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OPTIONS
Gas Control Options
The unit must be reviewed to determine if any of the listed gas
control options were supplied.
j Time Delay Relay
The optional time delay relay allows the gas controls to operate
for approximately 30 seconds before the blower starts. This
allows the heat exchanger a warm up period so that the initial
delivered air coming out of the ductwork is not cool. The time
delay relay also keeps the motor running for approximately 30
seconds after the call for heat has been satisfied to remove the
residual heat from the heat exchanger.
k Low Gas Pressure Switch
The switch monitors the gas pressure upstream of all the gas
controls and shuts off the electric supply to the ignition controller
and combination gas valve if low gas pressure is experienced.
The switch has an automatic reset so that if the gas pressure
is interrupted and then is returned, the switch will automatically
allow the unit to operate when gas conditions are returned to
the allowable range of the pressure switch. The pressure switch
range is 2" to 14" W.C. and should be set to insure that the
minimum inlet gas pressure is available (6" W.C. for natural gas,
11" W.C. for propane gas).
l High Gas Pressure Switch
The switch monitors the gas pressure downstream of all the gas
controls and shuts off the electric supply to the ignition controller
and combination gas valve if high gas pressure is experienced
right before the manifold. The switch has a manual reset so that
if the gas pressure is too high, a service person must check
the unit to make sure that none of the gas controls have been
damaged by the high gas pressure and then reset the switch to
allow the unit to operate when gas conditions are returned to
the allowable range of the pressure switch. The pressure switch
range is 2" to 16" W.C. and should be set to insure that the
maximum manifold gas pressure is not exceeded (3.5" W.C. for
natural gas, 10" W.C. for propane gas).
m Supply Air Fire Stat
The fire stat is factory installed in the duct furnace electrical
junction box with the sensor in the discharge air stream. In case
of elevated temperatures in the supply air, the manual reset
switch shuts down the entire unit. If the limit temperature is
exceeded, a service person must inspect the unit for the cause
of the high discharge temperature, take corrective action, and
then reset the switch.
n Timed Freeze Protection
The timed freeze protection system is factory installed in the
duct furnace electrical junction box with the sensor (30°-75°F
adjustable) factory installed in discharge air stream. On initial
start-up, the timed delay in the system allows the unit to go
through the normal ignition sequence. The timed delay is a
manual reset switch and adjustable for 1-10 minutes. In the
event that the unit fails to fire after this period, the discharge air
sensor will sense the cold air and will shut down the entire unit.
14
o Air Flow Proving Switch
The air flow proving switch is factory installed in the duct
furnace electrical junction box. The air flow proving switch
monitors the pressure differential between the duct furnace
and the atmosphere. The purpose of the air flow proving switch
is to cut power to the gas controls if a positive pressure is not
measured by the switch. This could be caused by a lack of air
movement through the heat exchanger.
NOTE: The air flow proving switch will prevent any heat
exchanger warm-up (the unit should not be equipped
with a time delay relay) because the gas controls can
not be energized until air flow is proven.
Setting the Air Flow Proving Switch
The range of the air flow proving switch is adjustable between
0.17" to 5.0" W.C.
1. Set the thermostat so that there is a call for heat. This should
start the blower and fire the burner.
2. Turn the set screw of the pressure switch clockwise until it
stops. This will set the pressure at 5.0" W.C.
3. Turn the screw counter-clockwise until the gas controls light
and then one additional full turn (This is approximately 0.25''
W.C.). This will allow for dirty filters or any other slight static
pressure increases in the system.
p Manual Reset High Limit
The manual reset high limit switch is factory installed in place
of the standard automatic reset high limit switch located in the
duct furnace electrical junction box. In case of a failure of the
blower motor, blockage of the inlet air, etc., the manual reset
switch prevents the unit from cycling on the high limit. If the limit
temperature is exceeded, a service person must inspect the unit
for the cause of the high discharge temperature, take corrective
action, and then reset the switch.
Figure 14.1 - Location of Gas Control Options
2. Low Gas Pressure Switch
3. High Gas Pressure Switch
4. Power Exhauster (Model Digit 3=P only)
5. Timed Freeze Protection
6. Ignition Controller
8. Control Relay
9. Time Delay Relay
10. Furnace Low Voltage Terminal Strip
12. Furnace Supply Power Terminal Strip
13. Control Step Down Transformer
Model DFP is shown, but also applies to all models.
5-564.4
Note: Wrap around gas train on
premium and electronic
modulation units only.
46. Electronic Modulating Amplifier
47. Electronic Modulating Gas Valve
48. Air Flow Proving Switch
49. High Limit Switch
50. Supply Air Fire Stat
51. Main Gas Valve
52. Burner Box
55. Differential Pressure Switch
(Model Digit 3=P only)
57. Control Terminal Board
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