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UBXC-UDXC-IOM (01-24) 1042980-0
MAINTENANCE—CONTINUED
Maintenance Procedures—Continued
Combination Gas Valve Maintenance
⚠ WARNING ⚠
The combination gas valve is the prime safety shutoff. All gas supply lines must be free of dirt or
scale before connecting them to the unit to ensure positive closure.
Inspect the combination gas valve (see Figure 20 for location), carefully remove any external dirt accumulation,
and check wiring connections. Check the valve annually to ensure that the valve is shutting off gas flow completely
as follows:
1. Close manual shutoff valve to prevent flow to combination gas valve.
NOTE: Use a water column manometer that is readable to the nearest tenth of an inch.
2. Connect manometer to 1/8-inch outlet pressure tap on combination gas valve (see Figure 22).
3. Open manual shutoff and combination gas valves.
4. Use finger to fully block main burner orifice for several seconds.
5. Observe manometer with orifice blocked. If any pressure is indicated, combination gas valve is leaking and must
be replaced before heater is restored to operation.
Pressure Switch Maintenance
If it is determined that the pressure switch needs replacing, use only the factory-authorized replacement part that
is designed for the model and size of heater being serviced. Replace the switch in accordance with the Pressure
Switch Replacement section.
NOTES:
• Depending on the date of manufacture and unit size, the pressure switch may not be in the
location indicated. Check the control bracket on the bottom of the compartment or further down
on the compartment wall.
• A unit operating above 6,000 feet (1,830 meters) in elevation requires a high-elevation pressure
switch (refer to Pressure Switch Replacement section).
High Temperature Limit Control Maintenance
If it is determined that the high temperature limit control needs replacing, use only a factory-authorized replacement
part that is designed for the size of heater. For the approximate limit control location, see Figure 20.
Transformer Maintenance
Use a voltmeter to verify that there are 24V output from the transformer. If the transformer is not functioning, it must
be replaced. Use a replacement transformer identical to the factory-installed model. For the transformer location,
see Figure 20.
Disconnect Switch Maintenance
If it is determined that the disconnect switch needs replacing, use only the factory-authorized replacement part that
is designed for the heater. Always replace the electrical box cover.
Vent or Vent/Combustion Air System Maintenance
Check the complete system at least once a year. Inspection should include all joints, seams, concentric adapter box
(separated-combustion units), inlet air guard or inlet air cap (separated-combustion units), and the vent terminal cap.
Clean all openings and replace any defective parts.
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