Control Operating Sequence - Modine Manufacturing DFP Installation And Service Manual

START-UP PROCEDURE
IMPORTANT
To prevent premature heat exchanger failure, with all control
systems, a blower starting mechanism must be provided so
that the blower is running or energized within 45 seconds of
the gas control operation.

Control Operating Sequence

Indoor power vented duct furnaces are supplied with intermittent
pilot systems with continuous retry as standard. On mechanical
modulation systems the main burner is turned off 100%
when the thermostat is satisified, but the pilot remains lit. For
intermittent pilot systems, except mechanical modulation both
the main burner and pilot burner are turned off 100% when
the thermostat is satisfied. Intermittent pilot systems, for both
natural and propane gas, the ignition controller is 100% shut-off
with continuous retry. On a call for heat, the system will attempt
to light the pilot for 70 seconds. If the pilot is not sensed for
any reason, the ignition control will wait for approximately six
minutes with the combination gas control closed and no spark.
After six minutes, the cycle will begin again. After three cycles,
some ignition controllers lockout for approximately one hour
before the cycle begins again. This will continue indefinitely until
the pilot flame is sensed or power is interrupted to the system.
NOTE: Gas Control Options (see page 14) could change the
listed sequence of operation based on their function.
The descriptions given are for the basic duct furnace.
Single-Stage Gas Controls
Utilizes a single-stage combination gas, an ignition control, and
a single-stage low voltage thermostat.
1. The thermostat calls for heat.
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 unit continues to operate 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).
8. If the unit was provided with a time delay relay, the blower
stops after 30 to 45 seconds.
Two-Stage Gas Controls
Utilizes a two-stage combination gas control, an ignition control,
and a two-stage low voltage thermostat. The unit fires at 50%
fire on low stage and 100% fire on high stage.
1. The thermostat calls for low stage heat.
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
12
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
50% fire.
6. If the unit was provided with a time delay relay, the blower
starts after 30 to 45 seconds.
7. If the temperature at the thermostat continues to fall, the
thermostat will call for high stage heat.
8. The main gas valve is opened completely and the main
burner is lit to 100% full fire.
9. The unit continues to operate until the high stage of the
thermostat is satisfied, at which time the main valve closes
to 50% fire.
10. The unit continues to operate until the low stage 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.)
11. If the unit was provided with a time delay relay, the blower
stops after 30 to 45 seconds.
Mechanical Modulating Gas Controls (Natural Gas Only)
Lo 1 2
Discharge
Air Temp. 58 62 65
(°F)
Utilizes a mechanical modulating gas control and an ignition
control. The mechanical modulating gas control is designed
for use in constant blower applications. The discharge air
temperature is maintained by setting the control dial of the
modulating gas valve in one of ten positions. A hydrostatic
sensing bulb located in the discharge air stream controls the gas
flow between 40% through 100% full fire. When the discharge
air hydrostatic sensing bulb is satisfied, the modulating valve
closes completely, but the pilot remains lit.
This control cannot be used with a room override thermostat.
1. Power is supplied to the unit through either an exhaust fan
interlock contact or a start/stop switch.
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 hydrostatic sensing bulb, integral to the mechanical
modulating gas valve, calls for heat. The main gas valve is
opened and the main burner is lit. The burner modulates
between 40% through 100% fire based on the call for heat.
6. If the unit was provided with a time delay relay, the blower
starts after 30 to 45 seconds.
7. The unit continues to operate until the hydrostatic sensing
bulb is satisfied, at which time the main valve closes 100%,
but the pilot remains lit.
8. The unit blower (and pilot) continues to operate until either an
exhaust fan interlock contact or a start/stop switch is opened.
If the unit was provided with a time delay relay, the blower
stops after 30 to 45 seconds.
5-564
Dial Number
3 4 5 6 7 8
69 72 76 79 83 86
Hi
90