13
68-0046—4
VR8200, VR4200
OPERATION
Step-Opening Pressure Regulation (VR8200C,P and
VR4200P)
Step-opening gas controls actually combine two pres-
sure regulators, one for the low pressure and one for the
full-rate pressure. When the thermostat calls for heat, the
automatic operator valve disc opens. The low pressure
regulator maintains outlet pressure at the preset step rate
for several seconds. Then the regulator valve is forced
fully open by the timing diaphragm, which is operated by
bleed gas. When the low pressure regulator is fully open,
the high pressure regulator maintains the desired full-rate
outlet pressure as described for the standard regulator.
The step model requires approximately 60 seconds to
reset once the main burner goes off. If it is re-energized
within 60 seconds, it may bypass or shorten the length of
the low pressure step. The burner may relight at the full
flow rate.
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. As
pressure inside the gas control and underneath the auto-
matic valve diaphragm equalizes, spring pressure closes
the second automatic valve to provide a second barrier to
gas flow.
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 disc further from the seat. Thus, flow of gas through
the second automatic valve is increased, and outlet pres-
sure rises to the desired level.
Slow-Opening Pressure Regulation (VR8200H and
VR4200H); Delayed and Slow-opening Regulation
(VR8200J)
Slow-opening gas controls function the same as
standard models except that when the thermostat calls
for heat, the second automatic valve opens gradually.
Opening is slowed because a gas flow restrictor in the
passage from the second automatic operator slows the
rate at which gas pressure is reduced under the second
automatic valve diaphragm after the automatic operator
opens. Outlet pressure to the main burner increases
gradually from 0 in. wc [0 kPa] to rated output pressure
within 3-6 seconds (for an 80,000 Btuh furnace at 7 in. wc
[1.8 kPa] inlet pressure and 3.5 in. wc [0.9 kPa] outlet
pressure). VR8200J has a six second delay from when the
thermostat calls for heat until the main gas valve opens.
Fig. 14—Position of gas control components during the thermostat off cycle.
GAS
CONTROL
OUTLET
M6920
1
1
FIRST
AUTOMATIC
VALVE
SOLENOID
GAS
CONTROL KNOB
GAS
CONTROL
INLET
SAFETY
SHUTOFF VALVE
RESET BUTTON
VALVE DISC
POWER UNIT
SECOND
AUTOMATIC VALVE
OPERATOR
SECOND
AUTOMATIC
OPERATOR
SOLENOID
SECOND
AUTOMATIC
OPERATOR
VALVE DISC
SERVO
PRESSURE
REGULATOR
SECOND AUTOMATIC
VALVE DIAPHRAGM
FIRST
AUTOMATIC
VALVE
PILOT
GAS
OUTLET
NOTE: SECOND AUTOMATIC VALVE OPERATOR AND
SERVO PRESSURE REGULATOR SHOWN OUTSIDE OF
GAS CONTROL FOR EASE IN TRACING GAS FLOW.
SLOW-OPENING GAS CONTROL HAS A GAS
FLOW RESTRICTOR IN THIS PASSAGE.
RED RESET
BUTTON