Bryant SERIES D 453 - Manual (Page 9)

through the pilot pressure switch to terminal F and
lights the pilot.
Approximately one minute after the pilot is ignited,
heat from the pilot will cause the pilot’s normally open
contacts to close and the normally closed contacts
to open.
If the pilot should go out, the normally open pilot con­
tacts (which were closed when the pilot was ignited)
will reopen, de-energizing the power relay (2G) and
the heat motor of the gas valve (5A). This will cause
both motors (3A & 3C) to stop and the gas valve to
close. The normally closed pilot contacts will close
again and complete the current path to reignite the pilot.
Cooling Operation
Two current paths are provided from chiller terminal Y
to chiller terminal F when the thermostat calls for
cooling and the pilot is ignited as described above.
Current flows from chiller terminal Y through hi-temp
limit switch (7H), closed contacts of circuit breaker
(2J), resistance coil of power relay (2G), the normally
open contacts of pilot assembly (6B), and through the
pressure switch (7P) to chiller terminal F. The ener­
gized power relay (2G) starts the fan motor (3C) and
the solution/water pump motor (3A).
Current also flows from chiller terminal Y through
hi-temp limit switch (7H), circuit breaker (2J) closed
contacts, sail switch (7D), part load control switch (7L),
the heat motor of gas valve (5A), normally open con­
tacts of pilot assembly (6B), through the pressure switch
(7P) to chiller terminal Fi The current flow through
the heat motor of the gas valve, energizes the gas valve
(5A) causing it to open and supply gas to the genera­
tor burner.
Part Load Control
The part load control circuit utilizes a temperature
sensitive switch and a long capillary tube which is
inserted a minimum of six inches in the chilled water
supply line. The part load control switch (7L) contacts
open when the chilled water supply temperature drops
below 40±1°F. The opening of the switch contacts
de-energizes the gas valve which shuts off the gas
supply to the burners. The power relay (2G) remains
energized; therefore, the fan motor (3C) and the
solution/water pump motor (3A) will continue to run.
The part load control switch contacts will close again
when the chilled water temperature rises to 42±1°F.
The gas valve is re-energized and the gas supply to
the burner is turned on again.
High-Temperature Cutoff
The high temperature cutoff circuit includes a hi-temp
limit switch (7H) located on the front of the generator,
plus a circuit breaker (2J) which is located in the con­
trol box.
If the generator becomes overheated, the hi-temp limit
switch will open causing all current in the Y leg to pass
through the heater element of circuit breaker (2J).
This current flow through the heater causes the nor­
mally closed contacts of the circuit breaker to open,
removing the 24-volts potential from the gas valve and
the power relay (2G).
The de-energized gas valve will turn off the gas supply
to the burner, and both the fan motor (3C) and
solution/water pump motor (3A) will stop running. The
circuit breaker is reset manually by pushing the red
reset button that protrudes through the bottom of the
control box. The reset button is accessible by removing
the left side access panel.
VII. adjusting gas input
The gas input must be checked and adjusted if neces­
sary to agree with the input shown on the unit’s rating
plate.
The natural gas units are equipped with adjustable gas
pressure regulators.
The propane gas units are not equipped with a pressure
regulator.
Check natural gas units by the following method:
1. Measure gas at meter. Be sure all other gas ap­
pliances are turned off. Increase or decrease input to
burner by adjusting the gas pressure regulator.
2. Set manifold pressure according to Table VII for
the Btu value and specific gravity of the gas to be
supplied to the unit. Connect manometer to 1/8-inch
pressure tap on gas valve and, with unit in operation,
set correct pressure by adjustment of the gas pressure
regulator.
Example: Natural Gas
1025 Btu
0.63 Specific Gravity
TABLE VII
BTU
Value
Specific Gravity
0.59 0.61 0.63 0.65 0.67
900 3.5 3.6 3.7 3.8 3.9
950 3.3 3.4 3.5 3.6 3.7
1000 2.85 2.9 3.1 3.15 3.2
1025 2.7 2.75 2.8 3.0 3.1
1050 2.55 2.6 2.7 2.75 2.8
1100 2.35 2.4 2.5 2.55 2.6
For manifold pressures exceeding 3.3 inches or less
than 2.7 inches, consult your local gas utility.
3. Refer to Table VII and note that the gas manifold
should be 2.8 inches wc for these values.
4. With manometer connected to the pressure tap, set
pressure to 2.8 inches by adjusting gas pressure
regulator.
Check propane gas units as follows:
The burner orifice is sized for the rated input with a