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29UH-PRC002-EN
Electric
Power
Sequence of Operation
Typical wiring diagrams and sequence of
operations for indoor units with
intermittent pilot ignition are shown
below. On duct furnaces, the fan motor
and associated controls shown in the
wiring diagrams are not integral to the
unit. It is essential, however, that the air
handling system be interlocked with the
duct furnace to prevent duct furnace
operation without airflow.
Single-Stage Control
With power applied to the unit:
1
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.
6
Main burners are lit at 100 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
The unit continues to fire until the
thermostat is satisfied and no longer
calls for heat.
9
The main and pilot valves close.
10
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.
• Refer to installation instructions for
venting, gas piping and start-up
procedures.
• - - - Indicates Field Wiring.
• ___ Indicates Factory Wiring.