Installation & Maintenance Manual Owner's manual

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GAS CONTROLS continued
ELECTRONIC MODULATING CONTROL
Units with electronic modulating control are provided
with an electronic modulating valve capable of firing from
100 percent to 40 percent of rated input. Ignition is at full
fire (100 percent of unit's rated input). The electronic
modulating valve is controlled by a room thermostat or
duct thermostat with remote setpoint adjustment which
modulates the gas input from 100 percent to 40 percent
of rated input.
An optional override room thermostat is available for use
with the duct thermostat. The override room thermostat
allows full fire and overrides the duct thermostat when
the room temperature falls below the override room
thermostat's setpoint.
With power applied to the unit, this system operates in
the following manner:
1. The thermostat calls for heat.
2. The pilot valve opens.
3. The ignitor sparks continuously to ignite the pilot.
4. The sensor proves pilot ignition and shuts off the
ignitor.
5. With the pilot lit, the main gas valve opens.
6. Main burners are lit at 100 percent of unit’s rated
input.
7. The fan time delay relay (optional on duct furnaces)
allows the heat exchanger to come up to operating
temperature. At this time the fan time delay relay
closes and activates the fan motor.
8. The unit is controlled by the electronic thermostat
which modulates the unit from 100 to 40 percent of
unit's rated input. The electronic thermostat can be a
duct thermostat or a room thermostat. An amplifier
receives an electrical signal from the thermostat and
converts this into a working voltage. This working
voltage determines the position of the modulating
valve. With no voltage applied to the valve, the valve
will be full open and full fire will occur. As increasing
voltage is applied to the valve, the valve will
modulate closed. At approximately 12 volts dc, the
valve will be at it's minimum fire position. As
temperature drops, the voltage also drops causing
the relay to reopen the valve. The unit will continue
to cycle in this manner until either an increase in the
unit's firing rate is required or the thermostat is
satisfied and no longer calls for heat. If the voltage
continues toincrease, indicating a further reduction
in the unit's firing is required, the increased voltage
closes a relay which closes the automatic gas valve.
9. When the thermostat is satisfied, the main and pilot
valves close.
10. The fan time delay relay remains closed keeping the
fan motor operating to dissipate residual heat from
the heat exchanger. At this time, the fan time delay
relay opens and deactivates the fan motor.
With power applied to the unit, this system operates in
the following manner:
1. The thermostat calls for heat.
NOTICE: If a thermostat is not supplied, steps 2
through 5 will be constant as long as power is applied
to the unit
2. The pilot valve opens.
3. The pilot sparks continuously to ignite the pilot.
4. The sensor proves pilot ignition and shuts off the
ignitor.
NOTICE: The unit firing is controlled by the hydraulic
sensing bulb on the mechanical valve. The
mechanically operated valve is in series with the
single-stage electrically operated valve.
5. With the pilot lit, the main electric gas valve opens.
At this time, the fan time delay (optional on duct
furnaces) is energized. The fan time delay relay
closes and activates the fan motor.
6. The unit is equipped with a bypass manifold
containing a single-stage electric valve which is in
parallel with the hydraulic modulating gas valve. The
bypass is activated by a thermostat and overrides
the hydraulic control. The override thermostat
should be a remote room thermostat. If the
temperature falls below the override thermostat's
setpoint, the thermostat opens the bypass valve,
and the unit operates at full fire. The unit will
continue at full fire until the override thermostat is
satisfied and no longer calls for heat, at which point
the bypass valve is closed. The unit is again
controlled by the hydraulic sensing bulb on the
mechanical valve.
7. The hydraulic sensing bulb on the mechanical valve
calls for heat.
8. The mechanical modulating valve opens allowing
gas flow to the main burners. Main burners are lit at
40 percent of the unit's rated input. The mechanical
modulating valve modulates between 40 percent
and 100 percent of firing rate, depending on the
temperature rise required.
9. The unit continues to fire until the hydraulic sensing
bulb is satisfied and no longer calls for heat.
10. The mechanical modulating valve closes shutting off
the main gas supply to the burners.
11. The room thermostat is satisfied and no longer calls
for heat. The main and pilot valves close. At this time
the fan delay relay opens and deactivates the fan
motor.