UH-PRC002-EN32
Electric
Power
Sequence of Operation
Hydraulic Modulating Control
With Bypass
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 unit is equipped with a bypass
manifold containing a single-stage
electric valve which is in parallel with the
hydraulic modulating gas valve. The
bypass is activated by a thermostat and
overrides the hydraulic control. The
override thermostat should be a remote
room thermostat. If the temperature falls
below the override thermostat’s setpoint
the thermostat opens the bypass valve,
and the unit operates at full fire. The unit
will continue at full fire until the override
thermostat is satisfied and no longer
calls for heat, at which point the bypass
valve is closed. The unit is again
controlled by the hydraulic sensing bulb
on the mechanical valve.
7
The hydraulic sensing bulb on the
mechanical valve calls for heat.
8
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.
9
The unit continues to fire until the
hydraulic sensing bulb is satisfied and
no longer calls for heat.
10
The mechanical modulating valve closes
shutting off the main gas supply to the
burners.
11
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.
