 gas controls, Figure 9, provide an “On-Off” manual control of gas flow. In he Off position, pilot and/or main burner gas flow is prevented. In the On position, pilot and/or main burner gas flow is under control of the thermostat, the Direct Spark Ignition System (DSI) module, Hot Surface Ignition System (HSI) module/intermittent pilot module and the gas valve.
Direct Spark Ignition/Hot Surface Ignition Applications
System operation
When the thermostat calls for heat, the DSI or HSI module is energized, Figures 10 & 11. The module activates the first and second automatic valves of the gas control, which allows main burner gas flow. At the same time, the DSI/HSI 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 main burner gas flow. Loss of power (thermostat satisfied) de-energizes the DSI/HSI module and closes the automatic valves. The system is ready to return to normal service when power is restored through the thermostat.
If the igniter-sensor stops detecting a flame at the main burner (lack of adequate microamp signal back to the module), then the trial for ignition is restarted. On DSI/HSI modules with lock-out timers, the automatic valves are de-energized and ignition stops after the lock-out period. On modules without lock-out timers, the trial for ignition continues indefinitely and the first automatic valves remain open.
If the main burner flame is restarted successfully, operation continues as described above. Gas control operation is described in more detail below.
Valve position during thermostat “Off” cycle
The valve is positioned as shown in Figure 12 when the:
• Gas control knob is in the On position.
• Thermostat is not calling for heat.
The first automatic valve is closed. The second automatic valve operator is de-energized, closing the channel to the pressure regulator, and opening a channel to the underside of the second automatic valve diaphragm. The combination of spring pressure under the second automatic valve diaphragm and lack of outlet pressure hold the diaphragm firmly closed. Both valves block the main burner gas flow.
When the thermostat calls for heat
When the thermostat calls for heat, the DI module generates a spark at the main burner and the first automatic valve and second automatic valve operators are energized (Figure 13). The first automatic valve opens, and the second automatic valve operator valve disk is lifted off its seat. This diverts gas flow from the second automatic valve diaphragm and causes a reduction of pressure on the underside of this diaphragm. The reduced pressure on the bottom of the automatic valve diaphragm repositions the diaphragm downward, away from the valve seat, allowing main burner gas flow.
Intermittent Pilot Applications
System operation
When the thermostat calls for heat, the S8600 Intermittent Pilot Module is energized. The S8600, in turn, activates the first automatic valve of the gas control, allowing pilot gas flow. Simultaneously, the S8600 module generates a spark at the igniter-sensor and lights the pilot. The pilot flame is then sensed by the igniter-sensor, and spark generation ends.
After the pilot is lit, the S8600 module energizes the solenoid for the second automatic valve operator (the first automatic valve remains energized). This is dependent upon an adequate microamp signal being produced as the flame is sensed by the igniter-sensor. That signal should be in the range of 2–10 microamps (3–5 being the normal range).
The second automatic valve diaphragm, controlled by a servo pressure regulator, opens and adjusts main gas flow as long as the system is powered and the pilot is burning. The servo pressure regulator monitors outlet pressure to provide an even main burner gas flow.
Loss of power (thermostat satisfied) de-energizes the S8600 module and closes both automatic valves. The system is then ready for the next cycle.
Loss of pilot flame, or when the flame is too small to reliably light the main burner, closes the second automatic valve operator. The S8600 module then attempts to restart the pilot. On S8600 modules with lock-out timers, the first automatic valve closes after the lockout period. On S8600 modules without lock-out timers, the trial for ignition continues indefinitely and the first automatic valve remains open.
If the pilot flame is restarted successfully, the main burner is reopened, and gas flows to the main burner as described above. Gas control operation is described in more detail below.
Valve position during thermostat “Off” cycle
The valve is positioned as shown in Figure 14 when the:
• Manual gas control now is in the On position.
• Thermostat is not calling for heat.
The first automatic valve is closed. The second automatic valve operator is de-energized, closing the channel to the pressure regulator, and opening a channel to the under-side of the second automatic valve diaphragm. The combination of the spring pressure under the second automatic valve diaphragm and lack of outlet pressure hold the diaphragm firmly closed (gas pilot burner gas flow is prevented by the first automatic valve and the main burner by both valves).
When Thermostat Calls for Heat
When the thermostat calls for heat, the trial for pilot ignition begins. The first automatic valve solenoid is energized by the module and opens, allowing pilot burner gas flow. Gas also flows to the second automatic valve operator, but is mechanically blocked at the operator (see Figure 12).
After the pilot lights and the pilot flame is sensed by the igniter-sensor, the second automatic valve solenoid is energized by the module, and the second automatic operator valve is lifted off its seat (see Figure 13). This diverts gas flow from the second automatic valve diaphragm and causes a reduction of pressure on the underside of this diaphragm. The reduced pressure on the bottom of the automatic valve diaphragm repositions the diaphragm downward, away from the valve seat, allowing main burner gas flow.
All Applications
During the On cycle, the servo pressure regulator provides close control of outlet pressure; even if inlet pressure and flow rate vary widely. Any outlet pressure change is immediately reflected back to the pressure regulator diaphragm, which repositions to change the flow rate through the regulator and, thus, through the automatic valve.
If outlet pressure begins to rise, the pressure regulator diaphragm moves slightly higher, allowing less gas flow to the gas control outlet. This increases gas pressure under the automatic valve diaphragm and repositions the valve disk closer to the seat. Thus, flow of gas through the second automatic valve is reduced and outlet pressure falls to the desired level.
If outlet pressure begins to fall, the pressure regulator diaphragm moves slightly lower, allowing more gas flow to the gas control outlet. This decreases gas pressure under the second automatic valve diaphragm and repositions the valve disk further from the seat. Thus, flow of gas through the second automatic valve is increased, and outlet pressure rises to the desired level.
When the call for heat ends
When the call for heat ends, the first automatic valve and the second automatic valve operator close, bypassing the regulator(s) and shutting off the main burner (and in the intermittent pilot application, the pilot gas flow). 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. ICM
