Direct Gas Make-Up Air
28
6. Set the Burner Air Pressure Differential
For units equipped with recirculation, reference the
Start-Up:
Direct Gas-Fired Heating, Optional Features,
Recirculation Operation
section in this Installation,
Operation and Maintenance Manual for more
information
.
For units equipped with variable air volume,
reference the
Start-Up:
Direct Gas-Fired Heating,
Optional Features, Variable Air Volume
section in this
Installation, Operation and Maintenance Manual for
more information.
With all access panels in place, the fan running
and discharging 70ºF
(21ºC)
air, connect a U-Tube
manometer to the burner differential sensing probes and
measure the static pressure across the burner.
Proper air velocity over the burner is critical on direct
gas-fired units. If the air velocity is not within the unit
specifications, the unit will not operate efficiently, may
have sporadic shutdowns, and may produce elevated
by-products of combustion.
The proper static pressure should be between:
• Natural Gas:
0.625 and 0.675 in. wg (156 and 168 Pa)
• Liquid Propane (LP) Gas:
0.8 and 0.9 in. wg (200 and 224 Pa)
Start-Up: Direct Gas-Fired Heating
All Units
2. Check the Optional High and Low Gas
Pressure Switches
The high pressure setting is typically 8 in. wg (2 kPa)
and the low pressure is setting is typically 3 in. wg
(0.7 kPa). The switches are set at the factory and should
not need adjustment. Adjust the setting only if needed.
Reference the
Pre-Start-Up: Direct Gas-Fired Heating,
Gas Train Identification
section in this Installation,
Operation and Maintenance Manual for the high and low
pressure switch location.
The purpose of the high and low gas pressure switches
is to automatically shut down the burner if the inlet gas
pressure is too low for the burner to safely light, or if the
manifold pressure is too high for the burner to operate
properly.
3. Energize the unit control circuits
(field-wiring
shown on diagram with dashed lines)
• Exhaust fan (optional) - Connect terminals R to H
• Supply fan - Connect terminals R to G
• Heating - Connect terminals R to W1
4. Verify inlet air sensor (TS4)
(if equipped)
Set inlet air sensor above the outside air temperature.
5. Verify supply fan rotation is correct
To reverse the rotation on three phase units, disconnect
and lockout the power, then interchange any two power
leads going to the motor. Check motor amp draw and
compare to motor nameplate Full Load Amperage (FLA).
Reduce airflow if amp draw is greater than FLA.
For proper unit function and safety, follow the start-up
procedure in the exact order that it is presented.
This start-up should begin after all of the installation
procedures and the supply fan start-up have been
completed.
1. Check the Supply Gas Pressure
Check the supply gas pressure and compare it with
the unit’s nameplate pressure requirements. Adjust
the supply regulator as needed until the supply gas
pressure is within the specified range. The nameplate is
located on the outside of the unit on the control panel
side.
Adjustable
Top Baffle
Airflow
Burner
Adjustable
Bottom Baffle
Burner and Baffles
4
5
6
7
0.625 - 0.675 in. wc
Measuring the Pressure Drop
Outer
Sensing
Probes
U-Tube
Manometer
Pressure
Switch
Direct Gas Nameplate
Type
of gas
Maximum
gas
pressure
Minimum gas pressure
for maximum output
