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UBX-UBZ-UDX-UDZ-IOM (07-23) 1034344-K
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, 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 14).
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 (see Figure 13) 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 (see Figure 19) 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 11.
Limit Control
Flame Rollout Switch
Door Switch
Transformer Disconnect Switch
Figure 19. Replaceable Components
Interlock Door Switch Maintenance (Models UBZ and UDZ Only)
⚠ CAUTION ⚠
Verify that the interlock door switch operates when opening the cabinet. While a temporary bypass
of the switch may be required for troubleshooting, always return the switch to normal operation
once troubleshooting is complete. Do not permanently bypass the switch.
If it is determined that the interlock door switch (see Figure 19) needs replacing, use only a factory-authorized
replacement part that is designed for the heater. For the approximate door switch location, see Figure 11.
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