5-565.1
17
start-uP Procedure
main burner. As the temperature drops, the voltage drops
causing the modulating valve to open further. If the discharge
air temperature increases, the voltage increases causing the
modulating valve to close allowing less gas flow to the main
burner. For further information regarding the operation of the
electronic modulating system, consult the literature provided
with the unit.
8. The unit continues to operate in this manner until the
thermostat is satisfied, at which time both the main and pilot
valves close 100%. (If the unit was not provided with a time
delay relay, the blower stops at this time.)
9. If the unit was provided with a time delay relay, the blower
stops after 30 to 45 seconds.
electronic modulating gas controls
two & three furnaces - model sizes 500-960
One Master furnace is provided with up to two Slave furnaces
that utilize electronic modulating/regulating gas controls,
combination gas valves, ignition controls, one multiple furnace
modulating amplifier, and either a modulating room thermostat
or modulating duct thermostat with remote temperature adjuster.
The thermostat controls can modulate the gas flow of all the
furnaces between 40% through 100% full fire. The amplifier
sends a signal to all of the gas valves so that they modulate
at the same percentage. When the thermostat is satisfied,
the amplifier cuts power to the combination gas valves which
prevents gas flow to both the main and pilot burners.
When duct sensing is utilized, a room override thermostat can
be added. When calling for heat, the room override thermostat
provides full fire operation until the space temperature is
satisfied. Control is then returned to the duct sensing control.
In this situation, either the duct sensor or the room override
thermostat can call for heat.
The sequence of operation for Electronic Modulating Gas
Controls - Two & Three Furnaces is the same as Electronic
Modulating Gas Controls - Single Furnace.
electronic modulating gas controls -
Building management control
(0-10 vdc or 4-20 ma signal)
Utilizes an electronic modulating/regulating gas control,
combination gas valve, an ignition control, modulating signal
conditioner, and an inverted (0 Vdc or 4 mA being high fire and
10 Vdc or 20 mA being low fire) 0-10 Vdc or 4-20 mA input
signal provided by a Building Management System (BMS).
The signal conditioner can modulate the gas flow between 40%
through 100% full fire. When the BMS thermostat (field supplied)
is satisfied, the BMS heat contact (field supplied) opens to cut
power to the combination gas valve which prevents gas flow to
both the main and pilot burners.
1. The BMS thermostat (field supplied) calls for heat and closes
the BMS heat contact (field supplied).
2. The power exhauster relay is energized starting the power
exhauster motor. Once the motor has reached full speed, the
differential pressure switch closes. The power exhauster relay
pre-purge timer closes after 20 to 40 seconds energizing the
gas control circuit.
3. The pilot valve opens and the spark ignitor sparks in an
attempt to light the pilot. (If the unit was not provided with a
time delay relay, the blower starts at this time.)
4. Once the pilot is lit, the flame sensor proves the pilot and
stops the spark ignitor from sparking.
5. The main gas valve is opened and the main burner is lit to
100% full fire.
6. If the unit was provided with a time delay relay, the blower
starts after 30 to 45 seconds.
7. The modulating gas valve is controlled by the BMS
thermostat. The thermostat can modulate the firing rate
between 40% through 100% full fire by modulating the
input signal between either 0-10 Vdc or 4-20 mA (The
signal conditioner can accept a 0-10 Vdc signal when all
the dip switches are in the “OFF” position and 4-20 mA
signal when all the dip switches are in the “ON” position).
The signal conditioner converts the input signal into a DC
voltage (0 to 12 volts DC with 0 volts high fire and 12 volts
low fire). The output voltage is applied to the modulating
gas valve to control the gas flow to the main burner. As the
temperature drops, the voltage drops causing the modulating
valve to open further. If the discharge air temperature
increases, the voltage increases causing the modulating
valve to close allowing less gas flow to the main burner. For
further information regarding the operation of the electronic
modulating system, consult the literature provided with the
unit.
8. The unit continues to operate in this manner until the
thermostat is satisfied, at which time the BMS heat contact
opens resulting in both the main and pilot valves closing
100%. (If the unit was not provided with a time delay relay,
the blower stops at this time.)
9. If the unit was provided with a time delay relay, the blower
stops after 30 to 45 seconds.
variable air movement applications
Units may be supplied with variable frequency drives for
applications where variable air volume is required. The minimum
air flow may be varied between 30 and 100% of the full speed
air flow depending on the controls selection of the unit. Due to the
extra restrictions of the controller all selections must be performed
with the AccuSpec configuration software. Within AccuSpec, three
variable frequency drive speed control changeover options are
available:
1. Two speed which may be controlled by a manual high/low
switch which may be factory mounted on the control panel or
shipped loose for field installation or by exhaust fan
interlocks.
2. Floating building pressure sensing which utilizes a photohelic
pressure controller to adjust the building pressure by
varying the amount of makeup air supplied to the the space.
3. Building management control which allows for an external
signal of 0-10VDC of 4-20mA to adjust the unit airflow.
Additionally, when the air mover supplied by others can provide
variable air movement (i.e. 2-speed or variable frequency drive
units), the allowable minimum CFM of the duct furnace can
be 66% of the minimum listed CFM in Table 26.1 if the unit is
applied as follows:
1. The unit is provided with 2-stage, mechanical modulation,
or electronic modulating gas controls. (see Model
Identification).
2. The unit is provided with a factory installed discharge air
controller.
3. The system does not include a room thermostat.
The factory installed discharge air thermostat will prevent the
unit from firing above the allowable 100°F rise when the unit is
at or above the minimum CFM by monitoring the discharge air
and going to low fire. A room thermostat, because it is located
remote from the unit, could cause the unit to over-fire.
