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UH-PRC002-EN30
Electric
Power
Two-Stage Control
With power applied to the unit:
1
The first stage of the thermostat calls for
heat.
2
The pilot valve opens.
3
The ignitor sparks continuously to ignite
the pilot.
4
The sensor proves pilot ignition and
shuts off the ignitor.
5
With the pilot lit, the main gas valve
opens to low fire.
6
Main burners are lit at 50 percent of
unit’s rated input.
7
The fan time delay relay (optional on
duct furnaces) allows the heat exchanger
to come up to operating temperature. At
this time, the fan time delay relay closes
and activates the fan motor*.
8
If additional heat is required, the second
stage of the thermostat calls for heat.
9
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.
10
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.
11
The main and pilot valves close.
12
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.
NOTE: This unit is equipped with a
blocked vent shutoff (spill) switch. If the
venting system becomes blocked or
there is continuous spillage, the vent
shutoff switch will shut off the unit
heater.
*See Fan Controls on page 27.
• 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.
• Hi-limit and blocked vent (spill) wires
minimum 200°C.
• Refer to installation instructions for
venting, gas piping and start-up
procedures.
• - - - Indicates Field Wiring.
• ___ Indicates Factory Wiring.