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689670-BIM-A-0311
Reset
Remove the call for cooling, by raising thermostat setting higher
than the conditioned space temperature. This resets any
pressure or freezestat flash codes.
Electric Heating Sequence Of Operations
The following sequence describes the operation of the electric
heat section.
Two-stage heating:
a. Upon a call for first stage heat by the thermostat, the
heater relay (RA) will be energized. After completing the
specified fan on delay for heating, the UCB will energize
the blower motor. If the second stage of heat is required,
heater relay (RB) will be energized. After completing the
specified fan on delay for heating, the UCB will energize
the blower motor.
b The thermostat will cycle the electric heat to satisfy the
heating requirements of the conditioned space.
Electric Heat Operation Errors
Temperature Limit
If the UCB senses zero volts from the high temperature limit,
the indoor blower motor is immediately energized.
This limit is monitored regardless of unit operation status, i.e.
the limit is monitored at all times.
If the temperature limit opens three times within one hour, it will
lock-on the indoor blower motor and a flash code is initiated
(See Table 27).
Safety Controls
The UCB monitors the temperature limit switch of electric heat
units.
The control circuit includes the following safety controls:
Limit Switch (Ls)
This control is located inside the heater compartment and is set
to open at the temperature indicated in the Electric Heat Limit
Setting Table 22. It resets automatically. The limit switch
operates when a high temperature condition, caused by
44
inadequate supply air flow occurs, thus shutting down the
heater and energizing the blower.
Table 22: Electric Heat Limit Setting
UNIT (TONS)
ZU-A3 (3)
ZU-A4 (4)
ZU-A5 (5)
ZU-A3 (3)
ZU-A4 (4)
ZU-A5 (5)
ZU-A3 (3)
ZU-A4 (4)
ZU-A5 (5)
LIMIT
HEATER
VOLTAGE
SWITCH
kW
OPENS °F
3
6
208/230
9
15
6
9
208/230
15
20
6
9
208/230
15
20
24
3
6
480
9
15
6
9
480
15
20
6
9
480
15
20
24
9
600
15
9
600
15
20
9
15
600
20
24
Johnson Controls Unitary Products
155
155
170
170
155
170
170
170
155
170
170
170
170
155
155
170
170
155
170
170
170
155
170
170
170
170
170
170
170
170
170
170
170
170
170
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