Indirect Gas-Fired Heat Modules
21
1. Connect a manometer to the test port on the burner
manifold.
2. Send the unit to high fire.
3. Check the gas pressure at the manifold test port. The
recommended pressure is 3.5 in. wg for natural gas
or 10.0 in. wg for LP gas.
4. Adjust the high fire screw on the valve.
Counterclockwise rotation will decrease the gas
pressure and clockwise rotation will increase the
pressure.
5. Send the furnace to low fire by disconnecting and
isolating the wire from the high fire terminal on the
valve.
6. Check the gas pressure at the manifold test port. The
recommended low fire manifold pressure is 7/8 in.
wg for natural gas or 2.5 in. wg for LP.
7. Adjust the low fire screw on the valve.
Counterclockwise rotation will decrease the gas
pressure and clockwise rotation will increase the gas
pressure.
8. Turn off furnace. Reconnect the wire to the high fire
terminal. Remove manometer and reinstall plug into
manifold test port.
9. Return the unit to normal operation.
IMPORTANT
Confirm that the discharge air sensor is installed in the
duct at least three duct diameters downstream of the
furnace.
Single Section
Burner Manifold
Two Stage
Valve
Remote Temperature
Sensor
A350
Controller
A350
JOHNSON
CONTROLS
Ignition
Controller
With 2 Stage Combustion
With Combination Turndown
Configurations
There are several different turndown options that may
be used in the units that are simply combinations of
basic configurations.
In all cases, all gas valves must be set for high fire
and low fire as part of the Start-Up process.
Inspect the furnace(s) and verify the type of valve(s) and
manifold used. Refer back to the previously described
configurations and find the one that most closely
matches the valve and manifold types present in this
unit. Adjust the high fire and low fire settings on all
valves before placing the unit in service.
p
Sensor
A350
JOHNSON
CONTROLS
A350
Controller
S350
JOHNSON
CONTROLS
S350
Controller
OR
OR
Eight Stage
Furnace
Single Stage
Furnace
Provides
16 Stages
Two Stage
Furnace
Two Stage
Furnace
Provides
4 Stages
Single Stage
Furnace
Single Stage
Furnace
Provides
2 Stages
Electronically
Modulated
4:1 Turndown
Furnace
Single Stage
Furnace
Provides
8:1 Turndown
FX Controller
Lead Furnace
A modulating furnace can also be combined with a
single stage furnace. Both furnaces are then controlled
by an FX controller.
In some cases, two furnaces may be installed in parallel
in one ventilating unit. In that case, the turndown is the
same as just a single furnace.
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