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Sequence of Operation
Operation Summary - Gas Heating
• When there is a call for heat, the main blower is turned “On” and the airflow switch is proven.
• The Flame Safety Controller (FSC-1) sends
120V AC
power to the line input of the power vent blower.
• Power vent blower is controlled by pin J17 on the control board. This varies a signal to the power vent
blower motor to initiate a
1 min
pre-purge at high speed.
•
24V AC
signal runs through the safety circuit (Power Vent Airflow Switch/High Temperature Limit/
Flame Roll-out Switch) and into FSC-1.
• FSC-1 initiates Trial for Ignition by sending a signal to the spark igniter to light the furnace and
24V AC
power to the On/Off gas valve and a signal to the HMI that it is sparking. This opens the On/Off gas
valve and triggers the start of the
17-seconds
from the HMI to the powervent blower, and modulating
valve.
• Flame is sensed by FSC-1’s remote flame sensor at the firing tube of furnace.
• HMI’s
17-second
high-fire off-delay time sequence runs out, and a variable voltage is sent to the
power vent blower motor.
• The control board continues to modulate the heat output of the unit by adjusting the 0-10V DC signal to
the modulating gas valve.
NOTE: If a high turndown furnace is present, this sequence occurs with FSC-2 if the call for heat
dictates a need for a second furnace.
There are different options for controlling the temperature output of these units. These include Discharge
Temperature Control, Space Temperature Control, Analog Control, and Direct Digital Control (DDC).
Discharge Control:
When used in discharge control, the MUA board receives a call to heat from the
intake sensor, the MUA board will modulate the discharge temperature until it hits the desired set point.
The user can choose whether discharge heating is activated based on intake temperature, space
temperature, either, both, or stat.
Space Control:
When the space control option has been selected, there may be an HMI (that contains an
internal temperature sensor) or a space thermistor. The user can choose whether the space heating is
activated based on intake temperature, space temperature, either, both or stat.
Analog Control/Direct Digital Control (DDC)
: A
0-10V DC
or
0-20mA
signal is sent to the MUA board
from the building control system to regulate the heating output of the unit.
In all cases, the MUA board controls the amount of gas to the burner based on the signal from the
temperature control components. When the modulating gas valve is all the way open, achieving the
maximum BTUs and temperature rise of the unit, the unit is in high fire.
