Catalogue
40 UH-PRC002-EN
Sequence of Operations
Two-Stage Control, Intermittent Pilot
With power applied to the unit:
1. The first stage of the thermostat calls for heat.
2. The pressure switch measures the flow through the vent system and energizes the intermittent
pilot when the flow is correct.
Important: CAUTION: THE PRESSURE SWITCH MUST NOT BE BYPASSED. THE UNIT MUST
NOT BE FIRED UNLESS THE FLUE VENT FAN IS OPERATING. If this procedure is not
followed, there may be a gas buildup that could cause an explosion.
3. The pressure switch closes and activates the flue vent fan.
4. The pilot valve opens.
5. The ignitor sparks continuously to ignite the pilot.
6. The sensor proves pilot ignition and shuts off the ignitor.
7. With the pilot lit, the main gas valve opens to low fire.
8. Main burners are lit at 50 percent of unit's rated input.
9. The fan time delay relay allows the heater to come up to operating temperature. At this time,
the fan time delay relay closes and activates the fan motor.
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10. If additional heat is required, the second stage of the thermostat calls for heat.
11. The main gas valve opens to full fire. The main burners are now at full fire. The unit continues
at full fire until the second stage of the thermostat is satisfied and no longer calls for heat.
12. The main valve closes to low fire. The main burners are now at low fire. The unit continues at
low fire until the first stage of the thermostat is satisfied and no longer calls for heat.
13. The fan time delay relay remains closed, keeping the fan operating to dissipate residual heat
from the heat exchanger. At this time, the fan time delay relay opens and deactivates the fan
motor.
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See “Fan Control,” p. 30.
• Caution - Disconnect power before
servicing.
• Unit must be grounded.
• Use copper conductors only.
• If any of the original wire as supplied
with the appliance must be replaced,
it must be replaced with wiring
material having a temperature rating
of at least 125°C.
• High-limit wires minimum 200°C.
• Refer to installation instructions for
venting, gas piping and start-up
procedures.
• - - - Indicates Field Wiring.
• ___ Indicates Factory Wiring.