Honeywell VR8345M Manual page 15

side of thesecond automatic valve diaphragm. The combi-
nation of spring pressure under the second automatic valve
diaphragm andd alck of outlet pressure hold the diaphragm
firmly closed. (Gas pilot burner gas flow is prevented by
the first automatic vlave and 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 ener-
gized by trhe module and opens, allowing pilot burner gas
flow. Gas also flows to the second automatic valve opera-
tor, but is mechanically blocked at the operator. See Fig. 13.
After the pilot lights and the pilot flame is sensed by the
igniter-sensor, the second automatic valve solenoid is ener-
gized by the module, and the second automatic operator
valve is lifted off its seat. See Fig. 14. This diverts gas flow
from the second automatic vlave 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 pro-
vides close control of outlet pressure, even if inlet pressure
and flow rate vary widely. Any outlet pressure change is
Fig. 13—Intermittent Pilot Applications: Position of gas control components during trial for pilot ignition.
GAS
CONTROL
INLET
FIRST AUTOMATIC VALVE
NOTE: SECOND AUTOMATIC VALVE OPERATOR AND
SERVO PRESSURE REGULATOR SHOWN OUTSIDE
GAS CONTROL FOR EASE IN TRACING GAS FLOW.
CONTROL
KNOB
FIRST
AUTOMATIC
VALVE
SOLENOID
SECOND AUTOMATIC
VALVE DIAPHRAGM
SLOW-OPENING GAS CONTROL HAS A GAS FLOW RESTRICTOR IN THIS PASSAGE.
1
15
immediately reflected back to the pressure regulator dia-
phragm, 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 pres-
sure inside the gas control and underneath the automatic
valve diaphragm equalizes, spring pressure closes the sec-
ond automatic valve to provide a second barrier to gas flow.
SECOND AUTOMATIC SECOND
VALVE OPERATOR AUTOMATIC
OPERATOR
SOLENOID
SECOND
AUTOMATIC
OPERATOR
VALVE DISC
1
OPERATION
SERVO PRESSURE
REGULATOR
PILOT
GAS
OUTLET
GAS
CONTROL
OUTLET
M9127
68-0160
VR8345M