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31UH-PRC002-EN
Electric
Power
Hydraulic Modulating Control
With power applied to the unit:
1
The thermostat calls for heat. NOTE: If a
thermostat is not supplied, steps 2
through 5 will be constant as long as
power is applied to the unit.
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 electric gas
valve opens. At this time, the fan time
delay (optional on duct furnaces) is
energized. The fan time delay relay
closes and activates the fan motor.
NOTE: The unit firing is controlled by the
hydraulic sensing bulb on the
mechanical valve. The mechanically
operated valve (not illustrated) is in
series with the single-stage electrically
operated valve.
6
The hydraulic sensing bulb, on the
mechanical modulating valve, calls for
heat.
• 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.
7
The mechanical modulating valve opens
allowing gas flow to the main burners.
Main burners are lit at 50 percent of the
unit’s rated input. The mechanical
modulating valve modulates between 50
percent and 100 percent of firing rate
depending on the temperature rise
required.
8
The unit continues to fire until the
hydraulic sensing bulb is satisfied and
no longer calls for heat.
9
The mechanical modulating valve closes
which shuts off the main gas supply to
the burners.
10
The room thermostat is satisfied and no
longer calls for heat. The main and pilot
valves closes. 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.