Indirect Gas-Fired Heat Modules
17
Furnaces with electronically modulated 4:1 turndown
use a combination valve for primary gas control. It
controls the high fire gas supply. In addition to the
combination valve, there is a modulating valve located
just after the combination valve. The modulating valve
is the device that modulates or changes the gas volume
that is being supplied to the furnace manifold.
Both of
these valves require adjustment at the time of unit
start-up.
The modulating gas valve has a built-in digital controller
that will accept user settings for High Fire and Low Fire
and will provide minimal hysteresis throughout the entire
range of modulation. The modulating valve controls the
amount of combustion gas that goes to the burners,
while the combination valve acts as an on/off switching
device. During normal use, the amount of combustion
gas will vary constantly, depending on the settings put
in by the owner. This allows the modulating valve to
regulate the heat output from the furnace and maintain
a constant space temperature with minimal variation, or
hysteresis.
The modulating valve is controlled by a user interface
known as the FX controller. The FX controller sends an
analog signal to the modulating valve that causes the
valve to send more or less gas to the furnace.
The combination valve is normally closed. It requires
24 VAC to hold it open.
With 4:1 Turndown Electronic
Modulation
Start-Up - Furnaces (all units)
After the gas piping has been installed and leak tested,
verify that all field-installed components such as an air
temperature sensor have been installed. Verify that ON/
OFF gas control knob on each stage-type gas valve is
turned to the “ON” position (see image).
Verify the unit turndown ratio as shown on page
2 of this manual. Consult the specific start-up
instructions as shown on the following pages to
accomplish the following steps:
Set Airflow
Airflow adjustment is done at the time of unit start-up.
Refer to the unit-specific Installation, Operating and
Maintenance manual (IOM) for instructions.
IMPORTANT
For the unit to function properly, ALL gas valves must
have their high fire and low fire settings adjusted for
field conditions.
IMPORTANT
Confirm the discharge air sensor is installed at least
three duct diameters downstream of the furnace.
IMPORTANT
Multi-furnace units may use a combination of the
available turndown options. Each furnace must be
set-up per the specific instructions for its control
type. For additional information, see
Gas-Fired Burner
Turndown
in this manual.
EXA Valve
Single Stage
Valve
Single Section
Burner Manifold
Remote Temperature
Sensor
Ignition
Controller
FX Controller
®
